Ncea Level Two Chemistry Revision Guide 2005

NCEA LEVEL TWO CHEMISTRY

REVISION GUIDE

2005 edition

Selected assessments with suggested answers

Really Useful Resources

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The suggested solutions and notes are those of the author. NZQA and the Sixth Form External Examination take no responsibility for them.

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Preface

This book is intended for students preparing for NCEA Level Two Chemistry. It includes a large number of appropriate assessments with suggested answers. There are useful hints included in both the question and answer sections to help students achieve with excellence.

The first section of the book is a series of questions in topic order following the seven achievement standards. The second section provides suggested answers with explanations. By practising questions based on each achievement standard, students will become more familiar with what is required.

Every effort has been made to make the revision guide consistent with the latest terminology and curriculum requirements. The 2005 edition takes into account the changes that arose from the 2004 review of the achievement standards. Obviously, we have tried to eliminate mistakes, but no doubt some remain so I apologise in advance for these. Please contact me if you dispute an answer. Constructive feedback is always welcome.

I sincerely thank the team who helped put the book together. They include Caroline Pau, Stephanie Woods, Anna Ferrier, Gabrielle Christenhusz, Helen Powell and Philip Arndt.

I sincerely hope that you will find this resource helpful and rewarding. I am sure that your results will improve with its use. Good luck in Chemistry this year and in the future, and I wish you every success in the NCEA Level 2 examination in November.

Gary Kennett

Editor

25 January 2005

CONTENTS

Carry out Qualitative Analysis (AS90305)

Guidel ines p5

Perform an Acid-Base Volumetric Analysis (AS90306)

Guidel ines p6

Gravimetric or Colorimetric Analysis (AS90763)

Guidel ines p7

Nature of Structure and Bonding (AS90308)

Quest ions p8 Answers and Notes p65

Structure and Reactions of Organic Compounds (AS90309)


Principles of Chemical Reactivity (AS90310)


Oxidation-Reduction Reactions (AS90311)

Quest ions p51 Answers and Notes p 110

Periodic Table at end of book

Feedback and Order Forms

Copies at the back of the book.

Qualitative Analysis (AS90305) 5

QUALITATIVE ANALYSIS (AS90305)

Carry out qualitative analysis (2.1) (3 credits - INTERNAL)

You will be expected to be able to:

• carry out procedures and use knowledge of precipitation reactions to determine ions present in solution.

Key Points:

1. A table of ions will not be provided.

2. A procedure, such as a flow chart, to assist in determining the unknown ions will be provided.

Solubility rules that are applicable will be provided.

3. Ions to be identified will be limited to: Ag+ , Al3+, Ba2+, Cu2+, Fe2+, Fe3+, Mg2", Pb2+ Na+ ,

N H / , Zn 2 t , СГ, CO32", I", NO3 , OH", and SO4

2". Na+ and NO3" are identified by a

process of elimination. NH4+ will be identified using its reaction with NaOH .

4. To gain Achievement the following will be required:

• Carry out given procedures to determine ions present in solution. Determination of the ions must be supported by experimental observations and identification of precipitates formed. This could include distinguishing between names pairs of anions or pairs of cations.

5. To gain Achievement with Merit the following will be required:

• Carry out given procedures to determine ions present in solution, and justify the identification. Determination of the ions must be supported by experimental observations and identification of precipitates formed. Justification must include balanced equations for the reactions where precipitates are formed.

6. To gain Achievement with Excellence the following will be required:

• Carry out given procedures, involving the formation of complex ion(s), to determine ions present in solution, and justify the identification. Justification must include balanced equations for the formation of complex ions. Complex ions may include FeSCN2+ and those formed when 0H"(aq) or NH3(aq) react with cations listed in 3. above, such as [Ag(NH3)2]+, [Al(OH)4]", Pb(OH)4]2", [Zn(OH)4]2", [Zn(NH3)4]2+, and [Cu(NH3)4]2+.

7. The assessment task will be set by your teacher. It is not appropriate to provide sample tasks and answers for this achievement standard.

6 Acid-Base Volumetric Analysis (AS90306)

ACID-BASE VOLUMETRIC ANALYSIS (AS90306)

Carry out an acid-base volumetric analysis (2.2) (3 credits - INTERNAL)


• carry out an acid-base volumetric analysis using a given titration procedure.

Key points:

1. The titration procedure and balanced equation will be provided.

2. The standard solution to be used in the titration will be provided.

3. In solving volumetric problems, the titration data used may be that collected you, or that provided by your teacher.

4. To gain Achievement you will be required to carry out an acid-base titration, and use titration data to calculate the concentration of a solution.

• At least two titre values must fall within a range of 0.4 mL. The average titre value must be within 0.8 mL of the expected outcome;

• The calculation must be carried out using an appropriate formula (not provided). Minor errors such as numerical errors or incorrect conversion of volumes will not be penalised.

5. To gain Achievement with Merit you will be required to carry out an acid-base titration with reasonable precision, and use titration data to correctly determine the concentration of a solution.

• At least three titre values must fall within a range of 0.4 mL. The average titre value must be within 0.5 mL of the expected outcome;

• A titration calculation must be carried out using only concordant titre values.

6. To gain Achievement with Excellence you will be required to carry out an acid-base titration with high precision, and use titration data from a reaction where the stoichiometry is not one-to-one, to correctly determine the concentration of a solution.

• At least three titre values must fall within a range of 0.2 mL. The average titre value must be within 0.2 mL of the expected outcome;

• A titration calculation where the stoichiometry is not one-to-one must be carried out correctly using only concordant titre values. The final answer must have correct units and an appropriate number of significant figures.

7. The assessment task will be set by your teacher. It is not appropriate to provide sample assessment tasks and answers for this achievement standard.

Quantitative Chemical Problems (AS90763) 7

Q U A N T I T A T I V E C H E M I C A L P R O B L E M S (AS90763)

Solve simple quantitative chemical problems (2.3) (2 credits - INTERNAL)


• Solve simple quantitative chemical problems using the relationships a n c l c =

Key points: 1. To gain Achievement you will be required to solve simple quantitative problems.

• Solving simple quantitative problems may involve using the relationships n - — and

M

с = ^ to calculate one variable given the other two. Some problems may require you to

determine molar masses from given atomic masses and formulae. 2. To gain Achievement with Merit you will be required to solve quantitative problems.

• You will be required to solve problems that require an application of the relationships

n = — and c = ^ . Example of suitable problems include:

- calculating % composition of a compound;

- determining empirical and molecular formulae;

- calculations involving determination of the mass of solid needed to prepare a given volume of a standard solution;

- determining, in moles, the amount of water removed on heating a sample of a hydrated salt to constant mass.

3. To gain Achievement with Excellence you will be required to solve complex quantitative problems.

• Complex problems will typically involve more than two steps, and the use of stoichiometric principles. Examples could include: - calculating the mass of a substance produced or consumed in a reaction given the

mass of another reactant or product and the balanced equation;

- determining the number of waters of crystallisation in the formula of a hydrated salt;

- determining the concentration of a solution given titration data and the balanced equation.

4. The assessment task will be set by your teacher. It is not appropriate to provide sample assessment tasks and answers for this achievement standard.

8 Structure and Bonding (AS90308)

S T R U C T U R E A N D B O N D I N G (AS90308)

Describe the nature of structure and bonding in different substances (2.4) (4 credits - EXTERNAL)


• describe bonding in simple molecules;

• describe the nature of various types of solids.

Key points:

1. Simple molecules have no more than four electron pairs about any atom (including multiple-bonded species). You may be required to draw Lewis structures and determine shape and polarity.

2. Types of solids are restricted to molecular, ionic, metallic and covalent networks. The nature of solids will be limited to the type of constituent particles (ions, atoms or molecules) and the attractive force between them (ionic, covalent or metallic bonds, or weak intermolecular forces).

3. Properties of solids include electrical conductivity, melting point, and solubility in polar nad non-polar solvents.

4. To gain Achievement the following will be required: • Describe the bonding in simple molecules and the nature of solids. A description requires

you to identify, name, draw, and give characteristics, or an account of, the bonding.

5. To gain Achievement with Merit the following will be required: • Link selected properties of simple molecules and different types of solids to their structure.

6. To gain Achievement with Excellence the following will be required: • Discuss properties of substances in terms of structure and bonding. A discussion requires

you to show an understanding as to how or why something occurs by linking chemistry ideas/principles. It may involve you justifying, relating, evaluating, comparing, contrasting and/or analysing the nature or structure of bonding.

QUESTION ONE (Answerp65)

The Lewis structure for hydrogen chloride, HCl, is «§- H

Draw a Lewis structure for each of the following molecules.

1. CO2

2. PH3

3. CH2Cl2

4. H2CO

5. F2O

Strucmre and Bonding (AS90308) 9

QUESTION T W O (Answerp65)

For each of the following elements or compounds:

(a) find the total number of valence electrons in the molecule;

(b) draw a Lewis structure;

(c) give a name for the shape.

1. NH3

2. H2O

3. O2

4. BF3

5. F2

6. Cl2O

7. N2

QUESTION THREE (Answer p66)

(a) For each of the following molecules:

(i) name the shape of the molecule;

(ii) draw a diagram to clearly illustrate the named shape;

(iii) state whether the molecule is polar or non-polar.

1. H2O

2. SO2

3. CCl4

4. NCl3

(b) Explain why the molecules CCl4 and NCl3 are polar or non-polar (as described in your answer to (a) above).

QUESTION FOUR (Answer p67)

For each of the Lewis structures below:

(a) predict the shape of the molecule (by name or 3-D diagram);

(b) state whether the molecule is polar or non-polar;

(c) explain why the molecule is either polar or non-polar.


I H - C - H

I H

2.

3. : o = c = o :

H - N - H

H

I F - S - F I

B I

I F t

I . H - C - F l

I F I

QUESTION FIVE (Answer p68)

When volcanoes erupt, a number of gases may be released. These include sulfur dioxide (SO2), carbon dioxide (CO2), hydrogen sulfide (H2S) and water vapour (H2O). At the surface, sulfur dioxide may also oxidise to form sulfur trioxide ( S O 3 ) .

Complete the table below by:

(a) (i) drawing a Lewis structure (electron dot diagram) for EACH of the formulae;

(ii) naming the shape for CO2, H2S, and SO3

Formula of molecule (i) Lewis structure (ii) Name of shape

SO2 bent or V-shaped

CO2

H2S

H2O bent or V-shaped

SO3

Structure and Bonding (AS90308) 11

(b) State whether the molecules CO2 and H2S from the table above are polar or non-polar. Explain your answers. (You may use Lewis diagrams in your explanation.)

(c) The shapes of SO2 and H2O molecules are both described as bent or V-shaped. The molecules are drawn below, with approximate bond angles shown.

O O H H

120° 109°

Explain why the bond angles in these two molecules are different.

(d) Following an eruption, lakes and streams become more acidic due to SO2 dissolving in the water and reacting with it. Use the structure and bonding in H2O and SO2 to explain why SO2 is soluble in H2O.

QUESTION SIX (Answer p69)

For each of the substances below:

(a) identify the type of solid it represents (metallic, ionic, molecular or extended covalent network);

(b) identify its name or formula (choose from magnesium chloride (MgCl2), water (H2O), silicon dioxide (SiO2), and silver (Ag).

Solid Melting point (0C)

Boiling point (0C)

Conduct electricity?

Soluble in water?

Type of solid

Name or formula

1. O 100 No Yes

2. 1700 2230 No No

3. 961 2210 Yes No

4. 714 1418 No Yes, and solution conducts electricity


QUESTION SEVEN (Answerp69) (SFEE)

The table below gives some properties of chlorides of the period 3 elements sodium, magnesium, silicon, phosphorus and sulfur.

Properties of the substances NaCl MgCl2 SiCl4 PCl3 SCl2

Melting point/°C 801 714 -70 -92 -78

Boiling Pointz10C 1465 1418 +58 76 59

Electrical conductivity when solid poor poor poor poor poor

Electrical conductivity when liquid good good poor poor poor

pH of a water solution 7 7 < 7 < 7 < 7

(a) Describe the bonding found in:

(i) NaCl;

(ii) SCi2.

(b) Explain the difference in melting point between NaCl (+8010C) and SCl2 (-78°C).

(c) Explain why MgCl2 does not conduct electricity as a solid but does conduct electricity when liquid.

QUESTION EIGHT (Answerp70)

Melting points of the chlorides of selected third-row elements are given in the table below.

Name of substance Formula Melting point ( C)

sodium chloride NaCl 801

magnesium chloride MgCl2 712

silicon chloride SiCl4 -68

sulfur dichloride SCl2 -80

(a) Describe the trend shown by the melting points of third-row chlorides in the table above.

(b) This trend in melting points is due to the type of bonding involved in each of the substances. For each of the substances below, describe the type of bonding that must be broken to melt the substance.

Name of substance Formula Melting point (*C) Type of bonding

sodium chloride NaCl 801

magnesium chloride MgCl2 712

silicon chloride SiCl4 -68

sulfur dichloride SCl2 -80


QUESTION NINE (Answer p70)

Complete the table below by:

(a) identifying the type of particle in EACH solid as atoms or ions or molecules.

(b) naming the attractive force that is broken when EACH solid melts.

(c) stating whether the relative melting point of EACH solid is high or low.

Name of solid Type of solid

(a) Type of particle

in solid - atoms or ions or molecules

(b) Attractive force

broken when solid melts

(C) Relative

melting point - high or low

ice (H2O)

molecular

silicon dioxide (SiO2)

covalent network

iron (Fe) metallic

potassium chloride (KCI)

ionic

QUESTION TEN (Answerp70)

For each of the following solid substances:

(a) state the type of particle found in the solid substance (atoms, ions or molecules).

(b) specify the attractive force that is broken when the solid substance melts.

(c) describe the attractive force existing between the particles of the solid as either weak or strong.

1. Sulfur(Se)

2. copper (Cu)

3. magnesium oxide (MgO)

4. diamond (C).


QUESTION ELEVEN (Answerp71) (SFEE)

(a) Consider the compounds HCN and NH3

(i) Draw the three-dimensional skeletal shapes of the compounds above.

(ii) Name the shape.

(iii) Give the approximate bond angle.

(iv) State whether the molecule is polar or non polar.

(b) Justify your choice of polarity for NH3 above.

QUESTION TWELVE (Answerp71) (SFEE)

Swimming pool water is constantly recirculated and filtered. The filtration takes out some of the pool debris, but chemical treatment is needed to remove the bacteria introduced by people's bodies. Without chemical treatment, the bacteria would thrive at pool water temperature, typically around 22°C-25°C.

Chlorine (Cl2) reacts with water to form hypochlorous acid (HOCl) as shown in the equation below:

Cli + H2O > H O C l + H t +Cl"

(a) Draw a Lewis diagram (electron dot diagram) for H0C1.

(b) Explain the angular/bent shape of the HOCl molecule.

QUESTION THIRTEEN (Answerp71)

(a) What name is used to describe diamond and graphite?

(b) Explain how you could account for the differences conductivity, between these two substances.

QUESTION FOURTEEN (Answerp71)

Diamond and graphite are allotropes of carbon. This means that they are both made only of carbon atoms but the atoms are arranged differently, which results in different physical forms of the same element.

Some uses of diamond and graphite are shown below.

Allotrope Use(s)

Diamond • Used to make saws to cut marble

Graphite • Used as a solid lubricant in machinery

• Used to make electrodes

Discuss the structure and bonding within diamond and graphite, and relate this to the uses shown in the table above.

(SFEE)

in physical properties, like electrical


QUESTION FIFTEEN (Answerp72)

The elements in Group 17 of the period table are called halogens.

(a) (i) Complete the table below to show the states of the first four halogens at room temperature (25°C).

Name of halogen Formula Melting point

0C Boiling point

0C State at 25°C

fluorine F2 -220 -188

chlorine Cl2 -101 -35

bromine Br2 -7 59

iodine I2 114 184 solid

(ii) Describe how the state of bromine at room temperature (25°C) can be determined from the data given.

(b) The table below shows three properties of iodine crystals (I2). Explain, in terms of the structure and bonding within the solid, why the solid has EACH of the properties stated.

Property Explanation in terms of structure and bonding within the solid

Iodine crystals will readily sublime (change from a solid to gas) when heated gently over a Bunsen burner.

Iodine crystals are more soluble in cyclohexane than in water.

Iodine crystals do not conduct electricity.

(c) Magnesium chloride (MgCl2) and sulfur dichloride (SCl2) are both chlorides of Row 3 elements. Describe the type of bonding present in solid samples of EACH of these chlorides.

QUESTION SIXTEEN (Answerp73)

For each of the following explain, in terms of structure and bonding within each solid, why the solid has the property describe:

(a) Solid magnesium chloride (MgCl2) is a poor conductor of electricity. However, when melted, magnesium chloride is a good electrical conductor.

(b) Chlorine (Cl2) has a low melting point of - 1 0 Г С .

(c) A piece of zinc (Zn) can be easily re-shaped without breaking into smaller pieces.


QUESTION SEVENTEEN (Answer p73) (SFEE)

(a) Draw Lewis diagrams of the following, indicating molecule shapes and bond angles. You will also need to correctly name each of the shapes and the predicted bond angle.

HCN H2CO CF4

(b) The type of bonding found in the carbon tetrafluoride you have just drawn is different from that found in substances such as KBr or MgCl2. Discuss this statement with reference to two types of bonding.

(c) Substances which have bonding like that found in NaCl have very high melting points and they conduct when molten or while in aqueous solution. Explain this statement.

(d) HCN and CF4 both contain polar bonds. Yet one is a polar molecule and the other is not. Explain both these statements.

QUESTION EIGHTEEN (Answer p74) (SFEE)

(a) Fill in the missing information on the table relating to bonding:

Substance Physical property Bonding type Other example

CaCl2

NH3

K

(b) Compare the following terms:

(i) Allotropes and isotopes, using carbon as an example;

(ii) A polar bond and a polar molecule, using ammonia and methane as examples;

(iii) Formation of an ion and an ionic bond.

QUESTION NINETEEN (Answerp74) (SFEE)

(a) Draw Lewis or Electron Dot Diagrams for the species below. Those which are in bold, the shape will need to be named, while the one in italics, the expected bond angle will need to be indicated.

(i) CO2 (ii) CH3CI (iii) PH3

(b) The electron arrangement of some elements is indicated below:

X 2,8,2 Y 2,8,4

(i) What type of bonding will Y most likely exhibit?

(ii) What would the formula be if X and Y were to combine?


(c) Explain the following observations:

(i) Magnesium oxide and water are both compounds involving the element oxygen. Magnesium oxide is a solid of very high melting point and water is normally a liquid and, as ice, has a reasonably low melting point.

(ii) Sand is predominately silica, SiO2; silica is characterised by its low solubility and reactivity. It can be melted only by heating to 1700°C. Another group 14 oxide, carbon dioxide, does not exist naturally as a solid, and when frozen as 'dry ice' it readily sublimes at -78°C. Carbon dioxide is more soluble and reactive than silica.

(iii) The conductivity of glacial acetic acid is very low. When glacial acetic acid is diluted with water, the conductivity increases.

QUESTION TWENTY (Answerp75)

Carbon and silicon are both elements found in Group 14 of the periodic table. Both elements show a combining power of +4 in forming oxides, with the respective formulae CO2 and SiO2.

Some of the properties of these oxides are as follows:

Oxide Melting point (0C) Conductivity of solid Hardness of solid

CO2 Sublimes at -78 poor brittle

SiO2 1700 poor very hard

Discuss the structure and bonding within carbon dioxide and silicon dioxide, and relate these to the properties shown in the table above.

QUESTION TWENTY-ONE (Answerp75) (SFEE)

(a) For each of the following facts, write a sentence of explanation:

(i) Sulfur has a low melting point.

(ii) Diamond is an extremely hard substance.

(iii) Petrol does not dissolve in water.

Write a clear definition of:

(i) an Ionic Bond:

(ii) a Covalent Bond.

QUESTION TWENTY-TWO (Answerp75) (SFEE)

(a) Draw Lewis Diagrams or Electron Dot Diagrams for ethyne (C2H2) and chloromethane ( C H 3 C I ) .

(b) Which of these molecules would have a dipole moment? (i.e. be polar)

(c) What effect does molecular polarity have on the boiling point of a compound? Explain why it has this effect.

(d) Describe the shapes of the following molecules.

(i) H2O. (ii) NH3.

(iii) CO2.


(e) Explain why CO2 is a non-polar molecule even though it has polar bonds.

(f) Write the formula of a compound which contains both ionic and covalent bonds

QUESTION TWENTY-THREE (Answerp76) (SFEE)

Use the periodic table at the back of the book. Select the correct element out of the first 36 elements (Hydrogen to Krypton) as answers to the following questions. In some cases more than one element will be correct:

(a) Which element is stored under oil?

(b) Which element exists as a green gas?

(c) Which element forms both a 2+ and a 3+ ion?

(d) Which element forms a giant covalent network solid?

(e) Which element has the highest electronegativity?

(f) Which element has a cation with an electron configuration of 2,8?

(g) Which element is a liquid at room temperature?

QUESTION TWENTY-FOUR (Answerp76) (SFEE)

Some elements such as gold, lead and sulfur have been known for thousands of years but others have been recent discoveries. In fact, early attempts to draw up the Periodic Table of the Elements resulted in gaps where the particular elements had not yet been discovered. Imagine it is your task to find element number 37 (which we now know as Rubidium). Using your Periodic Table, describe two important properties of this element that could assist you in searching for it and identifying it.

QUESTION TWENTY-FIVE (Answerp77) (SFEE)

(a) Use your knowledge of structure and bonding to explain the following properties:

(i) Magnesium oxide has a very high melting point.

(ii) Copper metal is malleable.

(iii) Oxygen is a gas at room temperature.

(iv) Silicon dioxide does not conduct electricity.

(b) Some molecules contain polar bonds but are non-polar molecules.

(i) Which of the following molecules would be an example of this? HCl NH3 CO2 H2O

(ii) Explain why the molecule you have chosen has no overall polarity.

(c) Phosphorus, like nitrogen, has five electrons in its valence shell. It forms a compound called phosphine (PH3) by reacting with hydrogen.

(i) Draw a Lewis structure (or electron dot diagram) for PH3

(ii) Name the shape of this molecule.

(iii) Which of the following solvents would you expect phosphine to be most soluble in? water cyclohexane petrol tetrachloromethane


(iv) Select from the following options to complete the two sentences below: strong weak intermolecular intramolecular

• Covalent bonds are forces that are .

• Van der Waals forces are forces that are

(d) (i) Explain why solid sodium chloride acts as an electrical insulator but molten sodium chloride acts as a conductor,

(ii) Draw a sequence of at least two labelled diagrams to show how particles in a crystal of sodium chloride dissolve in water.

QUESTION TWENTY-SIX (Answerp78) (SFEE)

(a) Explain why neon (Ne) is a gas under room conditions but carbon (C) is a solid.

(b) The following table represents the arrangement of particles in different types of substances at certain temperatures and gives some properties of the substances. Some numbered gaps have been left.

Structural diagram

ooooooooooo 0 0 0 0 0 0 0 0 0 0

ooooooooooo oooooooooo

OO 00

OO

o * o * o * o * o * o * o * o * o * o * o * o * o * o « o * o ®

(b)(i).

0 * 0 0 * 0

o * o o * o

Type of substance

Metallic Molecular (b)(ii). Simple atomic (b)(iii).

Example (b)(iv). Air (b)(v). Neon Water

Melting point

Usually high OjXvO- Very high (b)(vii). Low

Electricity conductor

Good conductor

Non-conductor

Conducts when... (b)(viii).

Non-conductor

Non-conductor

For (b)(i). to (b)(viii) above, give an appropriate answer to complete the table.

(c) Under the conditions of very high pressure at the core of the planet Jupiter, hydrogen exists as a solid and is thought to have a lattice structure similar to Lithium. Explain a property that such solid hydrogen would show on Jupiter which solid hydrogen would not normally show on the Earth.

(d) In New Zealand, common chemicals around hot springs are sulfur (Sg rings) and hydrogen sulfide (H2S) gas. (i) Draw a Lewis structure (or electron-dot diagram) for hydrogen sulfide and name its shape.

(ii) Draw a simple diagram for a sulfur molecule. (iii) What colour is sulfur under room conditions? (iv) Why is sulfur a solid and hydrogen a gas at 250C and 1 atmosphere pressure?

M/gmol ': (H)= 1 (S) = 32

Answers - Structural Formulae and Reactions (AS90309) 20

S T R U C T U R A L F O R M U L A E A N D REACTIONS OF C O M P O U N D S (AS90309)

Describe the structural formulae and reactions of compounds containing selected organic functional groups (2.5) (4 credits - EXTERNAL)


• describe structural formulae and reactions of formulae containing selected organic functional groups.

Key points:

1. Naming of organic molecules is done according to IUPAC convention.

2. Equations should be written using either names or structural formulae.

3. When writing the structural formulae, either the condensed form or the expanded form is acceptable.

4. Selected organic functional groups are limited to haloalkanes, alcohol, alkene, alkyne, ester, and carboxylic acid.

5. Compounds are limited to those containing no more than eight carbon atoms. Larger organic molecules may be used in questions involving the linking of structure and reactivity.

6. Isomerism is limited to structural and geometric (cis-trans) isomers.

7. Knowledge of primary, secondary and tertiary classification of alcohols and haloalkanes is expected.

8. Reactions are limited to: • the addition reactions of alkenes with H2ZPt, Cl2, Br2, H20/H+ and HCl (including the

identification of major and minor products on addition to asymmetric alkenes); • the reactions of alkenes with MnO4"; • polymerisation of alkenes; • halogenation of alkanes (limited to monosubstitution); • oxidation of primary alcohols to form carboxylic acids; • elimination of water from alcohols; • acid reactions of carboxylic acids; • formation of esters (may including triglycerides) from carboxylic acids and alcohols; • hydrolysis of esters.

9. To gain Achievement the following will be required: • Describe structures and reactions of organic compounds. A description requires you to

identify, name, draw, and give characteristics, or an account of, the structures or reaction.

10. To gain Achievement with Merit the following will be required:

• Link structure and reactivity of organic compounds.

11. To gain Achievement with Excellence the following will be required: • Discuss reactivity and structure of organic compounds. A discussion requires you to show

an understanding as to how or why something occurs by linking chemistry ideas/principles. It may involve you justifying, relating, evaluating, comparing, contrasting and'or analysing the structure or reaction.

Structural Formulae and Reactions (AS90309) 21

QUESTION ONE (Answerp79)

Complete the following table showing the structural formulae and IUPAC (systematic) names for some organic compounds.

Structural formula IUPAC name

(a) OH

I CH3—CH2—CH—CH3

(b) O //

CH 3—CH—CH 2—C

Cl OH

(c) Methyl propanoate

(d) 2,4-dichlorobut-1 -ene

QUESTION TWO (Answerp79)

\ ^

H H I I I

H - C C O I I

H H

H H H H I i i i

- C — C — C — C - H I l l l

H H O - H t i

(SFEE)

3. (CH3)2CHCH2CHj 4.

о / /

C H , — C

\ OH

6 .

CL H N /

C = C / \

H H C H , — C = C — C H ,


(a) Write the number of the compound above that is:

(i) an alkene;

(ii) an ester;

(iii) an alcohol.

(b) Identify the compounds numbered 1. to 6. above.

QUESTION THREE (Answerp80)

Vitamin C has the structure shown on the right.

(a) On the molecule indicate the section that would readily react to decolourise bromine water.

(b) Two of the - O H groups in the molecule have been labelled as (A) and (B).

Classify these -OH groups as primary, secondary or tertiary alcohol groups.

H O \

O H

QUESTION FOUR (Answerp80)

Refer to the following compounds to answer questions (a)-(g) below.

CH3CH2CH2CH2OH CH3CH=CHCH3 HCOCH2CH2CH3

O

A B C

CH3CH2CH2CH3 CH3CH2CH2COH CH3C=CH2

Il I O CH3

D E F

(a) What type of alcohol is compound A?

(b) State the type of reaction that will change compound A to compound E.

(c) Describe what would be observed if compound E is added to sodium carbonate.

(d) Contrast the reactions of compound B and compound D with bromine. Include relevant observations and equations, and identify the type of reaction that occurs in EACH case.

(e) Explain what is meant by the term structural isomer. Use examples from the above compounds to illustrate your answer.


Compound B has two geometric isomers (cis and trans). Draw the TWO isomers.

Compounds A, C and E are all colourless liquids. Describe tests that would identify EACH of these compounds if the labels were left off the bottles. Use the physical and chemical properties of these compounds to select suitable tests. Include any observations that would be made when carrying out these tests.

Complete the table by writing:

(i) the name of EACH functional group circled.

(ii) the systematic name for EACH compound.

Compound Name of functional group Systematic name of the compound

с н 3 с н / б о н \

с н / с о с н з

I o J

CH3

I

CH3CCH3

©

( ^ Ъ ^ 2 = С Н £ Н 2 С Н С Н 3

CH3

24 Structural Formulae and Reactions (AS90309)

QUESTION TWENTY-SIX (Answer p94) (SFEE)

The following flow diagram shows a series of reactions involving six carbon compounds.

(a) Complete the table below by giving the structural formulae and naming the compounds involved.

Compound Structural Formula Name

A

B

C

D O

Il

CH 3 CH3


E Propanoic Acid

F

(b) State the colour change that will occur in the mixture when C is converted to D. You need to state what the colour was originally and what it changed to.

(c) Give the name of the type of reaction that converts B into C.

(d) Predict the degree of solubility of compound A and B in water.

QUESTION SIX (Answerp83)

An unsaturated compound A, C3H6, reacts with water under acidic conditions to form two new products, B and C. Product B reacts with acidified potassium dichromate solution to form D. Product D reacts with a solution of sodium carbonate producing bubbles of gas. Product B reacts with D in the presence of sulfuric acid and compound E is formed. Compound E has a characteristic smell.

Identify the structural formula and name for each compound A, B, C, D and E.



Compound X is analysed and found to have the following percentage composition by mass: 38.40% carbon, 4.80% hydrogen and 56.8% chlorine. Compound X has molar mass of 125gmol"'. Relative atomic masses: H = I C = 12 Cl = 35.5

(a) Calculate the empirical formula of X.

(b) Determine the molecular formula of X.

(c) Compound X can exist as geometric isomers. Give the structural formula for a pair of geometric (cis-trans) isomers of X.

QUESTION EIGHT (Answerp84) (SFEE)

Hydrogenation is important commercially to convert unsaturated vegetable oils into solid margarine. When some of the unsaturated carbon bonds are hydrogenated, the melting point increases and the vegetable oil becomes a solid at room temperature.

(a) Explain what is meant by the term unsaturated.

(b) (i) Complete and balance the equation for the complete hydrogenation of pent-l,3-diene.

CH2 = CH - CH = CH - CH3 + H2 *

(ii) Give conditions and catalyst necessary for this reaction.

(c) Complete and balance the equation for the complete combustion of pent-1,3-diene.

CH2 = CH - CH = CH CH3 + 0 , >

QUESTION NINE (Answerp85)

(a) Polypropylene is the common name for a polymer used widely in clothing designed for warmth. It is an addition polymer made from propene (CH3CH=CH2). Draw a section of the polymer showing at least TWO repeating units.

(b) Propene can undergo other addition reactions, as given below. Complete the following equations, showing the structural formulae of the organic products. [For (ii), it is necessary to give BOTH of the possible organic products.]

(i) CH3CH=CH2 H2/Pt

H20/H*

(") CH3CH=CH2 and

(iii) Identify and explain which product in (ii) above is produced in the larger amount.


QUESTION TEN (Answerp85) (SFEE)

Saponification is a process by which an animal fat such as glyceryl tristearate (a molecule containing three ester groups) can be hydrolysed by aqueous sodium hydroxide. The glyceryl tristearate is heated with a strong sodium hydroxide solution for a long time.

(a) Complete the equation for the hydrolysis of the glyceryl tristearate molecule, giving the structural formula and the common or chemical name of product B.

O Il

H 2 C - O - C-(CH 2 ) 1 6CH 3

O H2C-OH Il I

H C - O - C - ( C H 2 ) I 6 C H 3 + 3NaOH(aq) > HC-OH + I

O H2C-OH Il

H 2 C - O - C-(CH 2 ) 1 6CH 3 glycerol

(Product A) (Product B)

(b) Give one use for each of the products above.

Glycerol: Product B:

(c) When glycerol is heated, it decomposes to the aldehyde, acrolein.

HCO

HC Ii

H2C (i) What other substance is formed when glycerol decomposes to acrolein?

(ii) Slowly, over time, a bottle of glycerol can become contaminated with acrolein. What chemical test could you perform to show the presence of acrolein? Give the reagent you would use and the result you would observe.

(iii) Acrolein (Mr = 56) has a boiling point of 52.7°C, while the corresponding alkene but-l-ene (Mr = 56) has a boiling point of -6.2°C. Explain why acrolein has a higher boiling point than but-l-ene.

Acrolein can undergo oxidation to form acrylic acid (propenoic acid) as shown below.

f0i > CH2 = CH-CHO CH2=CH-COOH

Acrylic acid is a very important industrial chemical and has a very pungent/acrid smell. It is used as a starting material to produce a number of important esters and polymers.

(iv) Acrylic acid can undergo additional polymerisation. Draw a length of poly (acrylic acid) showing at least three monomer units linked together.


QUESTION ELEVEN (Answerp86)

Two common polymers are polypropylene and polyvinyl chloride (PVC). A section of each polymer is shown in the table below. Draw the structural formula for the monomer molecule for each polymer.

(a)

(b)

Section of the polymer Monomer molecule

Polypropylene C H 2 C H I C H 1 C H J

ч / \ / \ / \ C H C H C H C H

I l l l С Н , C H , C H J C H 1

Polyvinyl chloride (PVC)

С Н , C H 1 C H 2 C H 2

\ C H C H C H C H

I l l l CL CL CL CL

QUESTION TWELVE (Answerp86) (SFEE)

(a) Alcohols are an important group of organic molecules. What is the general formula of alcohols?

(b) Draw the structural formulae and name three isomers of the alcohol having the molecular formula C 4 H 1 0 O .

(c) Another form of isomerism involves the lack of free rotation around the C=C bond at room temperature. Draw the structural formula of /rans-but-2-ene.

(d) Alcohols may be produced industrially by a hydration (addition) reaction with an alkene in the presence of a catalyst such as dilute sulfuric acid. Write the balanced equation showing the formation of ethanol from ethene.

QUESTION THIRTEEN (Answerp87) (SFEE)

Carboxylic acids are a group of organic molecules which also contain oxygen atoms. They may also be used in the laboratory to produce esters.

(a) What is the name of the ester drawn below?

о

C H 5

C H J

(b) Name the alcohol and the carboxylic acid this ester was produced from.

(c) State what different physical or chemical properties you could use to distinguish clearly between separate samples of the alcohol, the carboxylic acid in (b) above and the ester in (a) above?



(a) The flow diagram below may be useful to you in answering the following questions:

(i) Name the lettered substances of C and D.

(ii) What is the general formula of the homologous family that compound B belongs to?

(iii) Draw and name an isomer of compound D.

(iv) In the reaction where compound D is produced from compound B, what conditions and reagents are necessary?

(v) What is the name of the process in which compound C is produced from compound A?

(vi) Write the balanced equation for the formation of compound C from compound A.

(b) Define catenation.

(c) What is the electron configuration of carbon?

(d) What are 2 physical properties of the alkane C7H16?

(e) (i) Using structural formulae, write out the equation for the formation of propyl ethanoate.

(ii) What is the name of the homologous family that this compound belongs to?

(f) The formation of polyesters is a condensation reaction. Explain what is meant by this statement.

QUESTION FIFTEEN (Answer p88)

(a) Draw the structural formula of the organic product in each of the following reactions,

(i) OH CH3

с + CH,—CH, — CH,—OH »•


(ii)

C H

C H , 4

C H ,

H C I

(iii)

C H 3

I / 4

C H , C H 2

+ H 2 0 / H "

(iv)

H - O -

H H H I I I

- C — C — C - H + C B O 72 " / H ~

H I I

H H

(b) Name the functional group in each of the products of reactions (i) to (iv) above.

(c) Draw the structure of the major product formed when two molecules of HCl add to limonene (a substance found in citrus fruit).

C H ,

H J C C H

H ! C C H 2

Limonene 1 I + 2HC1 H 1 C /

T H

I A

H , C C H i

QUESTION SIXTEEN (Answer p89) (SFEE)

Some of the reactions of ethanol are illustrated in the following flow chart. The reagents and conditions needed are written over the arrows.

B & C

HT/Cr2O72

Cone. R S O , heat

2 4

CH 3 CH 2 OH

(a) Write the formulae for the products В, C, and D

(b) There are two possible products for A. Draw the structural formula for each product.


(c) Product D can be polymerised.

(i) Write the name of this polymer;

(ii) Draw a structural formula for this polymer.

(d) Explain why ethanol is very soluble in water.

QUESTION SEVENTEEN (Answer p90)

Compounds X, Y, Z, W, V and V' are involved in a series of reactions as shown by the scheme below. In this scheme only the organic reaction products are indicated. Molecular formulae for compounds X and W are given. Additional information about the compounds is listed in the box at right.

Complete a table that identifies the structural formula and name for EACH compound. (Hint: Begin with compound X, which is a straight chain molecule.)

QUESTION EIGHTEEN (Answerp91) (SFEE)

(a) Draw a structural formula for the primary, secondary and tertiary alcohol which all have the same molecular formula C 4 H 9 O H .

(b) In the homologous series what is the molar mass difference between any two consecutive numbers?

(c) Describe how you would distinguish between cyclohexane and cyclohexene using chemical tests.

(d) A student knows that three bottles which have lost their labels contain the following organic chemicals: Ethanoic acid, ethanol and ethyl ethanoate. Describe the simplest and quickest way you could decide what was in each bottle.


QUESTION NINETEEN (Answer p91)

Chemical test can be used to distinguish between pairs of compounds.

Identify tests to distinguish between the following pairs of compounds shown below and:

1. describe the test to be carried out;

2. describe the expected observations for the test used;

3. clearly explain how the test results can be used to distinguish between the molecules in each pair of compounds and why the test used is a suitable one.

(a) butan-1 -ol and but-2-ene.

(b) butanoic acid and methylbutanoate.

Crude oil is a complex mixture of hydrocarbon molecules. Many of these molecules belong to a homologous series called the Alkanes. The lowest members of the alkane series are gases and occur underground as a mixture called natural gas.

(a) What is the meaning of the term hydrocarbon?

(b) Write the general formula of the alkane homologous series.

(c) What is the name and molecular formula of the alkane molecule in unrefined natural gas which has three carbon atoms in its molecule?

(d) One fraction of refined petroleum is called petrol. Petrol contains some alkane molecules with six carbon atoms in them. Draw TWO possible structural isomers of these alkane molecules.

(a) Draw the structural formula of ethene.

(b) (i) Draw the structural formula of the substance formed when ethene reacts with bromine

(ii) What type of organic reaction is this?

(c) What observation would be made if the original reactants were compared to the final reaction

QUESTION TWENTY (Answer p91) (SFEE)

QUESTION TWENTY-ONE (Answer p92) (SFEE)

water.

mixture?

QUESTION TWENTY-TWO (Answer p93) (SFEE)

Lactic acid is an acid found in the body. The structural formula for lactic acid is shown below:

H - C — с — с

H

(a) Draw a circle around the hydrogen atom that dissociates most easily from the molecule.


(b) Suppose you wrote the formula for lactic acid as HLa. Using this shorthand for the formula, write the dissociation equation for this weak acid in water.

(c) Lactic acid contains two functional groups. One of these is called the carboxylic acid group. What is the name of the other functional group?

QUESTION TWENTY-THREE (Answerp93) (SFEE)

Analysis of aspirin tablets involves dissolving aspirin tablets in a known volume of standard sodium hydroxide. The following reaction takes place:

2NaOH(aq)+HOOCC6H4COOCH3(s) * NaOOC6H,COH(aq)+CH3COONa(aq)

(a) Name one of the products.

(b) Three bottles of organic liquids cyclohexene, ethanol and ethanoic acid have lost their labels. Using only aqueous solutions of bromine and sodium carbonate, explain how you could identify each chemical.

QUESTION TWENTY-FOUR (Answerp93) (SFEE)

(a) Copy out the numbers A.(i) - A.(v) and write down the appropriate names or formulae which would complete the table below.

Fuel Components (names and formulae)

Name Formula Name Formula

CNG A.(i). CH4 and A.(ii). C2H6

LPG A.(iii). A.(iv). and Butane A.(v).

(b) (i) Explain what is meant by the term 'functional group' in organic chemistry.

(ii) Give the names or formula of two different functional groups. (Do not name the homologous series).

QUESTION TWENTY-FIVE (Answer p94)

(a) A compound of molecular formula C2H2CI2 can exist as cis-trans (geometrical) isomers. Draw, and name, the cis-trans isomers.

(b) There is another structural isomer of C2H2CI2 that cannot exist as cis-trans isomers. Draw the structural formula for this molecule and explain why this molecule cannot exist as cis-trans isomers while the one you have drawn above in (a) can.



Study the flow chart diagram for the production of soap below and answer the questions following it.

(a) Give the formula for the two raw materials.

(b) Name another substance that could be used instead of sodium hydroxide.

(c) What type of reaction is Reaction 1?

(d) What is the role of the steam?

(e) What is reagent X?

(f) Give one use for glycerol.

QUESTION TWENTY-SEVEN (Answerp95)

Fats and oils are triester molecules. Hvdrolvsis of a fat can be represented bv the equation below.

O Il

CH2OC(CH2)I6CH3

O O Il Il

CHOC(CH2)I6CH3 + 3NaOH > 3CH3(CH2)i6CONa + Compound M

O

CH2OC(CH2)I6CH3

(a) Circle ONE of the ester groups in the fat.

(b) Write the name of the functional group that would be present in compound M.


Unsaturated fats are usually considered to be healthier than saturated fats.

(c) What is meant by the term unsaturated?

(d) Describe a test, using a solution of bromine in a non-polar solvent, that could be carried out in the laboratory to compare the degree of unsaturation of two fats.

QUESTION TWENTY-EIGHT (Answerp95) (SFEE)

Study the flow chart diagram for the production of margarine below and answer the questions following it. Substance

X Sodium Hydroxide

Hydrogen

Extraction and Heating

( Extraction and Heating

f Commercial V. Margarine

Steam at 210° C

Additives

(a) (i) What type of impurity is removed by adding sodium hydroxide?

(ii) Identily substance X.

(iii) What is meant by "volatile"?

(iv) How does the steam treatment remove volatile impurities?

(v) Name the type of reaction involved for Treatment 2.

(vi) Name two substances added to raw margarine to make commercial margarine.

(b) Fats and oils are esters of long chain fatty acids and glycerol.

(i) What is meant by "a long chain of fatty acid"? Use a diagram to illustrate your answer.

Oils are considered to be polyunsaturated.

(ii) What is meant by the term "polyunsaturated"?

(c) (i) Reducing sugars can be tested for by using Fehling's solutions A and B. Describe what colour change you would expect to see for a positive Fehling's test.

(ii) What is the structural difference between soluble and insoluble starch?

Answers - Chemical Reactivity (AS90310) 36

CHEMICAL REACTIVITY (AS90310)

Describe thermochemical and equilibrium principles (2.6) (5 credits - EXTERNAL)


• understand the principles of chemical reactivity by describing and using simple thermochemical and equilibrium principles.

Key points:

1. Thermochemical principles are limited to: • classification of reactions as exothermic or endothermic; • determination of enthalpy changes; • factors affecting rates of reaction - restricted to changes in concentration, temperature,

surface area, and the presence of a catalyst.

2. Equilibrium principles are limited to: • dynamic nature of equilibrium and the effect of changes to the system. This is restricted to

changes in temperature (A r H given), concentration, pressure, or addition of a catalyst;

• equilibrium constant expressions for homogeneous systems and the significance of the magnitude K;

• the nature of acids and bases in terms of proton transfer; • calculations involving Ar

ii and pH;

• properties of aqueous solutions of strong and weak acids and bases including ionic species such as NH4

+. The properties are restricted to conductivity, rate of reaction, and pH.

3. To gain Achievement the following will be required: • Describe thermochemical and equilibrium principles. A description requires you to

identify, name, draw, and give characteristics, or an account of, the principles.

4. To gain Achievement with Merit the following will be required: • Interpret information about thermochemical and equilibrium principles. An interpretation

requires you to provide reasons for how and why.

5. To gain Achievement with Excellence the following will be required: • Discuss information about thermochemical and equilibrium principles. A discussion

requires you to show an understanding as to how or why something occurs by linking chemistry ideas/principles. It may involve you justifying, relating, evaluating, comparing, contrasting and/or analysing the thermochemical or equilibrium principle.

Chemical Reactivity (AS90310) 37

QUESTION ONE (Answerp97) (SFEE)

A commercially bought cold pack, which can be used for sports injuries, consists of a sealed outer plastic bag containing ammonium nitrate crystals and a thin inner plastic bag containing water. When the outer bag is bent and twisted, the inner bag breaks, allowing the ammonium nitrate crystals to come into contact with the water and the following dissolving takes place:

N H 4 N O 3 ( S ) » NH4+(aq)+N03"(aq) AH = +25kJmol''

(a) Which of the following are the most energetically stable species in the above dissolving process?

(i) Reactants or products.

(ii) Justify your answer.

(b) If the outer bag contains 20g of ammonium nitrate, calculate the number of moles of ammonium nitrate present. Relative atomic masses: N = 14 H = I 0 = 1 6

(c) Calculate the energy change caused by the dissolving of 20g of ammonium nitrate crystals.

(d) If the inner bag contains IOOmL of water, calculate the concentration of the resulting solution.

QUESTION TWO (Answerp97) (SFEE)

The curves shown below show the rate of production of hydrogen gas in various stated reactions. The curve labelled B involves the reaction between 20ml of ImolL 1 HCl and excess magnesium turnings at room temperature.

(a) Explain why excess magnesium turnings were used in each case.

(b) Match up the following reactions with the letter of a curve on the graph above.

(i) 40mL of ImolL"1 HCl and excess magnesium turnings.

(ii) 20mL of 2molL"' HCl and excess magnesium turnings.

(iii) 20mL of 0.5molL'' HCl and excess magnesium turnings.

38 Chemical Reactivity (AS90310)

(c) A catalyst of Cu2+ ions can be used to speed up the rate of reaction. Explain how a catalyst can increase the rate of reaction.

(d) Name two other variables, not mentioned above, that could also affect the rate of the reaction.

QUESTION THREE (Answerp97)

A 1 g lump of calcium carbonate was added to a 250 mL beaker containing 100 mL of 1.0 molL1

hydrochloric acid solution, at room temperature (25°C). Bubbles of carbon dioxide were produced.

The experiment was repeated under different conditions, as given below.

(a) For EACH change, describe how the reaction rate would be affected. Use the words 'increase', 'decrease' or 'remain the same' in each answer.

Change 1: The temperature of the reaction mixture was increased to 40°C.

Change 2: 100 mL of water was added to the acid. 100 mL of this diluted acid solution was added to a 1 g lump of calcium carbonate.

Change 3: The 1 g lump of calcium carbonate was ground to form a powdei, and then 100 mL of the 1.0 molL"1 hydrochloric acid was added.

Change 4: A 500 mL beaker was used instead of the 250 mL beaker, but the same amounts of reactants were used.

(b) Explain the effect on the reaction rate for Change 1 and Change 2 above by referring to the collisions of particles.

QUESTION FOUR (Answerp98) (SFEE)

Consider a gas equilibrium reaction: reactants^) ~~ productS(g)

The following information for percentage conversion of reactants to products was obtained at different temperatures and pressures. The results are shown in the table below.

Temperature Pressure/kPa Temperature

1.0 2.0 3.0

400°C 15% 25% 35%

600°C 10% 20% 30%

800°C 5% 15% 25%

(a) State Le Chatelier's Principle.


(b) Use the information in the table above to determine the correct equation below, which could represent the gaseous equilibrium.

1. A(g)+B(g) < 1 3C(g)+2D(g) Д H i s positive

2. A(g)+B(g) » 3 C(g)+2D(g) AWis negative

3. 2А($+2В(й v C(g)+2D(g) AH is positive

4. 2A(g)+2B(g) < > C(g)+2D(g) Д H is negative

(c) For your chosen equilibrium in (b) above, write an expression for the equilibrium constant, Kc.

(d) For your chosen equilibrium equation in (b) above, use Le Chatelier's Principle to answer the following questions:

(i) How would the equilibrium position be affected if extra reactant B was put into the reaction vessel? Explain.

(ii) How would the equilibrium position be affected if the temperature of the reaction mixture decreased? Explain.

QUESTION FIVE (Answer p98) (SFEE)

We may use chemicals in our everyday lives - cleaners, medicines, foods etc. Sulfuric acid is used in car batteries, sodium hydroxide is the active ingredient in drain cleaners, iodine is the active ingredient in Betadine® and the ethanoic acid is present in vinegar.

Sulfuric acid is produced by the Contact Process which involves, in the final stage, the addition of water to oleum, H 2 S 2 O 7 , as shown in the equation below.

H2S207(l)+H20(l) > 2H1SO4(I)

(a) What mass of the sulfuric acid can be obtained when 8.9g of oleum is reacted with excess water? Relative atomic mass: 0 = 1 6 S = 32 H = I

(b) If the mass of sulfuric acid in (a) above is dissolved in IOOOmL of water, a solution of concentration 0.1 molL'1 is formed. Calculate the pH of this solution.

(c) A 25mL quantity of a different drain cleaner fluid, with a concentration of 5.5molL'' sodium hydroxide, is added to 50mL of hydrochloric acid of concentration 3.OmolL"1.

(i) Write an equation for the reaction between sodium hydroxide and hydrochloric acid.

(ii) Calculate the number of moles of sodium hydroxide in 25mL of the drain cleaner fluid of concentration 5.5molL"'.

(iii) Calculate the number of moles of hydrochloric acid in 50mL of 3.OmolL 1 hydrochloric acid.

(iv) Calculate the pH of the final mixture.


QUESTION SIX (Answerp99)

Consider the equilibrium equation for the conversion of SO2 to SO3 in the Contact Process:

(SFEE)

2S02(g)+02(g> 2SO,(g> AH = - 1 OOkJmol •1

(a) Write the equilibrium expression for this reaction: Kc =

(b) In this industrial process, both vanadium catalysts, as VO3" or vanadium pentoxide V 2 O 5 are used. Explain how a catalyst works in a chemical reaction.

(c) What effect do catalysts have on the equilibrium position for the formation of sulfur trioxide? Explain your answer carefully.

(d) Explain two factors which would increase the production of sulfur trioxide. Justify your answer with reference to Le Chatelier's Principles.

(e) IfATc = 2300 at 400°C, what does this tell you about the relative concentrations of SO2 and SO3 in this reaction?

(a) On a graph, draw and label an energy diagram for the production of sulfur trioxide. Show reactants, products, activation energy and energy change. The x-axis is labelled Reaction Co-ordinate, and the>>-axis is labelled Enthalpy.

(b) If 1500 tonnes of SO3 are produced, how many moles of oxygen are consumed''

(c) What would be the value of AH in the above reaction producing 1500 tonnes of SO3?

QUESTION EIGHT (AnswerplOO)

Classify EACH of the following processes as either endothermic or exothermic.

(a) H2(g)+'/202(g) -» H2Ote, ArH= -286 kJ mol"1.

(b) Photosynthesis - food-making process in plants.

(c) Freezing of water.

(d) Dissolving sodium hydroxide in water (the temperature increases).

(e) Sublimation of solid carbon dioxide to carbon dioxide gas.

QUESTION SEVEN (AnswerplOO) (SFEE)


QUESTION NINE (Answer plO1)

Octane is a key component in petrol, and burns according to the following equation:

C 8 H m o + 12^0 2 ( g ) 8C02(g ) + 9H2O0 ) A1H = -5500 kJmol"1

(a) Explain whether burning octane is an exothermic or endothermic reaction.

(b) 1.00 litre of octane contains 6.12 moles of the fuel. Calculate the energy released when 1 litre of the fuel is burnt.

(c) Using hydrogen gas (H2) as a fuel for cars, rather than octane, is viewed as better for the environment.

Calculate the mass of H2 required to produce the same amount of energy as 1.00 litre of octane, as calculated in (b) above. State your answer to three significant figures.

Н а д + | o 2 ( g ) H 20 ( g ) ArH = -286 kJmol-1

QUESTION TEN (AnswerplOl)

Hydrogen peroxide, a common bleaching agent, decomposes as follows:

H2O20) • H 2 Q m + ИОад ArH= -98.2 kJmol"1

(a) Calculate how much energy is released when 5 moles of hydrogen peroxide decompose.

(b) Calculate how much energy is released when 1.0 g of oxygen is formed by the decomposition of hydrogen peroxide.

(c) Calculate the mass of hydrogen peroxide that must decompose to produce 600 kJ of energy.

QUESTION ELEVEN (AnswerplOl) (SFEE)

(a) Define an acid, in terms of the Bronsted-Lowry definition.

(b) Hydrochloric acid can be described as a strong acid, whereas ethanoic acid is described as a weak acid. Explain this statement.

(c) Calculate the pH of a hydrochloric acid solution that has a concentration of 0.14molL~'.

QUESTION TWELVE (Answerpi02)

The pH of a 0.10 mol L 1 solution of acid HX and the pH of a 0.10 mol L"1 solution of acid HY are

measured. The pH of acid HX is 3 and the pH of acid HY is 1.

(a) Which of the two acids, HX or HY, is the stronger acid?

(b) Justify your answer to (a) in terms of the measured pH. (c) Describe another test that could be carried out to confirm that the acid you selected in (a) above is

the stronger of the two. Describe what you would do and what you would expect to observe.


QUESTION THIRTEEN (Answerpi02)

Two acids of the same concentration, hydrochloric acid (HCl) and propanoic acid ( C H . . C H 2 C O O H ) , have properties as shown below:

Property Hydrochloric acid (0.100 molL"1) Propanoic acid (0.100 molL"1)

Relative conductivity of solution

High Low

pH of solution 1.00 2.93

(a) Complete the following equations to show the reaction of both hydrochloric acid and propanoic acid with water.

HCl+H 2 O +

CH3CH2COOH + H2O ^ +

(b) Explain the differences in the conductivity and pH of the two acids. In your explanation include reference to the species present in each solution.

QUESTION FOURTEEN (Answerpi02) (SFEE)

When hard water is boiled in a jug, over time a deposit of calcium carbonate (scale > will form on the bottom and sides of the jug. This can be removed at home using vinegar. Sam decides to take a more analytical approach and brings an old metal jug into chemistry class. He determines the jug contains 4.5g of calcium carbonate. He decides to use 0.50molL"' hydrochloric acid to dissolve the scale.

(a) Write a balanced equation for the reaction Sam is carrying out.

(b) List an acid-base conjugate pair in the following equation:

HCO ^H1O* ч — - » H2CO+H2O

(c) How many moles of calcium carbonate has Sam estimated to be in the jug?

(d) What volume of hydrochloric acid will Sam need to use?

(e) In the laboratory, we often use sodium carbonate to prepare a standard solution. Why would sodium carbonate be preferable to calcium carbonate?

QUESTION FIFTEEN (AnswerplOi) (SFEE)

Four colourless solutions were investigated by pupils and the following data was obtained for them:

• Solution A - pH = 7 and no reaction with HCl;

• Solution B - pH = 13 and neutralised with HCl;

• Solution C - pH = 9 and fizzed when HCl added:

• Solution D - pH = 11 with a strong odour. Neutralised with HCl.


The teacher told the students that the names of the 0.1 molL"' solutions were amongst the following list of chemicals:

• Water;

• Ammonia;

• Potassium hydroxide;

• Ethanoic acid;

• Sodium chloride;

• Sodium bicarbonate;

• Sulfuric acid;

• Ammonium chloride;

• Copper nitrate.

Mary identified them as follows:

• Solution A - copper nitrate;

• Solution B - potassium hydroxide;

• Solution C - sodium bicarbonate;

• Solution D - ammonium chloride.

(a) The teacher said that Mary had made two mistakes. Identify the ones she got wrong and select the correct solutions to replace them.

(b) Write an equation which shows why the pH of solution Di s 11.

(c) List one other solution that you think could qualify for being solution A.

(d) Write an equation which describes the reaction that happens when a few drops of pure sulfuric acid are added to water.

QUESTION SIXTEEN (Answerpi03) (SFEE)

(a) When carbon dioxide is bubbled through water the solution becomes slightly acidic. Using appropriate equations explain how this acidity is caused.

(b) (i) Write the equation for the reaction of the weak acid C H 3 C O O H with water,

(ii) Write the acid equilibrium constant expression (Ka) for this equilibrium.

QUESTION SEVENTEEN (Answerp 103)

Write the equilibrium constant expression for EACH of the following reactions:

(a) 2S02(g , + 02(g) =*==* 2S03(g)

(b) 4NH3(g) + 5 0 а д 4NO,gl + 6H20 (g )

(c) CH3COOH,.,,, < ir ( a i l ) • CH jCOO laq,


QUESTION EIGHTEEN (Answerpi04) (SFEE)

Using your knowledge of the collision theory, explain the following observations:

(a) Fine sawdust (wood) powder suspended in the air in saw mills can lead to explosions if it is accidentally sparked.

(b) Magnesium metal burns more vigorously in oxygen than it does in air.

(c) A mixture of oxygen gas and hydrogen gas will not react, but if a spark enters the mixture they react explosively.

QUESTION NINETEEN (Answer p 104)

The reaction between 20.0mL of 0.500 molL"1 hydrochloric acid and 20.0mL of 0.250 molL"1 sodium thiosulfate solution at room temperature (25°C) produces a precipitate of sulfur that makes the solution go cloudy after about 5 minutes.

(a) How would the time taken for the solution to go cloudy be affected if the reaction were carried out in a water bath at a temperature of 50° C ?

(b) With reference to the collisions of particles, explain why the reaction is affected in this way.

QUESTION TWENTY (Answer p 104) (SFEE)

Sketch an appropriate energy diagram for an endothermic reaction that has a significant activation energy. Add to your diagram (using a different colour pen) the change made to it when a catalyst is used to help the reaction. Label your diagram clearly.

When a solution containing iron III ions (Fe3+) is added to a solution containing thiocyanate ions (SCN") at room temperature, a blood red product forms. The product is a complex ion with formula [FeSCN]2+. The test tube and its contents showed a slight increase in temperature during the reaction.

(a) Write the equilibrium equation for this reaction.

(b) Describe what you would observe, and explain why it happens, if more iron III ions are added to the original mixture.

QUESTION TWENTY-ONE (Answerpi04) (SFEE)

QUESTION TWENTY-TWO (Answerp 104) (SFEE)

What effect does increasing the temperature of an equilibrium reaction have on:

(a) The speed of the forward reaction?

(b) The speed of the reverse reaction?


QUESTION TWENTY-THREE (Answerpi05) (SFEE)

For the gaseous reaction, 2NO, * \ ' ; 0 4 , explain what the effect would be of increasing the pressure.

QUESTION TWENTY-FOUR (Answerpi05) (SFEE)

A sample of stomach acid (HCl) was tested with universal indicator paper and gave a pH reading of 2.

(a) Would you classify HCl as a strong or weak acid?

(b) Write the equation for the dissociation of HCl in water.

(c) Assuming the pH of the acid is 2, determine its hydrogen ion concentration.

(d) Use your answer in (c) to determine what the concentration of hydrochloric acid is in your stomach.

QUESTION TWENTY-FIVE (AnswerplOS) (SFEE)

A student was investigating the reaction between two chemicals X and Y. In the reaction a colourless gas was produced which turned lime-water milky. The volume of gas production was recorded at regular time intervals giving a measure of the rate of reaction. The reaction was repeated using a different concentration of X but the same original concentration of Y and the results recorded below.

Concentration of X = 0.5molL

Volume of gas produced (mL) 0 9 18 24 28 30 30 30 30

Time taken (s) 0 10 20 30 40 50 60 70 80

Concentration of X = 0.2 moll/1

Volume of gas produced (mL) 0 7 14 20 25 28 29 30 30

Time taken (s) 0 10 20 30 40 50 60 70 80

(a) What is the most obvious conclusion that can be drawn from these results?

(b) What is the name of the gas produced?

(c) What is the chemical name for lime-water?

(d) Which of the two chemicals, X or Y is in excess?

(e) Why does the reaction rate slow down as each reaction proceeds?

(f) Chemical Z was added to the mixture and it increased the rate of the reaction. If Z was acting as a true catalyst, state TWO things about the way it works.

(g) (i) How would the rate of reaction between X and Y change if the temperature was increased?

(ii) Explain why the higher temperature causes this change.


QUESTION TWENTY-SIX (AnswerplOS) (SFEE)

In New Zealand ammonia is produced on an industrial scale by reacting nitrogen gas with hydrogen gas as shown in the equation below.

N2(g)+3H,(g) * 2NH,(g) ArH = -92kJmor ' (of reactants as shown)

(a) What name is given to this industrial process?

(b) Is a high or low industrial pressure used for this process?

(c) From the information provided, how do you know that the production of ammonia is exothermic?

(d) What effect on the amount of ammonia produced does:

(i) adding a catalyst have?

(ii) increasing the concentration of hydrogen have?

The graph below shows the effects of pressure and temperature on % yield of ammonia. (The pressures shown are 600, 300, 200, 100, 50 and 10 atmospheres.)

Effects of Pressure and Temperature on the Percentage Yield of Ammonia

- 600atm - 300atm

- 200atm -1 OOatm

- 50atm - IOatm

350 400 450 500 550 600 650 700 750

Temperature C

(e) What is the % yield for the process at 350°C and 300 atmospheres?

(f) Why is the process carried out at 450°C despite greater yields at lower temperahires?

(g) How is the overall efficiency of the entire process increased?

(h) If 28,000kg of nitrogen gas is introduced into the reaction chamber, what mass of ammonia would be formed (assuming the conversion was 15% efficient)? (Mgmol"': (N) = 14, (H) = 1)


QUESTION TWENTY-SEVEN (Answerpi06) (SFEE)

(a) Shown below are three possible fuels for car engines. Values for their molar mass (M/gmol1) and the energy released on complete combustion (as kJmol"1 and kJg"1) are also shown:

Fuel Molar mass/

gmol"1

Energy of combustion Fuel Molar mass/

gmol"1 kJmor1 kJg '

Hydrogen H2 2 285 X

Methanol CH3OH 32 755 23.6

Octane CgH i8 128 6118 47.8

(i) Calculate x - the maximum energy (in kj) that could be released on burning Ig of hydrogen.

(ii) Why is hydrogen not commonly used as a fuel for cars?

(iii) Assuming that the total energy efficiency of burning l.Og of octane in car engines is 15%, how efficient must the burning of l.Og of methanol be to release the same amount of energy? (Hint - compare the amounts of energy released on burning Ig of each fuel.)

(b) Consider the following possible exhaust products from car engines:

H2O CO CO2 SO2

(i) Which is/are poisonous?

(ii) Which cause(s) global warming?

(iii) Which gas is a reactant in the catalyst converter used in many car exhaust systems?

QUESTION TWENTY-EIGHT (Answerpi06) (SFEE)

(a) Methanol can be produced commercially by reacting hydrogen with carbon monoxide.

C O ( g ) + 2 H 2 ( g ) - CH3OH(g)

(i) Write out the equilibrium expression for this reaction: Kc -

(ii) Using Le Chatelier's Principle and the information supplied above, outline two factors which would favour the production of methanol.

(b) Methanol undergoes complete combustion in oxygen, yielding 360kJ mol"1.

(i) Write out the balanced chemical equation for this reaction.

(ii) Is this reaction exothermic or endothermic?

(iii) Draw and label an energy graph showing this combustion reaction.

(iv) For every tonne of methanol produced, what mass of hydrogen gas must be supplied?

(c) Explain one possible method for increasing the rate of the following reactions. Choose a different method in each case:

(i) A large cube of zinc is added to 0.1 molL"1 of hydrochloric acid;

(ii) Hydrogen peroxide spontaneously decomposes to water and oxygen.


QUESTION TWENTY-NINE (Answerpi07)

Two oxides of nitrogen exist in the following equilibrium system:

2N02(g) v — ^ N2O4(Q) ArH = -57kJmor ' and Kc =6 .3*10" ' a t227°C

(a) Which oxide (NO: or N2O4) would you expect to be present in the greater concentration at equilibrium?

(b) Justify your answer to (a) above.

(c) N O 2 is a brown gas and N 2 O 4 is a colourless gas. What would you expect to observe when the following changes are applied to the equilibrium system? In each case, justify your observation.

(i) The temperature is increased to 500°C (without changing the pressure).

(ii) More NO2 gas is added to the system.

QUESTION THIRTY (Answerpi08)

(a) The following equilibrium system is established when thiocyanate ions (SCN'i are added to iron (III) ions (Fe ). The resulting aqueous solution is a dark red colour. The equation representing the equilibrium system and the colours of each species involved are given below.

Fe3 +( a q ) +SCN (aq) ^ F e S C N 2 ^

pale orange colourless dark red

(i) Complete the equilibrium constant expression (Kc) for the above reaction.

(ii) When iron (III) ions (Fe3 ') are removed from the equilibrium mixture (by adding sodium fluoride), a colour change is observed. Describe the colour change you would expect to see and explain why it occurs.

(b) Ammonia is produced industrially according to the Haber process as shown below:

N2(g) + 3 H 2 ( g ) ^ 2 N H 3 ( g )

(i) Complete the equilibrium constant expression (ATc) for the above reaction.

(ii) The pressure of the system at equilibrium is increased (by decreasing the total volume of the system).

Describe the effect of this change on the amount of NH3 in the system. Explain you answer.

(iii) The percentage of NH3 present in equilibrium mixtures at different temperatures and at constant pressure is shown in the table below.

Temperature ( 0 C ) Percentage NH3 present in equilibrium mixture

200 63.6

300 27.4

400 8.7

500 2.9

Justify whether the reaction in which NH3 is formed, is endothermic or exothermic.

QUESTION THIRTY-ONE (Answerpi08)

HC0 3 (a4) is a species that may act as an acid or a base

HC0 3 (aq )+H 2 0( l ) " H2C03 (aq)+0H"(aq)

(a) Is HC03"(aq) acting as an acid or a base?

(b) Justify your answer to (a) above.

QUESTION THIRTY-TWO (Answer pi 08)

Chickens make egg shell, CaCO3, using carbon dioxide gas from the air. The carbon dioxide forms carbonic acid (H2CO3), which then reacts to form the carbonate ions (CO3

2') needed to make egg shell. Two equations showing part of this process are given below.

Equation 1: H2C03(aq ) + H2Off l ^ HC03" (aq) + H30+( a q )

Equation 2: HC03~(aq) + H2O,;, ^ C0 32

(aq) - Н , 0 Ц ,

(a) Identify three conjugate acid-base pairs in the equations above.

(b) HCO3" can act as both an acid and a base. Specify which equation above (Equation 1 or 2) shows HCO3" acting as an acid. Give a reason for your answer.

QUESTION THIRTY-THREE (Answer pi09)

A sample of solid ammonium chloride, NH4CI is dissolved in water. The solution formed is tested and is found to be acidic. Explain why the solution is acidic. Include appropriate equation(s) in your answer.

QUESTION THIRTY-FOUR (Answerpi09)

Complete the following table by showing the:

• hydronium ion concentration for both solution A and solution B • hydroxide ion concentration for solution B

• pH of solution A.

К* = [H3O+] [OH ] =1.00 x IO"14


. Consider the equilibrium system:

Solution [H3O+] / mol L 1 [OH ] / mol L"1 pH

A 0.0288

B 5.24


QUESTION THIRTY-FIVE (Answerpi09)

(a) The table below shows four different solutions. Each solution contains both hydronium ions (H3O+) and hydroxide ions (OH").

Complete the table, showing the relative concentrations of these two ions in solution, by placing H3O+ ions and OH" ions in the appropriate box. The first one has been done for you.

Solution Concentration OfH3O+ and OH" in solution

Greaterthan IxlO 7molL"' Less than IxlO 7InolL"1

Na2CO3 0.1 OOmolL1 OH H3O+

HNO3 0.100 molL"1

NaOH 0.100 molL"1

NH4Cl 0.100 molL"1

(b) Calculate the pH of a solution with a hydronium ion concentration, [H3O+], of 0.0350 molL"1. State your answer to three significant figures.

(c) If a solution of sodium hydrogen carbonate has a pH of 9.20, calculate the concentration of hydroxide ions, [OH ], present in the solution. State your answer to three significant figures.

Kw = LOOxlO'14

51 Answers - Oxidation-Reduction Reactions (AS90311)

OXIDATION-REDUCTION REACTIONS (AS90311)

Describe oxidation-reduction reactions (2.7) (3 credits - EXTERNAL)

You will be expected to be able to

• describe oxidation-reduction reactions.

Key points:

1. Knowledge of appearance and state of the following reactants and their products is required.

• Oxidants are limited to O2 ,12 , Cl,, H' Fe3*, H2O1, MnO4 Iaq) /F f , Cr,072"(aq) /H+;

• Reductants are limited to metals, C , CO, H, , Fe2+, Br , r , SO2 , (HSO,~);

• Appropriate information relating to any other oxidants or reductants will be provided.

2. Aspects of oxidation-reduction include: • determine oxidation numbers; • write balanced oxidation-reduction equations; • identify oxidants and/or reductants;

• recognise the ability of halogens to act as oxidants in reactions with other elements, water or halide ions.

3. Oxidation-reduction reactions may be assessed in the context of simple electrolytic cells. Knowledge of preferential discharge is not required.

4. To gain Achievement the following will be required: • Describe oxidation-reduction reactions. A description requires you to identify, name,

draw, and give characteristics, or an account of, the reactions.

5. To gain Achievement with Merit the following will be required: • Apply oxidation-reduction reactions.

6. To gain Achievement with Excellence the following will be required: • Discuss oxidation-reduction reactions. A discussion requires you to show an

understanding as to how or why something occurs by linking chemistry ideas/principles. It may involve you justifying, relating, evaluating, comparing, contrasting and/or analysing oxidation-reduction reactions.

52 Oxidation-Reduction Reactions (AS90311)

QUESTION ONE (AnswerpllO) (SFEE)

Hypochlorous acid (HOCl) is a weak acid. The hypochlorous acid reacts with ammonia (which can come from swimmers' urine) in a redox reaction to produce chloramine:

NH +HOCl > NH2CH-H2O

Give the oxidation state of the following atoms:

(a) Oxidation state of chlorine in: HOClandNH2Cl: (NB: NH2Cl = -I).

(b) Oxidation state of nitrogen in: NH3 and NH2Cl.

QUESTION T W O (AnswerpllO)

(a) Write the oxidation number of the underlined element in each of the following species.

(i) Mn2+

(ii) H2O2

(iii) Br2

(iv) SO2

(v) CO

(b) Two oxidation-reduction reactions were carried out and the observations recorded. Use the information provided to answer the questions that follow.

Reaction One

Reactants Observation

Acidified hydrogen peroxide, H2O2caq)

and

bromide ions in solution, Br"(aq).

Both reactant solutions were colourless but when added together an orange colour was observed.

(i) Identify the species oxidised and the species reduced in the above reaction.

(ii) Use oxidation-reduction processes to explain why the solution goes orange.

Reaction Two

Reactants Observation

Chlorine gas, С1ад, was bubbled into a solution containing iron (II) ions, Fe2+(aq)

The pale green solution changed to a pale orange colour.

(iii) Explain this observation in terms of oxidation-reduction processes.

Oxidation-Reduction Reactions (AS90311) 53

QUESTION THREE (AnswerpllO) (SFEE)

Chlorine can be used to remove other impurities in the swimming pool water besides bacteria. Some metal ions such as Fe2+, can discolour the water and cause staining of the pool area. These can be removed as solids when the recirculating water passes through the filters.

d + 6 H , 0 + 2 F e 2 " > 2Cl"+2Fe(OH)3+6H'

(a) What species is the precipitate?

(b) What colour is the precipitate?

(c) The chlorine has undergone either oxidation or reduction. Choose one and justify your answer.

(d) Cb can be reacted with aqueous sodium hydroxide to form the bleaching agent sodium hypochlorite. Write a balanced equation for this reaction.

QUESTION FOUR (Answerplll) (SFEE)

Tablets containing a small quantity of potassium permanganate can be used to sterilise water supplies when you are camping in areas where the water is not treated. The quantity of potassium permanganate (KMnO4) in the tablets can be determined by titration with a colourless solution of standardised sodium oxalate (Na2C2O4).

(a) Write the half reaction for the reduction of the permanganate ion under acid conditions.

(b) Write the half reaction for the oxidation of the oxalate ion to carbon dioxide.

(c) Combine the two half reactions above to write the overall redox reaction between the acidified permanganate ion and the oxalate ion.

QUESTION FIVE (Answerplll)

When sulfur dioxide gas (SO2) is bubbled into a solution of acidified potassium dichromate solution, a colour change is observed. The unbalanced equation for this reaction is given below.

Cr2O72- + SO2 -> SO4

2" + Cr3+

(a) Identify the reductant in this reaction. Justify your answer using oxidation numbers.

(b) Write balanced half-equations for the oxidation and reduction reactions that occur.

(c) Combine the half-equations in (b) to give the balanced equation for this oxidation-reduction reaction.

(d) Describe the observations that would be expected when this reaction occurs. Explain these expected observations by referring to the species involved in the reaction.


QUESTION SIX (Answerplll)

Iodine (I2) can be produced from the reaction between potassium iodate (KIO3) and potassium iodide (KI). The iodide ions (I") and iodate ions (IO3 ) react together to form the iodine (I2).

(a) Iodide ions (I") and iodate ions (IO3 ) both contain the element iodine (I), but in two different oxidation states. What is the oxidation number of iodine (I) in each species?

(b) Identify the oxidant and reductant in this reaction.

(c) Complete and balance the half-equations for the reactions of iodide ions (I") and iodate ions (IO3 ) forming iodine (I2).

Г: Г I2

IO3": IO3- I2

(d) Combine these two half-equations to give a balanced equation for the reaction.

QUESTION SEVEN (Answerpi 12) (SFEE)

Electrolysis is a very important industrial process and has many applications for extractions and purification of metals, electroplating and the production of other important chemicals.

(a) For the electrolysis of the following substances, complete the table by giving the product formed at the anode and the product formed at the cathode.

Product at positive anode

Product at negative cathode

Cone, sodium chloride solution Hydrogen Molten lead bromide Bromine Dilute sulfuric acid

(b) (i) Write an equation for the reaction that occurs at the negative cathode in the electrolysis of the concentrated sodium chloride solution.

(ii) Write an equation for the reaction that occurs at the positive anode in the electrolysis of lead bromide.

(c) In the membrane cell for the electrolysis of sodium chloride solution, a third commercially important product is produced. This product is not directly given off at the anode or the cathode. Give the name or formula of the product.

QUESTION EIGHT (Answerpi 12) (SFEE)

Redox reactions are occurring all around you, and inside you. Combustion of fuels is a redox reaction. Write a balanced equation for the complete combustion of propane.


QUESTION NINE (Answer pi 12) (SFEE)

Acidified potassium permanganate may be used in the laboratory to oxidise iron (II) to iron (III). The permanganate is reduced to manganese (II) ions.

(a) What name would you give to the potassium ions in this reaction? Explain your answer.

(b) Write a balanced equation for the above reaction, using the ion-electron half equation method.

(c) What is the oxidation number of sulfur in:

(i) sulfate ion?

(ii) sulfur dioxide?

(d) Some alcohols may be oxidised readily. Dichromate ions may be used to oxidise butan-l-ol. Write the balanced ion-electron half equation for the oxidation of butan-l-ol to butanoic acid.

(e) Write out the balanced ion-electron half equation involving the ion responsible for the colour change you would observe in the above reaction.

(f) Now write out the complete balanced equation for this reaction.

Electrolysis is an important industrial process used to reduce metals or to plate other metals. In the laboratory, we can demonstrate this process using a power source, an electrolyte, and electrodes with suitable glassware and connectors.

(a) Explain what an electrolyte is.

(b) The electrolysis of magnesium from seawater involves, in its final step, the use of molten magnesium chloride. Write out the redox reaction which occurs at the anode.

A student decides to investigate electroplating for her practical investigation. In her initial experiments, she copper plates some iron nails. Draw and label a diagram showing how she might set up her experiment in a beaker. Be sure to include in your drawing:

• A power source;

• An electrolyte - state what she will use;

• Suitable electrodes (including the nail);

• Be sure to label the cathode and anode.

QUESTION TEN (Answerpi 13) (SFEE)

QUESTION ELEVEN (Answerpi 13) (SFEE)


QUESTION TWELVE (Answerpi 13) (SFEE)

(a)

(b)

(c)

(d)

(e)

Magnesium metal is combusted in excess oxygen. Explain why this is a redox reaction.

Silver plates can 'tarnish' after time. The tamish (Ag2S) is caused from sulfur and sulfur compounds in the atmosphere and foods.

(i) Write out the balanced half-equation for the oxidation reaction above.

(ii) Write out the balanced half-equation for the reduction reaction above.

(iii) Write the completely balanced redox equation for this reaction.

In the breath-alyzer test, ethanol is oxidised to ethanoic acid by acidified dichromate. The police officer looks for a colour change to verify his suspicions.

What are the colours of the following ions - Cr2O?2 and Cr3+.

(i) Write out the balanced half-equation for the oxidation of ethanol by dichromate.

(ii) Write out the balanced half-equation for the reduction of dichromate by ethanol.

(iii) Write the balanced equation for this reaction, using your two half equations.

Metal ores can be reduced by chemical means, for example, reducing copper from copper sulfate. Electrolysis usually is the only method to reduce very reactive metals such as magnesium.

Why, then, is electrolysis the method used to reduce aluminium?

In New Zealand, most of the raw product is shipped to the South Island for electrolysis. What is the name of the raw material, and where is its place of origin?

Indicate on the following electrolytic cell where the anode, and cathode are found.

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

Indicate on the diagram above where the reduction occurs in the electrolysis.

Give one important use for the aluminium produced.

Explain what physical property of aluminium allows for the use outlined in (v) above?

Give another example of a physical property of aluminium.


QUESTION THIRTEEN (AnswerplN) (SFEE)

A 5.Og sample of brass (an alloy of zinc and copper) was reacted with concentrated nitric acid in a fume cupboard. A brown gas was generated, and the resulting solution was a green colour. The cooled solution was mixed with sodium hydroxide and a blue precipitate was filtered off. After drying, this precipitate was found to have a mass of 4.95g, and was heated in a crucible over a Bunsen burner. The resulting black solid was weighed and the resultant mass was 4.05g.

(a) What is the name of the brown gas generated?

(b) What is the name, and symbol, of the ion resulting in the green colour of the solution?

(c) This reaction is an example of a redox reaction. What is the oxidation half-equation?

(d) Write the balanced reduction half equation.

When a piece of copper metal is placed into a solution of silver nitrate, a deposit forms on the surface of the copper.

(a) Describe one other observation you would make as the reaction proceeded over a period of time.

(b) Write the ion-electron half equation for the redox reaction taking place.

(c) Write the overall ionic equation for the reaction.

(d) Write down the oxidation state of each element present in silver nitrate. (Ag, N and 0 )

(e) In this reaction between copper and silver nitrate identify which species is acting as the oxidant (oxidising agent) and which is acting as the reductant (reducing agent).

(f) In another redox reaction, a greenish coloured gas was bubbled through a solution of potassium iodide in a fume cupboard and the solution turned brown. What is the brown coloured chemical produced by this reaction?

QUESTION FOURTEEN (AnswerplH) (SFEE)

copper г—'-'

__ deposit on surface of the copper

/ Silver nitrate solution


QUESTION FIFTEEN (Answerpi 15)

Chlorine is made industrially by the electrolysis of brine, a concentrated solution of sodium chloride. A simplified diagram of the cell is shown below.

(a) Label the anode and the cathode clearly in the dashed boxes on the diagram above.

(b) The two half-reactions occurring in the cell are shown below. Complete and balance the equation for Reaction Two.

Reaction One: 2 H 2 0 + 2e" -» 20H +H2

Reaction Two: СГ —> Cl2

(c) Gases are produced at each electrode as the electrolysis proceeds. Identify which gas (Gas A or Gas B) in the diagram above is chlorine.

Justify your choice by discussing the reactions that occur during electrolysis, the electrodes where these reactions occur, and the products that are formed.

QUESTION SIXTEEN (Answerpi 15) (SFEE)

Hydrated magnesium chloride is crystallised from MgCl2 solution. It is heated and then mixed with some sodium chloride and calcium chloride before being electrolysed. The mixture is heated sufficiently to

(b) Using the terms, left hand electrode or right hand electrode, state which of them is the anode.


(c) Magnesium will be produced by one electrode. Which one?

(d) Write the ion-electron half equation for the reaction occurring at the other electrode.

(e) Suggest a reason why the electrolysis is carried out at a temperature high enough to melt the magnesium chloride.

(f) Graphite electrodes obviously conduct electricity. Suggest another reason why they are used.

QUESTION SEVENTEEN (Answerpi 16) (SFEE)

When iron is extracted from magnetite, scrap iron consisting of such things as old car bodies, structural steel and railway lines is added towards the end of the process. Obviously, some of this scrap iron is rusty and the rust is converted back to iron in the process. The formula for rust is Fe2O3 and the chemical that causes it to change back into iron is carbon monoxide gas.

(a) Write an equation for the reaction which takes place.

(b) What are the oxidation states of iron and oxygen in Fe2O3?

(c) State one reason why the reaction works only at high temperatures.

A MCEA Level 2 Chemistry student came up with a novel way of recycling scrap iron. He suggested that the scrap should be put into a bath containing sulfuric acid which would dissolve out both the iron and its oxides and leave behind any other materials to be filtered off and discarded.

(d) Assuming there was both iron and iron (III) oxide in the scrap, write balanced equations for the two reactions which would occur with the sulfuric acid.

(e) The solution resulting from the acid treatment of the scrap contained sulfates of iron. The student suggested that the solution could be reacted with magnesium metal to form iron.

(i) Write an ion-electron half equation showing what would happen to the magnesium metal in this reaction.

(ii) Write a similar half equation showing what would happen to Fe2+ ions in this reaction.

(iii) Add these two half equations together to make an ionic redox equation.

(iv) Which metal, magnesium or iron, is the stronger reducing agent?

(v) Suggest TWO reasons why this student's recycling process would not be a good economic proposition.

(f) One of the reactions in (d) produces a gaseous by-product. The student suggested this by-product could be used to make hydrochloric acid by reacting it with chlorine gas. Write a balanced equation for his proposed reaction.

Another student, after hearing of this novel approach to recycling, suggested that it might be used to reclaim other important elements from their oxides. Her list of possible oxides was:

ZnO CuO SiO2 SO2

(g) Select one from her list which would react in the required way with the sulfuric acid.

(h) Choose one of the four oxides which will not work and explain why.


QUESTION EIGHTEEN (Answer pi 17)

The following oxidation-reduction reaction was carried out in the laboratory.

Iron (II) sulfate solution was added to acidified potassium permanganate solution.

For this reaction:

(a) What colour is potassium permanganate solution?

(b) Explain why the potassium permanganate needs to be acidified.

(c) Write the half equation for the oxidation of the iron (Il) ions.

(SFEE)

QUESTION NINETEEN (Answerpi 17)

H'lh

/ Electrode A - h

Electrode B

Electrolyte solution

(SFEE)

(a) (i) Do cations in the electrolyte move towards electrode A or B?

(ii) Name an electrolyte other than NaCl.

(iii) At which electrode does oxidation occur?

(iv) Name the electrodes, A and B.

(b) If A and B are made of graphite and the electrolyte is concentrated sodium chloride solution, write the half equation representing what takes place;

(i) At electrode A;

(ii) At electrode B.

(c) (i) Complete the diagram of the following electroplating cell by drawing a spoon at the electrode you wish to plate with copper metal.

(ii)

(iii)

Draw and label what the other electrode should be made of.

Add a label showing what solution you would use.


QUESTION TWENTY (Answer pi 17) (SFEE)

Read the following passage on fuel cells and answer the questions which follow.

A fuel cell directly reacts a fuel such as hydrogen with an oxidant such as oxygen to produce electrical energy. The chemicals are supplied continuously and the electrical energy is generated under control for as long as the raw materials are supplied. In the fuel cell used on some space-craft such as the Space Shuttle, hydrogen gas is supplied to one electrode and oxygen gas to the other electrode, as shown in the diagram below.

At the negative electrode, hydrogen reacts with aqueous hydroxide ions to form water and releasing electrons. At the positive electrode, oxygen gas removes electrons for the nickel electrode and reacts with water to regenerate the hydroxide ions.

To generate electricity, fuels cells are much more efficient than combustion reactions because chemical energy is converted directly into electrical energy. Combustion reactions are used to convert chemical energy to heat energy to mechanical energy and then to electrical energy.

Fuel cells using other fuels such as methanol (CH3OH) and hydrazine (N2H4) have been developed. The advantages of fuel cells is that they are mostly pollution free, noiseless and produce a high energy return, but disadvantages include their expense and in some cases the toxicity of the fuels (i.e. hydrazine).

(a) Suggest a label for Y on the diagram.

(b) Accurately name the oxidant and reductant.

(c) Write ion-electron equations of the two half reactions.

(d) Give an overall balanced equation for the overall fuel cell reaction.

(e) Why are fuel cells a more efficient method of converting chemical energy into electrical energy than a generator involving combustion and dynamos?

(f) Explain one advantage this fuel cell would have over lead accumulator batteries for space flights.

When using certain reagents in the laboratory, some chemicals must be acidified.

(a) Identify two such chemicals which must be acidified.

(b) Name precisely the acid that is used to acidify these chemicals and give the reason why this acid

hydrogen gas

QUESTION TWENTY-ONE (Answerpi 18) (SFEE)

is chosen.


QUESTION TWENTY-TWO (Answerpi 18) (SFEE)

Aluminium metal is extracted from aluminium oxide ore shipped into Bluff from Australia.

(a) To extract and purify the aluminium oxide from the ore, sodium hydroxide solution is needed. What property of the aluminium oxide allows this method of extraction and purification?

Shown below is a simple diagram of the electrolytic cell used in the extraction of aluminium.

solid crust graphite anode (+) carbon cathode (-) lining the ceil

molten aiuminia dissolved in cryolite

steel casing

molten aluminium flows out through tap hole

(b) Write an ion-electron equation for each electrode reaction, identifying the electrode and the products.

(c) Explain the function of the cryolite (Na3AlF6) used in the electrolysis.

QUESTION TWENTY-THREE (Answerpi 18)

When acidified potassium permanganate solution is added to freshly prepared iron(II) nitrate solution, a reaction occurs. The unbalanced equation is shown below:

MnO4 ' + Fe2+ • Fe3+ + Mn2+

(a) Is the MnO4 ' ion oxidised or reduced? Use oxidation numbers to justify your answer.

(b) Identify the oxidant and reductant in the reaction above.

(c) Write balanced equations for the oxidation and reduction half reactions.

(d) Combine the half equations in (c) to give the balanced equation for this oxidation-reduction reaction.

(e) Clearly describe what would be observed when this reaction is carried out, linking the observations to the species involved in the reaction.

(f) Complete the table below for EACH combination of species.

Species to be mixed Does a reaction occur? (yes/no)

I fa reaction occurs, describe what would be

observed.

If a reaction occurs, write a balanced equation

for the reaction.

( i ) Ch(aq) + B r ( a q)

(ii) Cl ( a q ) + h ( a q )


QUESTION TWENTY-FOUR (Answerpi 19)

Wines often contain a small amount of sulfur dioxide that is added as a preservative. If too little is added, the wine is oxidised to vinegar. If too much is added, it affects the taste of the wine. The sulfur dioxide content of wine can be determined using a reaction with iodine, I2, to produce sulfate and iodide ions.

(a) What is the oxidation number of sulfur in EACH of the following?

(i) SO2

(ii) SO42"

(b) Write ion-electron half equations for the oxidation and reduction reactions occurring between SO2

and I2.

(c) Combine the half equations in (b) above to form a balanced equation.

(d) The alcohol content of the wine can be determined by a reaction with acidified potassium dichromate. What colour change would be observed if excess wine is added to acidified potassium dichromate solution? Link the colour change observed to the species responsible for the observed colour. (Neither the alcohol nor its oxidation product is coloured.)

QUESTION TWENTY-FIVE (Answerpi 19)

Aluminium is produced by the electrolysis of aluminium oxide that has been extracted from bauxite ore. A typical electrolysis cell is shown in the diagram below. The oxygen gas produced reacts with the carbon (graphite) electrodes, so that the electrodes have to be replaced from time to time.

Carbon (graphite) SoW crust of ETORTROLYTE K O H I S O H top

Tapping hole

г Steei tank witft carbon (graphite) lining

Molten Gteclrolyte consisting of aluminium oxide dissolved in cryolite

Molten aluminium is tapped oft at intervals

(a) On the diagram above:

(i) clearly label the anode and the cathode.

(ii) show the direction of movement of ions in the electrolyte.

(b) Write balanced half equations for the reactions occurring at:

(i) the positive electrode.

(ii) the negative electrode.


(с) Write an equation for the reaction that occurs between the oxygen gas and the carbon electrode, and hence explain why the electrode must be replaced from time to time.

QUESTION TWENTY-SIX (Answer pi20)

An electrolytic cell can be used for electroplating. Electroplating is a process where a metal object is coated with a thin layer of another metal.

This process is shown in the diagram below. The spoon is electroplated with silver metal and the silver electrode gets smaller.

Power supply

silver metal

electrode Ag I spoon

Using principles of electrolytic cells, discuss how the layer of silver is plated onto the spoon. Include relevant oxidation-reduction half equations in your discussion.

QUESTION TWENTY-SEVEN (Answerpi20)

Sodium hypochlorite, NaOCl, reacts with hydrochloric acid to give sodium chloride (NaCl) and chlorine gas (Cl2). It is the reaction between the СГ and ОСГ ions that produces the chlorine gas.

(a) What are the oxidation states of the chlorine in EACH of the following species?

(i) OCl"

(ii) Cl-

(iii) Cl2

(b) Write balanced ion-electron half equations for the formation of chlorine, Cl2, from:

(i) chloride ion (СГ).

(ii) hypochlorite ion (OCl ).

(c) Identity the reaction in part (b) above that is an oxidation reaction, and give reasons for your answer.

(d) If warmed, or left to stand, a solution of sodium hypochlorite decomposes as follows:

2NaOCl • 2NaCl + O2

Clearly identify the element that is being oxidised and the one being reduced. Justify your decision.

Answers - Structure and Bonding (AS90308) 65

STRUCTURE AND BONDING (AS90308)

We have chosen not to identify each 'answer' as 'achieved', 'merit' or 'excellence'. Students who wish to do so can determine the 'level' by referring back to the standard specified at the beginning of the 'question' chapter.

QUESTION ONE (Question p8)

I- CO2 * = c = < 5 .

2- Р Н з H - P - H I H

3. CH2Cl2 л

H - c — S b I " H

4. H2CO

5- f O »?-"-?» W N N

QUESTION TWO (Question p9)

Formula of element or compound

Total number of valence electrons in molecule

Lewis structure Name of shape

1. NH3 8 H - N - H I H

Trigonal Pyramid or

triangular pyramid

2. H2O 8 'O4

Hx 4H

Bent (planar)

or

V-shaped

3. O2 12 Linear


m 24

I I F t • •

Trigonal planar

or

triangular planar

5. F2 14 M M I F - F l M aa

Linear

6. Cl2O 20 [Should be drawn 'bent')

М М М ICI — O — C l l « • « • « • Bent

7. N 2 10 I N = N I Linear

QUESTION THREE (Questionp9)

(a)

Molecule Shape Polar or Non-polar

H2O Bent / V-shaped / angular

'o* н ' 4H

Polar

SO2 Bent / V-shaped / angular

M M

Polar

CCl4 Tetrahedral

ICIt

ICI — C — Sil " I "

ICIl m

Non-polar

NCl3 Triangular Pyramid

ICI — N — C l l M M

ICII M

Polar


(b) 1. It has four polar bonds but its tetrahedral shape means that the molecule is symmetrical and so the bond dipoles (or polarity) cancel. Therefore, CCl4 is non-polar.

2. It has three polar bonds and is not a symmetrical shape (since the nitrogen has non-bonding valence electrons) and so the bond dipoles cannot cancel. Therefore, NCl3 is polar.

QUESTION FOUR (Question p9)

Lewis structure Shape Polarity Explanation 1.

H I

H - C - H I

H

Tetrahedral Non-Polar 4 polar bonds; Tetrahedral shapes means molecule is symmetrical. Bond dipoles cancel.

2.

Linear Non-polar

Atoms of similar electronegativity; No polar bonds.

3.

: ° = c = o : Linear Non-polar

2 double bonds (polar); Molecule is symmetrical. Bond dipoles cancel.

4.

H - N - H I

H

Trigonal pyramid or

triangular pyramid Polar

3 polar bonds; Shape is not symmetrical due to lone pair of electrons. Bond dipoles do not cancel.

5. М М М

• F - S - F l М М М

Bent (planar) or

V-shaped Polar

2 polar bonds; Not symmetrical molecule. Bond dipoles do not cancel.

6.

' F 4

% - B - • I

S F t M

Trigonal planar or

triangular planar Non-polar 3 polar bonds;

Symmetrical molecule. Bond dipoles cancel.

7.

I ~ l I -

H - C - F l I ~

I F I • •

Tetrahedral Polar (slightly)

4 polar bonds; Shape is slightly unsymmetrical due to different bond dipoles. C is more strongly attracted to F than H. Bond dipoles do not cancel.

8. M N M

• C l — P — C l l «• J M

ICIt M

Trigonal pyramid or

triangular pyramid Polar 3 polar bonds;

Not a symmetrical shape. Bond dipoles do not cancel.


QUESTION FIVE (Question plO)

(a)

Formula of molecule (i) Lewis structure (ii) Name of shape

SO2

W W bent or V-shaped

CO2 ф = с = ф linear

H2S H — H m bent or V-shaped

(should be drawn bent)

H2O H ' 4H

bent or V-shaped

SO3 «Si

J = S - S i

(other structures are possible; could draw S without electrons)

triangular planar /

trigonal planar

(b) CO2 is a non-polar molecule. CO2 contains polar bonds as atoms have different electronegativity, but the molecule is symmetrical due to the double bonds. This means that the bond dipoles cancel.

H2S is a polar molecule. H2S contains polar bonds and the molecule is not symmetrical. This means that the bond dipoles do not cancel.

(c) SO2 has three regions of electron density (clouds) around the S and repulsion results in a bond angle of 120°. H2O has four regions of electron density (clouds) resulting in a bond angle of 109". The greater repulsion means O atoms are further apart in the SO2 molecule and hence are less bent.

(d) SO2 and H ; 0 are both polar molecules because they contain polar bonds. They don't have symmetry to cancel the dipoles because their central atoms have non-bonding valence electrons. Since "like dissolves like" SO2 is soluble in H2O. Furthermore, hydrogen atoms can form between the oxygen atoms in SO2 and the hydrogen atoms in H2O, increasing the solubility of SO2

in H2O.

QUESTION SIX (Question pi 1)


Solid Melting point (0C)

Boiling point (0C)

Conduct electricity?

Soluble in water?

Type of solid

Name or formula

1. 0 100 No Yes Molecular H2O

(water)

2. 1700 2230 No No Covalent network

SiO2

(silicon dioxide)

3. 961 2210 Yes No Metallic Ag (silver)

4. 714 1418 No Yes and solution conducts electricity

Ionic MgClj

(magnesium chloride)

QUESTION SEVEN (Question pi2) (SFEE)

(a) (i) NaCl is made up of Na+ and Cl" ions with strong electrostatic forces of attraction/ionic bonds between them.

(ii) The S and Cl share electrons to complete valence levels in a covalent bond.

(b) NaCl: strong ionic bonds hold the ions together, therefore lots of energy is needed to overcome them. This means a high melting point of 8010C.

SCl2: weak forces of attraction between molecules, therefore less energy is required to overcome them. This means a lower melting point of-78°C.

(c) As a solid, the ions are fixed in space and cannot move, therefore cannot conduct electricity. As a liquid the ions are free to move and therefore can conduct electricity.

70 Answers - Structure and Bonding (AS90308)

QUESTION EIGHT (Question pl2)

(a) The melting point decreases across the row (from left to right), with a significant decrease from Mg to Si.

(b)

Name of substance Formula Melting point ( 'C) Type of bonding

sodium chloride NaCl 801 Ionic bonding

magnesium chloride MgCl2 712 Ionic bonding

silicon chloride SiCl4 -68 covalent

sulfur dichloride SCl2 -80 covalent

QUESTION NINE (Questionp!3)

Name of solid Type of solid

(a) Type of particle in solid - atoms

or ions or molecules

(b) Attractive force

broken when solid melts

(c) Relative

melting point - high or low

ice (H2O)

molecular molecules van der waals

forces / hydrogen bonds

low

silicon dioxide (SiO2)

covalent network molecules covalent bonds high

iron (Fe) metallic atoms metallic

bonding high

potassium chloride

(KCI) ionic ions ionic bonding high

QUESTION TEN (Question pl3)

N a m e of solid subs tance Type of par t ic le in solid At t rac t ive force b roken when solid melts

At t ract ive force between part ic les

Sulftir (S8) molecules van der Waals weak

Copper (Cu) atoms metallic bonding strong

Magnesium oxide (MgO) ions ionic bonding strong

Diamond (C) atoms covalent strong


QUESTION ELEVEN (Question pi 4) (SFEE)

(a) HCN NH3

(i) Skeletal Shape

H - C = N I

(i) Skeletal Shape

H - N - H I H

(ii) Name of shape: Linear (ii) Name of shape: Trigonal pyramid

(iii) Approx bond angle: 180° (iii) Approx bond angle: 109°

(iv) Polar or non polar: polar (iv) Polar or non polar: polar

(b) One of the following points is required:

• the bond dipoles do not cancel out;

• vector addition of the bond forces gives a non-zero resultant.

QUESTION TWELVE (Question pl4)

(a)

(SFEE)

(b) There are 4 groups of electrons around the central oxygen atom. Due to e -e" repulsion, they go towards the corners of a tetrahedron. Since two positions are occupied by lone (non-bonding) pairs, the shape is bent.

QUESTION THIRTEEN (Question pi4) (SFEE)

(a) Allotropes.

(b) Bonding - diamonds are made up of C atoms covalently bonded to 4 other C atoms forming a strong 3-D network. No free e" - no conductivity.

Graphite has a 2-D structure made of layers of covalently bonded C atoms with delocalised 4,h e' holding layers loosely together via Van der Waals forces. The 4th e" is free to conduct.

QUESTION FOURTEEN (Question pi4)

In diamond, each carbon atom forms single bonds with four adjacent carbon atoms at the corners of a tetrahedron. This results in a rigid, covalent, three-dimensional structure that makes diamond the hardest known substance. Because diamond is so rigid and hard it can be used to cut other very hard substances, such as marble.


Graphite consists of layers of two-dimensional carbon sheets. Each layer is made up of six-member rings, and these rings have mobile electrons attached to them. The presence of mobile electrons means that graphite is a good conductor of electricity and can be used to make electrodes. Also, although there are weak van der Waals forces, there is still a relatively significant gap between the layers. This gap makes graphite slippery and makes it suitable as a lubricant.

QUESTION FIFTEEN (Question pi5)

(a) (i)

Name of halogen Formula Melting point

0C Boiling point

0C State at 25°C

fluorine F2 -220 -188 gas

chlorine Cl2 -101 -35 gas

bromine Br2 -7 59 liquid

iodine Ь 114 184 solid

(ii) Its melting point is - 7 ° C so at 25°C it has melted from a solid to a liquid. Its boiling point is 590 C so at 25 ° C it has not yet boiled to become a gas. Therefore at room temperature bromine is a liquid.

(b)

Property Explanation in terms of structure and bonding within the solid

Iodine crystals will readily sublime (change from a solid to gas) when heated gently over a Bunsen burner.

I2 molecules contain a covalent bond. However, during sublimation it is the intermolecular forces that break. These are very' weak van der Waal's instantaneous dipole attractions since I2 molecules are non-polar. Therefore the attractions are easily broken so sublimation occurs at low temperatures.

Iodine crystals are more soluble in cyclohexane than in water.

I2 is non-polar since it contains two identical atoms. Water is polar and cyclohexane is non-polar. Since "like dissolves like" I2 is more soluble in cyclohexane than water.

Iodine crystals do not conduct electricity.

In order to conduct electricity there must be mobile ions or electrons. I2 molecules have neither of these since they are covalently bonded I atoms, are discrete molecules and are not ionic. Therefore they do not conduct electricity.

(c) MgCl2 is ionic so has ionic bonds (both intermolecular and intramolecular forces) between molecules and ions (i.e. forms an ionic lattice when solid). It is ionic because it contains a metal bonded to a non-metal.

SCl2 is covalently bonded between the S and Cl atoms (intramolecular forces). This is because S and Cl are both non-metals. Between the molecules (intermolecular forces) are instantaneous dipole weak van der WaaIs attractions.


QUESTION SIXTEEN (Question pi5)

(a) Solid magnesium chloride (MgCl2) is a poor conductor of electricity. However, when melted, magnesium chloride is a good electrical conductor.

Solid magnesium chloride is an ionic solid. In this state the ions are held in fixed positions but when melted the ions are free to migrate (due to increased energy) ad so are free to conduct electricity.

(b) Chlorine (Cl2) has a low melting point of - ЮГC .

Chlorine is a covalent, molecular substance. The attractive forces between molecules (van der Waals) are weak and so require little energy to be broken.

(c) A piece of zinc (Zn) can be easily re-shaped without breaking into smaller pieces.

Zinc is held together by non-directional, attractive forces between an array of positive ions and a sea of electrons. Its shape can be changed without disrupting the forces.

QUESTION SEVENTEEN (Question pi6) (SFEE)

(a) HCN H,CO CF4

и ; с s : N: * x • •

OR:

H - C = N I

H x

" c ! ; o* H

OR:

H

> = < 5 H

X X * Q • 1Sc x®^ I/1

Xr-X V >

OR:

l " l . I .

I F - C - I I m I m

I F I m

Name of Shape: Linear Name of Shape: TrigonalPlanar Name of Shape: Tetrahedral

PredictedBondAngle: 180° PredictedBondAngle: 120° PredictedBondAngle: 109°

(b) CF 4 : covalent bonding between two non-metals; e shared to complete valence shells. MgCl1 /KBr: ionic bonding between metal and non-metal. Electrostatic attraction between oppositely charged ions.

(c) Ionic bond forms from strong attraction between cations and anions. There are no mobile ions or e therefore they do not conduct as solids.

In solution, ions are free to conduct. In molten state an increase in energy allows ions to become mobile and therefore conduct mobile ions. At melting point a lot of energy is needed to separate ions due to the strong attraction.

(d) HCN is a polar molecule as dipoles created by unequal sharing (polar bonds) do not cancel due to the unsymmetrical shape of the molecule.

CF4 contains polar bonds but due to symmetry, dipoles cancel and the molecule is non-polar.


QUESTION EIGHTEEN (Question p 16) (SFEE)

(a) Substance Physical property Bonding type Other example

CaCl2 High melting points Crystalline solid

Ionic NaC 1, PbI2

NH3 Colourless gas Low boiling point

Covalent (molecular) CO,, O

K Conducts electricity Metallic Zn, Al

(b) (i) Diamond - each C covalently bonded to 4 atoms and graphite (3 C atoms) are different structures of the same element (allotropes). Isotopes 12Cand 14C have the same atomic number and different mass number due to different number of neutrons.

(ii) NH j and CH1 both contain polar covalent bonds, e" are attracted more to N and C atoms setting up a partial charge but since CH4 is symmetrical, the partial dipoles cancel and overall CH4 is a non-polar molecule. NH j is asymmetrical in shape and therefore a polar molecule.

(iii) An ion is an atom (or group of covalently bonded atoms) which have gained or lost e". An ionic bond is formed by the electrostatic attraction of oppositely charged ions.

QUESTION NINETEEN (Question pi 6) (SFEE)

(i) CO2 (ii) CH3Cl

i o i i c i : O* .Hx

H • c ' f t Shape: Linear * '

H

BondAngle: 1097109.5° (iii) PH3

• X

h : P : X •

Shape: Trigonal Pyramidal

(b) (i) One of the following points is required: • Covalent; • Ionic.

(ii) One of the following points is required: • X 2 Y ;

X Y 2


(c) (i) MgO ionic compound has a high melting point due to a strong electrostatic bond

H2O is molecular and melting involves breaking a weaker intermolecular bond.

(ii) SiO, is a giant covalent lattice/covalent solid and has a high melting point, due to strong intermolecular forces.

CO2 are discrete molecules with weak Van der Waals forces between them.

(iii) CH jCOOH weak acid dissociates poorly. It exists as molecules and therefore does not conduct. With dilution the acid dissociation increases and the free ions in solution increase conductivity.

QUESTION TWENTY (Question pi 7)

Carbon dioxide is a covalent, molecular substance and so contains discrete molecules with weak van der Waals forces between them. Very little energy is required to break these bonds and so sublimation occurs at low temperatures. As it exists as non-polar molecules, it therefore does not conduct electricity. The weak van der Waals forces are directional and easily broken so the solid line is brittle.

Silicon dioxide is a covalent network solid. The electrons are all used to make covalent bonds and are not able to move freely to conduct electricity. The covalent bonds between molecules are strong and so require a lot of energy to be broken. Therefore, SiO2 has a high boiling point and is very hard.

QUESTION TWENTY-ONE (Question pi 7) (SFEE)

(a) (i) Sulfur is a covalent molecular substance. The weak Van der Waals forces between the molecules are easily broken.

(ii) Diamond is a covalent network substance with strong covalent bonds in all directions. To break diamond, these covalent bonds must be broken.

(iii) Water is a polar solvent. The molecules in petrol are non-polar and cannot disrupt the strong intermolecular forces between water molecules.

(b) (i) A strong attractive force (electrostatic) between oppositely charged ions in a crystal structure (lattice).

(ii) A strong attractive force between two atoms created by the electron pair they are sharing.

QUESTION TWENTY-TWO (Question pi 7) ( S F E E )

(a) C2H2

H - C = C - H H J J C * * C * H OR

CH3Cl

H I W

H - C Cl» I -

H

H • X

H j c ; c£

OR


(b) Chloromethane.

(c) Increases it. Molecular polarity increases the strength of the intermolecular forces. (Higher temperatures are required to separate the molecules).

(d) (i) bent/v-shaped;

(ii) trigonal pyramid;

(iii) linear.

(e) The two bond dipoles cancel each other out (due to its symmetrical shape).

(f) Could include one of the following:

• CaCO3 ;

• NaNO 3 ;

• NH Cl. 4

QUESTION TWENTY-THREE (Questionpi8) (SFEE)

(a) One of the following points is required:

• Li; . Na;

• K.

(b) Cl.

(C) Fe.

(d) One of the following points is required:

• Si; • C.

(e) F.

( 0 One of the following points is required:

• Na;

• Mg; • Al.

(g) Br.

QUESTION TWENTY-FOUR (Question pi8) (SFEE)

Two of the following points are required: • any metallic property( s); • will have soluble compounds; • forms a+1 ion.

QUESTION TWENTY-FIVE (Question pi 8)


(SFEE)

(a) (i) The following points are required:

• magnesium oxide is an ionic solid;

• all the ions are held together by strong ionic forces needing high temperatures to separate.

(ii) Copper metal is held together by non-directional attractive forces between an array of positive ions and a "sea" of electrons. Its shape can be changed without disruption of the forces.

(iii) Oxygen is a covalent molecular substance. The attractive forces between the molecules are weak (Van der Waals) and so it has a very low boiling point.

(iv) Silicon dioxide is a covalent network solid. The electrons are all used to make covalent bonds and are not able to move freely.

(b) (i)

(ii)

(c) (i)

CO2

The bond dipoles are equal and opposite and cancel each other out.

H - P - H I

H

* • • » и, p ; и X • H

OR

(ii) Trigonal pyramidal.

(iii) Water.

(iv) Covalent bonds are intramolecular forces that are strong.

Van der Waals forces are intermolecular forces that are weak.

(d) (i)

(ii)

In solid sodium chloride the ions are held in fixed positions, but in molten sodium chloride they are free to migrate (are mobile).

O / N H H

H H \ / O

dissolved ions


QUESTION TWENTY-SIX (Question pl9) (SFEE)

(a) Neon consists of separate atoms with very weak attractions between atoms. Carbon atoms are strongly bonded to each other - covalent bonds.

(b) (i) O

O °

(ii) Ionic.

(iii) Molecular.

(iv) One of the following points is required:

• copper; • iron.

(v) One of the following points is required: • sodium chloride; • potassium chloride; • magnesium oxide; • magnesium chloride.

(vi) Low.

(vii) Low.

(viii) One of the following points is required:

• liquid; • molten; • aqueous; • dissolved.

(c) Conduct electricity - because of free moving electrons.

(d) (i) V-shaped or bent.

H H

• x

I s - H I S - H OR

(«) с с

^ S S

(iii) Yellow.

(iv) Sulfur has much larger molecules (M/gmol1: (Ss)=256, (H, )=2).

Larger molecules usually need more energy to separate them.


STRUCTURAL FORMULAE AND REACTIONS OF COMPOUNDS (AS90309)


QUESTION ONE (Question p2l)

Structural formula IUPAC name

(a) OH

CH 3—CH 2—CH—CH 3 Butan-2-ol

(b) O / /

C H 3 — C H — C H 2 — C

I \ Cl OH

3-chlorobutanoic acid

(c) C H V CH3

V O

Methyl propanoate

(d) Cl \

C — C H 2 — C H 2 — C l / /

CH2

2,4-dichlorobut-1 -ene

QUESTION T W O (Question p21) (SFEE)

(a) (i) 5

(ii) 1

(iii) 2

(b) 1. ethyl propanoate.

2. butan-2-ol (or 2-butanol).

3. 2-methyl butane.

4. ethanoic acid.

5. 3-chloroprop-l-ene.

6. but-2-yne (or 2-butyne)


QUESTION THREE (Question p22)

(a)

(b) A Primary

B Secondary

QUESTION FOUR (Question p22)

(a) Primary

(b) Oxidation

(c) Fizzing would be observed as a gas is produced.

(d) Compound B is an alkene and will undergo an addition reaction to form a 2,3-dibromobutane (СНзСНВгСНВгСНз). The orange/brown Br2 solution will therefore turn colourless.

Compound D is an alkane. Under UV light, it will undergo a substitution reaction to form an alkylhalide СНзСН2СН2СН2Вг (1-bromobutane). The orange/brown colour of Br2 will slowly turn colourless as the substitution reaction is a very slow process. The gas released is HBr which is an acidic gas and can be detected by placing damp blue litmus paper at the mouth of the test tube. The litmus paper should turn red.

(e) A structural isomer is a molecule that has the same molecular formula as another, but a different structural arrangement of atoms (e.g. molecules B & F are structural isomers as are C & E).

(f)

H H I /

H - C — C

« v „ • 4

H H I /

H - C — C H H C — C - H

H7 A

cis isomer trans isomer


Compound A: alcohol (primary). Add acidified potassium permanganate dropwise. Colour will change from purple to colourless, [or, K2Cr2O7ZHt - orange -> green change. ]

Compound C: aldehyde. Add silver nitrate solution (Tollens reagent). A silver mirror will be produced on the inside of the test-tube.

Compound E: carboxylic acid. Add water and test with litmus. Blue litmus is turned red.

Add water - A and E will dissolve; C will not.

Compound Name of functional group Systematic name of the compound

c h 3 c h / c o h \

Ю J carboxylic acid propanoic acid

сн/соснз 1W ester methyl ethanoate

C H 3

I Cthcch3

( o h )

alcohol 2-methyl propan-2-ol

( ^ с н ^ = а ^ с н 2 с н с н 3

C H 3 alkene 4-methyl pent-l-ene


QUESTION FIVE (Question p24) (SFEE)

(a) Compound Structural Formula Name

A CH3CH1CH2OH Propan-l-ol

B

CH2

/ / CHj CH

Propene

C OH

CH3 CH CH3

Propan-2-ol

D

O

/ < 4 CH3 CH3

Propanone

E H H ' O t I I / /

H - C — C — C I I \ H

H H ^

Propanoic Acid

F O W

C — O — C H 2 — C H 2 - C H 3

/ C H 2

/ /

C H 3

Propyl Propanoate

(b) The colour changes from orange to green.

(c) One of the following points is required:

• hydration;

• addition.

(d) A is soluble in water; B is insoluble in water.


QUESTION SIX (Question p25)

Compound Structural Formula Name

A H H

\ / C = C

» . / 4H

Propene

B H I

H H ° I I

H - C — C — C - H I I I

H H H

Propan-l-ol

C H

H ° H I I I

H - C — C — C - H I I I

H H H

Propan-2-ol

D H H O I I //

H - C — C — C 1 1 \ H H n O

\ H

Propanoic acid

E

У \ ^ H

H H H H \ i l l

C — O — C — C — C - H // I I I o ' H H H

Propyl propanoate

84 Answers - Structural Formulae and Reactions (AS90309)

QUESTION SEVEN (Question p26)

(a)

(b)

(c)

% +A1

C

38.40

38.40

+ smallest

H

4.80

48.80

1

= 4.8

M 1.6

= 3

12

= 3.2

12 1.6

= 2

Empirical formula: C2H3Cl

M(C2H3CI) = 62.5. So Molecular Formula = C4H6Cl2.

(Need to do (a) and (b) in order to answer(c))

Cl ^H3

\ / C-C

/ \ CH3 Cl

CH3

\ / C = C

/ \ Cl Cl

Cl

56.8

56.8

35.5

= 1.6

L6

1.6

= 1

CH3

(SFEE)

Cl Cl

\ /

У H \ H

" Л

Cl

Cl

/ = C

У \ ^H

A

QUESTION EIGHT (Questionp26)

(a)

(b)

A compound that has double or triple bonds between carbon atoms.

(i)

(ii)

CH2 = CH - CH = CH - CH3 + 2H2 CH CH CH C H C H

One of the following catalysts is required: • Ni; • Pt; • Pd.

One of the following conditions is required: • high temperature; • high pressure.

(SFEE)

(c) CH2= CH - CH = C H - C H 3 + 7 0 . • • 5CO. + 4H,0


QUESTION NINE (Question p26)

(a) CH3 H CH3 H

- п т г H H H H

(b) (i)

H H H

H - C I

— C I

— C - H I

H I

H I

H

(ii)

H H H H H H I I I I I I

H - C — C — C - H and H - C — C — C - H I I I I I I H ,A-HH н H .A-H

m —

(iii) The left-hand structural formula [CH3CH(OH)CH3] will be produced in the larger amount. The carbon 'richest' in hydrogen atoms gets 'richer' in them (Markovnikov Rule) and therefore in this case the OH adds to the 'poorer' carbon.

QUESTION TEN (Question p27) (SFEE)

(a) / 0

3CH3(CH2)i6 C

ONa

One of the following points is required:

• sodium stearate;

• sodium octadecanoate.


• glycerol:

— cosmetics;

— lubricant;

— explosives.

• Product B:

— soap for washing.


(c) (i) Water/H20.

(ii) Bromine water - acrolein will decolourise, (orange - colourless).

(iii) Acrolein is a polar molecule with stronger forces between its molecules than the non-polar but-l-ene.

(iv) H H H H H H

I _ I _ I _ I _ I _ I _

"I CI CI CI CI I H C O O H H C O O H H C O O H

QUESTION ELEVEN (Question p28)

(a)

(b)

Section of the polymer Monomer molecule

Polypropylene C H I C H I C H I C H 1

\ C H C H C H C H

I L L L C H , C H , C H , C H ,

IL

x^x

Polyvinyl chloride (PVC) C H I C H 1 C H 1 C H 2

4 C H C H C H C H

I L L L CL CL CL CL

IL

QUESTION TWELVE (Questionp28) (SFEE)


• CnH,2n+i)0H; • C„H(2n+2)0.

(b)

H H H H I l l l

H - C — C — C — C O - H I l l l H H H H

H H H H I l l l

H - C - C C C - H I l l l

H 1 H H 0 1

H

H H H I I I

H - C - C — C — O - H I I I

h H - C - H h

I H

Butan-l-ol Butan-2-ol 2-methylpropan-l-ol [or show as

2-methvlpropan-2-ol]


" H

H I ' -H H H

(d) H2O(I) + CH2 = CH2 • - ) CH3CH2OH

QUESTION THIRTEEN (Questionp28) (SFEE)

(a) Ethyl Propanoate.

(b) Alcohol - ethanol;

Carboxylic acid - propanoic acid.

(c)

Molecule Alcohol Carboxylic Acid Ester

Property: • Solubility

• Cr2O72TH+

• pH

• bicarbonate or carbonate

• Smell

Test Results: • Soluble in H2O • Colour change

from orange to green

• < 7

• gas been given off, i.e. bubbles.

• Sweet

QUESTION FOURTEEN (Question p29) (SFEE)

(a) (i) C = 2,3 dibromopentane;

D = pentan-2-ol. (or 2 pentanol).

(ii) CnH2n

(iii) One possible answer is:

- O - H

pentan-l-ol

(iv) AdddiluteH SO or H1PO, and heat. v ' 2 4 3 4

(v) One of the following points is required: • substitution; • bromination; • halogenation.

(vi) CH3CH,CH2CH2CH+2Br, , '^ i > CH3CH2CHBiCHBiCH5+2HBr.

H H I

H I

H I

H I

I H - C

I

I — C -

I

I - C —

I

I C I

I — C

I I H

I H

I H

I H

I H


(b) Ability of C atoms to join / bond together in long chains.

(c) 2,4.

(d) Two of the following points are required:

• liquid at room temperature;

• non-conductive;

• colourless;

• immiscible.

(e) (i)

. " „ Н

H H 1T H + H - O - H I I I C - C - O t

H H

(i) Ester.

(f) Polyester is formed by alternating carboxylic acids and alcohols (i.e. esters). Condensation involves the removal of the H2O molecule.

QUESTION FIFTEEN (Questionp29)

(a) (0

OH C H

с + CH—CH—

H H H H H Oi I l l l I //

H - C - С — C — O i + H - C - C I 1 " 1 \ M

H H H H O

H I

H - C -

H

H I

H - C -I H

H H у I I //

- C — C — O - C

V

(ii)

CH3 CH2

HCI H H H I I I

H - C — C — C - H 1 I 1 H d, H

(iii)

CH3

CH1 CH,

+ H.O / H"

OH

I C H 3 — C — C H 3

I CH,


(c)

(iv)

H H H H 1 1 1 4 O H - O - C — C — C - H +Cr2O7

2VH* ° H H I I I \ I I H H H C — C — C - H

// 1 1 A' H H

(b) (i) Ester

(ii) Haloalkane

(iii) Alcohol

(iv) Carboxylic acid.

Limonene

C 1 H 1 C H 1 CL

H / 4 / C \ X H

H 1 C C H I I J C C ^ H

I + 2HC1 • I H ' C C H 1 H ! C C H ,

\ H / N C 1 H X

/ Ч H I C C H I H , C J H I CL

C

QUESTION SIXTEEN (Question p30) (SFEE)

(a) B = CO 2 ;

C = H2O;

D = CH2CH2.

(b) The following structural formulae are required:

H O

H O H - C - C I // I \

H - C - C H о

H H And

(c) (i) Polythene,

(ii)

H I

. - C -I

H

H I

H I

H H I I

H I I

C -I

I - C -

I I I

- C C I I

I C -I I

H I

H I I

H H I H


(d) Ethanol is a polar molecule and so can dissolve in water which is a polar solvent.

OR, the - O H bonds allow for H bonds to adjacent molecules and H2O making ethanol very soluble in water.

QUESTION SEVENTEEN (Question p31)

Compound Structural formula Name

X H H H H H I l l l l

H - C — C C — C 0 « I I I I "

H H H H

b u t a n - l - o l

Y H H H !^t I I I //

H - C — С — C — C I I I \ H

H H H ^ m

b u t a n o i c a c i d

Z H H V H

H H H H 4 C ^ \ I I I / / H V - ^ \

H - C — C — C — C \ H I I I \ ^ c H

H H H ^ N h

b u t y l b u t a n o a t e

W V ?

C — C — C - H // I '

H - C H H \ H

b u t - 1 - e n e

V ? H H H I l l l

H - C C C C - H I l l l

H H H H

1 - c h l o r o b u t a n e

V' H H H H I l l l

H - C — C — C — C - H I l l l

H I1 H H

2 - c h l o r o b u t a n e

Note: V' is produced in greater amounts than V. V and V' can be reversed in table above.


QUESTION EIGHTEEN (Question p31) (SFEE)

(a) Primary: H I I

H - C — C I I H

H

H

H H I I

- C — C — O - H

(b) 14.

Secondary:

H H H H ! I I I

H - C — C — C — C - H I l l l

H H I H 0 1 H

Tertiary:

H

HH-{-HH I I I

H - C — с — c-1 I 1

H 1 H 0 1 H

(c) The following points are required: • react each with bromine water; • cyclohexene causes brown colour to turn colourless immediately; • cyclohexane produces no colour change.

(d) Add blue litmus/indicator to each and colour change to red (or appropriate for indicator chosen) identifies the ethanoic acid.

Add the remaining two chemicals to water - the one that dissolves is ethanol, leaving ethyl ethanoate as the remaining one. A fruity smell is the Ester.

QUESTION NINETEEN (Question p32)

(a) butan-l-ol and but-2-ene.

Test: Add to bromine solution.

Observations: But-2-ene will decolourise the solution; there will be no change in the solution's colour with the addition of butan-l-ol.

Explanation: The but-2-ene reacts with the bromine solution (addition reaction) to produce 2,3-dibromobutane. The alcohol does not react with the bromine solution.

(b) butanoic acid and methylbutanoate.

Test: Blue litmus paper.

Observations: Blue litmus paper will turn red with butonoic acid; there will be no change with methylbutanoate.

Explanation: The COOH group means that СНзСН2СН2СООН is acidic and so will turn blue litmus paper red. There will be no change with methylbutanoate.

QUESTION TWENTY (Question p32) (SFEE)

(a) A compound containing the elements hydrogen and carbon only.

(b) C„H2n+2-

(c) Name - propane.

Molecular formula C3H8 .


(d) Two of the following structures are required:

H H H I I

H H I

H I I

H - C -I I

- C — C — C -- C I

— C - H I I

H I I

H H H I

H I

H

Hexane

H H H H H I i i i i

H - C C C C C - H I l l l l

H H H - i - H H H

I H

3-methyl pentane

H

HH-{-HH I I I

H - C C — C — C - H I l l l H 1 H H H - C - H

I H

2,2 dimethyl butane

H H H H H I l i i i

H - C — C C C — C - H

1 W - H " I

H

2-methyl pentane

H

H H " ? - H H H I I I I

H - C — C — С — C - H I

2,3 dimethyl butane

QUESTION TWENTY-ONE (Question p32) (SFEE)

(a)

(b) (i)

H I

H - C - - C - H

Br Br

(ii) Addition.

(c) The brown/orange colour of bromine would have gone colourless.


QUESTION TWENTY-TWO (Question p32) (SFEE)

H H O I I //

H - C — C — C _

i А - Л ©

(b) HLa+H^O v 4 H30*(aq)+La'(aq)


• alcohol;

• hydroxyl;

• hydroxy.

QUESTION TWENTY-THREE (Question p33) (SFEE)

(a) One of the following points is required: • sodium ethanoate; • sodium acetate; • sodium salicylate; • sodium hydroxybenzoate.

(b) Cyclohexene: • does not dissolve in aqueous solutions; • two miscible layers;

• decolourises orange aqueous bromine.

Ethanoic acid: • fizzes;

• gas released with Na,C03(aq).

Ethanol: • unreactive sample; • dissolves only.

QUESTION TWENTY-FOUR (Question p33)

(a) (i) Methane;

(ii) Ethane;

(iii) Propane;

(iv) C3H8;

(v) C4H10.

(b) (i) Characteristic groups of atoms an properties.

(SFEE)

bonds which gives the molecule its characteristic


(ii) Two of the following points are required: double carbon to carbon bond;

-COOH/carboxyl group;

-OH/hydroxyl group;

-COO-ester linkage/group.

QUESTION TWENTY-FIVE (Question p33)

(a)

Cl Cl

Cis-1,2-dichloroethene Trans-1,2-dichloroethene

(b) Cl

Cl

This isomer cannot exist in cis-trans isomers because both chlorine atoms are attached to the same carbon. For geometric isomers to exist, there must be two different 'groups' on each carbon of the double bond. In (a) above we had a hydrogen atom and a chlorine atom attached to each carbon, but in this isomer we have the same 'groups' attached to each carbon.

(a) The following points are required:

NaOH;

• CH, (OCOR)CH(OCOR')CH2 (OCOR");

• R/R'/R' ' = C17H35 or suitable alternative.


QUESTION TWENTY-SIX (Question p34) (SFEE)

potash;

potassium hydroxide; sodium carbonate; potassium carbonate.


saponification;

hydrolysis.


(d) One of the following points is required:

• raise temperature;

• boils reaction mixture.

(e) Brine.

(f) One of the following points is required:

• cosmetics;

• paint;

• foodstuffs.

QUESTION TWENTY-SEVEN (Question p34)

СНЮО|СН2),6СН3

о о Il Il

CHOC(CH2)I6CH3 + 3NaOH > 3CH3(CH2)i6CONa + Compound M

O Il

CH2OC(CH2)16CH3

(b) alcohol (3 of them).

(c) The term unsaturated means that the compound contains at least one non-single bond. Therefore the compound has at least one double or triple bond present.

(d) Firstly, each of the fats would need to be homogenised and added to a non-polar solvent such as cyclohexane. Add excess bromine solution to each of the fat-containing solutions. The bromine solution is originally orange/brown coloured. Bromine reacts with carbon double bonds by an addition reaction. The more double bonds that there are in the fat-containing solution, the faster the solution will lose its colour. Therefore the degree of unsaturation can be determined by observing the degree of decolourisation of the orange/brown colour, or the amount of time taken for each solution to become colourless.

QUESTION TWENTY-EIGHT (Questionp35) (SFEE)

(a) (i) Free fatty acids.

(ii) One of the following points is required: • soaps; • sodium salt of fatty acid; • RCOONa.


(iii) One of the follow ing points is required:

easily vaporises;

low boiling point.


steam increases temperature;

low boiling point impurities evaporate.


hydrogenation;

reduction.

Two of the following points are required: vitamins; water; emulsifiers; citric acid; mineral salts; salt;

flavourings; colour (beta-carotene).

One of the following points is required: long chain carboxylic acid; organic acid with a large CH or alkyl group. C17H35COOH (or)

H I

H - C -I

H

(c)

H H I I

- C C -I I

H H

H I

H I

H I

H I

H I

H I I

C I

I

I I

C I

! C I

I C I

I

I I H

I H

I H

I H

I H

I H

//

V» ( " )

(i)

(ii)

Containing more than one carbon to carbon double bond / multiple bond.

Blue to orange/yellow/red/pink.


• soluble starches contain fewer glucose units than insoluble starch. • insoluble starch has more branching in its structure.


CHEMICAL REACTIVITY (AS90310)


QUESTION ONE (Question p37) (SFEE)

(a) (i) Reactants.

(ii) Since forward reaction is endothermic, reactants must be of lower energy, therefore energetically more stable.

(b) M(NH 4 N0 3 ) = 80

__m _ 20 80

= 0.25mol

(c) 1 mole needs 25kJ

0.25 Therefore 0.25 mol needs x 25

1

= 6.25kJ

... n 0.25 (d) с = — =

V 0.1

= 2.5molL1

QUESTION TWO (Question p37) (SFEE)

(a) To ensure that all the HCl has reacted or that the reaction goes to completion.

(b) (i) D. (ii) E.

(iii) A.

(c) A catalyst offers an alternative pathway of lower activation energy.

(d) The following points are required:

• temperature;

• surface area.

QUESTION THREE (Question p38)

(a) Change I: Increase

Change 2: Decrease

Change 3: Increase

Change 4: Remain the same

98 Answers - Chemical Reactivity (AS90310)

(b) Change 1:

As temperature is increased, the average kinetic energy of the particles is increased. More particles have sufficient energy to overcome the reaction's activation energy, collide and react. Therefore there are more successful collisions per second and more product forms per second. The reaction rate has increased.

Change 2:

The concentration of acid has been decreased so there are less acid particles present in the lOOmL. There will be fewer collisions per second and less product forming per second. The reaction rate has decreased.

QUESTION FOUR (Question p38) (SFEE)

(a) If any change is made to a system at equilibrium, the equilibrium will shift in the direction to oppose/minimise the change.

(b) 4. 2A(g)+2B<g) •- ^ C(g)+2D(g>. AH is negative,

r _ [C][D]2 (c)

[A]2[B]2

(d) (i) Move to RHS/products favoured. If concentration of B was increased, the equilibrium will respond by using up some of the extra added B, thus moving to RHS.

(ii) Move to RHS/products favoured. If we cool the system down, the equilibrium will shift in exothermic direction to produce more heat, therefore forward direction favoured.

QUESTION FIVE (Question p39) (SFEE)

(a) 178g of H2S2O7 produces 196g of H2SO4

8 9 Therefore 8.9g of H,S,O7 produces — x l96 =9 .8g l l ,S0 4

178

(b) PH = -Iog [H3O+]

= -log (0.2)

= 0.7

(c) (i) NaOH+HCl * NaCl+H,0

(ii) /i(NaOH) = cx V = 5.5x0.025

= 0.1375moles

(iii) /!(HCl) = C x V = 3 .0x0 .05

= 0.15moles


(iv) и(НС1) in the final mixture = 0.15 - 0.13 75

= 0.0125moles

n 0.0125 c(HCl) in the final mixture = — =

V 0.075 = 0.167molL"'

PH = -IogtH3CN

= -log(0.167) = 0.78

QUESTION SIX (Question p40) (SFEE)

[SO3]2

(a) K c [ S 0 2 ] 2 [ 0 2 ]

(b) A catalyst provides an alternative pathway which has a lower activation energy, therefore more particles colliding will have sufficient E a to react. A catalyst is not used up in the reaction.

(c) No effect.


• catalysts speed up forward and reverse reactions equally;

• catalysts only speed up the time to achieve equilibrium.

(d) Two of the following points are required:

• increase [SO2 ] and/or [O, ] according to Le Chatelier, equilibrium will shift to use up

additional reactants producing more product SO3 ;

• remove [SO3 ]. A decrease in [SO3 ] will shift equilibrium to produce more, therefore increasing production of [SO3 ];

• increase in pressure shifts equilibrium to the side with less volume (i.e. least number of gaseous particles) therefore producing more [SO3 ];

• In exothermic reactions, heat is produced and therefore decreasing the temperature favours the forward reaction producing more [SO ].

(e) Kc = 2300 is very large, therefore [SO3 ] is much greater than [SO, ].

1OO Answers - Chemical Reactivity (AS90310)

QUESTION SEVEN (Question p40)

(a)

(b)

(c)

с ш

2mol SO j from Imol 0 .

M(SO3)= 32+ (16x3)

= 80gmol l

160gS0 3 from 1 mol O,

0.16kg => ImolO2

0.16

= 9.375 x IO6 mol

Imol SO3 (80g) liberates IOOkJ

0.08kg SO3 =^lOOkJ

l . S x l O ^ k g ^ 1 5 x 1 d ^ - 1 0 0

0.08

(SFEE)

2S0j (g ) products

Reaction co-ordinate

= - 1 . 8 7 x l 0 9 k J

QUESTION EIGHT (Question p40)

(a) exothermic

(b) endothermic

(c) exothermic

(d) exothermic

(e) endothermic


QUESTION NINE (Question p4Ij

(a) In exothermic reactions heat is produced (i.e. ДH is negative). AH is negative where

-^products"^rcacunts- Negative AH means that reactants have more energy than products

(i.e. heat is lost).

(b) 6.12 molx-5500 kJmor ' = -33660 kJ. Therefore, 33660 kJ is released.

(c) 1 mol H2 gives 286 kJ, so for 33660 kJ we need = 117.69 moles OfH2 (2 dp). 286

m n = •—

M

117.89= m

(2x1)

m = 117.69xf2xl)

= 235g (3sf)

QUESTION TEN (Question p41)

(a) Sx-ArH = S mol x98.2 = 491 kJ

(b) n = ——- — ^ 7 = 0.03125 mol

M (2xl6)gmol

The given equation shows Vi mol O2 being formed so the energy released when 0.03125 mol O2 is

0.03125 „ „ „ , , „ _ . x98.2 = 6.1375

0.5 = 6.14 kJ (3sf)

(c) 98.2 kJ from 1 mol H2O2. 600 kJ from — = 6.11 mol H 2 O 2 . 98.2

m n = —

M m = 6.11x34

= 208 g

QUESTION ELEVEN (Question p41) (SFEE)

(a) An acid is defined as a proton donor.

(b) HCl dissociates completely in solution giving off its proton to form H jO" (low pH) whereas ethanoic acid does not dissociate fully existing mainly as molecules - less FT given off; higher pH.

(c) pH = -log [FT ]

= -log (0.14)

= 0.85.


QUESTION TWELVE (Question p41)

(a) HY is the stronger acid.

(b) The two acids have the same concentration. pH is a measure of the H3O+ concentration. A lower pH means more H3O+ ions are present since pH = -log[H30+]. HY has a lower pH than HX, so its solution contains more H3O* ions. This means that it dissociates more in solution HY than HX, so HY is the stronger acid.

(c) There are a number of possible answers. Two possible tests are as follows:

• Add magnesium ribbon or calcium carbonate to the two solutions. In both solutions the magnesium will react, as seen by the fizzing, but the magnesium will fizz more violently in the stronger acid, and will stop reacting in the stronger acid first (because it reacts quicker).

• Titrate the two acids against NaOH and measure the pH at the equilibrium point. The pH for the titration with the stronger acid will be lower than for the titration with the weaker acid.

• Test the conductivity of each acid HY. The stronger acid will be a better conductor.

(b) The hydrochloric acid has a high acidity and high conductivity because in aqueous solution it completely dissociates

The propanoic acid, however, is a weak acid - it only partially dissociates resulting in an equilibrium. Propanoic acid has a lower concentration of H3O+ and therefore has a lower acidity. Propanoic acid has a lower concentration of ions in general so therefore has a lower conductivity than the hydrochloric acid solution.

(a) CaCO+2HC1 * CaCI,+C02+H,0 .

(b) Acid - H1O+;

Conjugate Base - II1O

OR:

Acid - H1CO3;

Conjugate Base - HCO3 .

(c) M(CaCO3)= 40 + 12 + (16x3)

QUESTION THIRTEEN (Question p42)

(a) HCl+ H2O -> H30+( a q i +Cr ( a q )

CH3CH2COOH + H2O ^ CH3CH2COO (aq) + H3O ( 4 ) n 3 u (aq)


= IOOgmol -i

n =


(d) 2 x 0.045 = 0.09 moles HCl needed to react with 0.045 moles of CaCO3.

" л cr\ „ - 1 0.09 mol с = — => 0.50molL =

V V(L) V = 0.180 L

(e) One of the following points is required:

• sodium carbonate is more soluble (i.e. Na" does not form a precipitate);

• calcium carbonate is insoluble (i.e. Ca2+ reacts with some anions to form a precipitate).

QUESTION FIFTEEN (Question p42) (SFEE)

(a) The following two points are required:

• Solution A - should be sodium chloride or water.

• Solution D - should be ammonia.

( b ) NH 3 (aq)+H,0 V NH/(aq)+OH-(aq)

(c) Water or sodium chloride (must be different to answer to first part of (a))

(d) H 2S0 4+2H ,0 » 2H,0*(aq)+S0J2"(aq)

QUESTION SIXTEEN (Question p4i) (SFEE)

( a ) C 0 2 ( g ) + 2 H 2 0 v H 3 0*(aq)+HC0 3 - (aq) ;

(b) (i) CH3C00H(aq)+H,0 4 CH3C00-(aq)+H30\aq)

[CH 3 COOHH 3 O*] (") ° [CH3COOH]

QUESTION SEVENTEEN (Question p43)

( a ) K c = - J ^ [ S 0 2 ] 2 [ 0 2 ]

rw к Inq]4 (D) K e = „ , „ l 4 r r . l5

(C) Kc =

[NH3] [O2]

[CH3COO ][H*]

[CH3COOH]


QUESTION EIGHTEEN (•Question p44) (SFEE)

(a) The wood powder presents an enormous surface area in contact with oxygen. This gives a high collision rate and a very fast reaction.

(b) The concentration of oxygen is greater in pure oxygen than in air. This produces a higher collision rate and a faster reaction.

(c) At room temperature the activation of energy is too high to overcome. The spark provides this energy and the reaction, once started is rapid.

QUESTION NINETEEN (Question p44)

(a) The time taken would reduce.

(b) The increased temperature means that the particles have more energy and move faster. This results in more collisions between particles. Both factors (more energy and more collisions) mean that more particles will collide with sufficient energy to form the product.


Energy uncatalyscd path

products

catalysed path

reactants

Reaction Coordinate

QUESTION TWENTY-ONE (Question p44)

(a) Fe3*(aq)+SCN"(aq) <• " [FeSCN]2"(aq) [ AH is exothermic.]

(SFEE)

(b) Solution turns a darker red.

Addition of Fe3t causes equilibrium to shift to the right.

QUESTION TWENTY-TWO <Question p44)

(a) Increases it.

(b) Increases it.

(SFEE)


QUESTION TWENTY-THREE (Question p45) (SFEE)

The equilibrium would shift to the right. This would reduce the number of gaseous molecules present.

QUESTION TWENTY-FOUR (Question p45) (SFEE)

(a) Strong.

(b) HCH-H2O — - > H30+(aq)+CI (aq)

(C) [H3O"]= IxlO"2 molL'1.

(d) Ix 10~2molL"'.

QUESTION TWENTY-FIVE (Question p45) (SFEE)

(a) Decreasing the concentration of X decreases the rate of reaction.

(b) Carbon dioxide.

(c) Calcium hydroxide.

(d) Y.

(e) As the reactants are used up, their concentration decreases.

(f) The following two points are required: • it lowers the activation energy; • it is not 'used up' in the reaction.

(g) (i) Increase.

(ii) Particles move faster and collide more frequently and/or with greater energy.

QUESTION TWENTY-SIX (Question p46) (SFEE)

(a) Haber Process.

(b) High Pressure.

(c) ArH value is negative - exothermic reaction.

(d) (i) No effect.

(ii) Increases the amount of ammonia produced.

(e) 56-59%.

(f) One of the following points is required: • reaction is too slow; • not enough particles have activation energy required.


(g) One of the following points is required: • removing ammonia quickly; • recycling unreacted hydrogen and nitrogen.

(h) N2 2NH3

Mgmor 1 28 17

Reaction ratios 28 34

100% conversion of 28,000 kg of nitrogen, gives 34,000 kg of ammonia.

15% of 34,000 = 5100 kg.

QUESTION TWENTY-SEVEN (Questionp47) (SFEE)

9RS (a) (i) = 142.5

(ii) One of the following points is required:

• explosive combustion;

• car accidents;

• large volume for small distance;

• storage difficulties/dangers.

(iii) Octane combustion 15% efficiency yields - 47.8x0.15 = 7.17kJg"'

Same yield for methanol 7.17 = 23.6 x % efficiency

% efficiency = — x 100 = 30.38 = 30.4% 23.6

OR

x 15% = 30.4% 23.6

(b) (i) The following are required:

• C O ;

• SO, .

(ii) CO 2 .

(iii) CO.

QUESTION TWENTY-EIGHT (Question p47) (SFEE)

[CH,OH] (a) (i) Kc

[CO][H,]"


(ii) The following points are required:

• Increase pressure - products have 1 mol compared to 3 mol for reactants and according to Le Chatelier's Principle increasing pressure will favour the side with lower number of gaseous moles;

• Increase [reactant] - system will act to minimise stress by using up reactants and [product] increases;

• Removal of product - system will oppose the change by shifting equilibrium to the right increasing the production of methanol;

(b) (i) 2CH30H+302 * 2C0 ,+4H,0 ДЯ = - 3 6 0 kjmol"1

(ii) Exothermic.

(iv) M(CH3OH)=32 OR 0.032kg CH jOH from 4 x 10 3 kg H2

4 x 10"3 xIO3

% composition 1000kg => 0.032

^ • x 1000=125kg = 125kg

(c) (i) Increase temperature of acid=> increases kinetic energy of reactant allowing more effective collisions.

(ii) Add a catalyst => (MnO2) provides an alternative pathway requiring a lower activation energy therefore increasing the rate of decomposition.

QUESTION TWENTY-NINE (Question p48)

(a) NO2

(b) The Kc value is so small and since it is the ratio of products to reactants this indicates that there are more reactants present than products. Therefore NO2 is present in a greater concentration at equilibrium than N2O4.


(c) (i) You would expect to observe the gas system going browner. This occurs because increasing temperature adds energy to the system, favouring the endothermic reaction (the one that produces NO2Ig)) to absorb energy. Equilibrium shifts to the left and the gas system turns browner due to the excess N02 ( g ) produced.

(ii) You would expect to observe the gas system to go darker brown. Addition OfNO2 leads to increased NO2 concentration and increased pressure. Equilibrium position will move to minimise this, and decrease pressure, by forming fewer molecules and use up some of the NO2. Note that some (but not all) NO2 will react to form N2O-I. There will be more NO2 in the new equilibrium mixture and it is observed as a darker brown.

QUESTION THIRTY (Question p48)

[FeSCN2+, ,] (a) (i) K c = ^

[FeJ+(aq)][SCN-(aq)]

(ii) The solution would become paler (less red). The equilibrium shifts left to replace the removed ions (and use up the excess FeSCN2+).

(b) (,) [N2][H2]3

(ii) The amount of NH3 in the system will increase. The increase in pressure shifts the equilibrium to the right (producing more NH3), because this is the side with fewer moles of molecules (i.e. two versus four).

(iii) At higher temperatures the system will move in the endothermic direction to use up the excess energy. At higher temperatures we have less NH3 present so the system must favour the left-hand side (i.e. the reverse reaction is endothermic), so the reaction in which NH3 is formed must be exothermic.

QUESTION THIRTY-ONE (Question p49)

(a) It is acting as a base.

(b) A base is a proton acceptor. HCO3" accepts a proton from H2O to become H2CO3. It is therefore acting as a base. Also, because it produces OH" ions this indicates that it is a base and not an acid (which would produce H+ or H3O+ instead).

QUESTION THIRTY-TWO (Question p49)

(a) 1. Acid H2CO3; base HCO3".

2. Acid H3O+ base H2O.

3. Acid HCO3 ; base CO32".

(b) Equation 2 is acting as an acid because it is acting as a proton donor (it gives its proton to the H2O tO form H3O", aq)).


QUESTION THIRTY-THREE (Question p49)

When NH4Cl dissolves in water the following reactions occur:

NH 4 Cl w NH4+

(aqJ + Cr (aq)

then NH4*+H20 * 4 NH3+H3O*

i.e. the NH4+ ions produced on dissolving OfNH4Cl in water reacts with the water to produce H3O+ ions.

It is therefore going to produce an acidic solution (pH < 7) because of the H3O t ions produced.

QUESTION THIRTY-FOUR (Question p49)

Solution [H3O+] / mol L 1 [OH ] / mol L"1 pH

A lxlO"1 4

0.0288

3.47x IO13 (3 sf)

0.0288 I o g f l x l 0 M 1 = 12.5(3 sf)

[ 0.0288

B IO"5'24 =5.75x IO 6 (3 sf) l x l 0 " ' 4 - 1 .74 x IO"9 (3 sf) 10

5.24

QUESTION THIRTY-FIVE (Question p50)

(a) „ Solution Concentration of H3O+ and OH" in solution

Greaterthan IxlO 'molL 1 Lessthan lxlO 'molL"1

Na2CO3 0.100 molL-1 OH H3O+

HNO3 0.100 molL-1 H3O+ OH'

NaOH 0.100 molL"1 OH H3O+

NH4Cl 0.100 molL"1 H3O+ OH

(b) pH = -log[H30+] = -log(0.035) = 1.46 (3 sf)

(c) 9.2 =-IogfH3O+]

[H3O+] = IO"'2

[H 3 0 + ] = 6 .31xl0 1 0 molL"1 (3 sf)

= 1.58 x IO"5 molL"1 (3 sf)


OXIDATION-REDUCTION REACTIONS (AS90311)


QUESTION ONE (Question p52) (SFEE)

(a) Oxidationstateofchlorine in: HOCl = + 1 NH2Cl = -I

(b) Oxidation state of nitrogen in: NH3 = -3 NH,Cl = -1

QUESTION TWO (Question p52)

(i) Mn2+ +2

(ii) H2O2 -1

(iii) Br2 O

(i) SO2 +4

(ii) CO +2

(b) (i) Species oxidised , Br-; species reduced, H2O2.

(ii) Br" is oxidised to form Br2 which is orange.

H2O2 + 2H+ + 2e" —» 2 H 2 0

2Br" Br2 + 2e"

H2O2 + 2H~ + 2Br" -> Br2 + 2 H 2 0

(iii) 2(Fe2+ ->Fe 3 + +le")

Cl2 + 2e" 2СГ

2Fe2++Cl2 -> 2Fe3+ +2СГ

The Fe2+ ions, which are pale green, are oxidised to form Fe3* ions which are omage.

QUESTION THREE (Question p53) (SFEE)

(a) Fe(OH)3.


• orange;

• brown;

• rusty.

(c) Reduction.

Oxidation state has decreased from O to -1 . Chlorine atom gains an e".

(d) Cl+2NaOH * NaOCb-NaCKH1O .

Answers - Oxidation-Reduction Reactions (AS90311) 111


(a) MnO4" + 8FT + 5e~ »Mn2 t + 4 H , 0 .

(b) C2O42" * 2 C 0 + 2 e " .

(c) 2MnO; + 16Ы + ) 8 ^ * 2Mn2* +8H20 .

5C,042- > IOCO2 + XJ^.

2MnO; + 16H* + 5C ,0 2" *• 2Mn2" + 8H,0 + IOCO,. 4 2 4 2 2

QUESTION FIVE (Question p53J

(a) The reductant is SO2 because it is oxidised. It has gained oxygen or, equivalently, the oxidation number of S has increased from +4 to +6.

(b) Oxidation: SO2 + 2 H 2 0 SO42" +4H* + 2e~

Reduction: Cr2O72" +14H+ +6e~ -> 2Cr3+ + 7 H 2 0

(c) 3S0 2 + 6 H 2 0 + Cr2O72" + 14H+ + 6e" -> 3S04

2" +12H+ + 6e" + 2Cr3+ + 7 H 2 0

3S0 2 + Cr2O72" + 2H+ -> 3S04

2" + 2Cr3+ + H2O

(d) The solution would turn from orange (due to the Cr2O?*" ions) to green (due to the Cr3+ ions produced).

QUESTION SIX (Question p54)

(a) (i) Г: -1

(ii) IO3": +5

(b) Oxidant: IO3'

Reductant: Г

(c) I": 21" —• I2 + 2e~

IO3": 2 I0 3 "+12H + +10e" - » I 2 + 6 H 2 0

(d) Ю Г + 2Ю3" + 12Н + ->6 I 2 + 6 H 2 0

51" + IO3" +6H + ->3I 2 + 3H 2 0


QUESTION SEVEN (Question p54) (SFEE)

(a) Product at positive

anode Product at negative

cathode

Cone, sodium chloride solution Chlorine Hydrogen

Molten lead bromide Bromine Lead

Dilute sulfuric acid Oxygen Hydrogen

(b) (i) One of the following points is required:

• 2H+ + 2e~ -> H2 /

• 2 H 2 0 + 2e" —» H2 + 20H".

(ii) 2Br" * Br, + 2e".


• NaOH;

• sodium hydroxide;

• caustic soda.

QUESTION EIGHT (Question p54) (SFEE)

C3Hg + 5 0 , * 3C0 , + 4H,0

QUESTION NINE (Question p55) (SFEE)

(a) Spectator ions.

Not directly involved in the reactor or the oxidation number does not change in the reaction.

(b) Reduction half equation: MnO4" + 8H+ + 5e" -> Mn2+ + 4 H 2 0

Oxidation half equation: Fe2+ Fe3+ + e"

Balanced equation: MnO4" +8H + +5Fe2+ -> Mn2+ + 4 H 2 0 + 5Fe3+

(c) (i) +6.

(ii) +4.

(d ) C H 3 C H 2 C H 2 C H 2 O H + H 2 O > C H 3 C H 2 C H I C O O H + 4H* + 4E"

(e) Cr2O72"+ 14FT + 6e *• ICxs* + 7H,0

(f) 2Cr2072' + 16H* + З С Н З С Н ^ С ^ С ^ О Н » 3CHtiCH2CH2COOH+4Cr3' + 11H2O



(a) A soluble or molten substance which is capable of carrying a charge.

(b) 2Cl"(aq) * Cl2(S) + 2e".

QUESTION ELEVEN (Questionp55)

C u anode

h

Nail (cathode)

-CuSO4(J4)

-•beaker

(SFEE)

QUESTION TWELVE (Question p56) (SFEE)


• Mg gains 0 ;

• oxidation number of Mg increases.

(b) (i) Ag * Ag* + e"

(ii) S + 2e" *S2 '

(iii) 2Ag + S > 2Ag* + S2"

(c) Cr2O72- - orange

ГЛ 3 + Cr - green.

(d) (i) CH3CH2OH + H2O * CH3COOH + 4H* + 4e "

(ii) Cr2O72" + 14H* + 6e ' * 2Cr3* + 7H20

(iii) 3CH3CH,OH + 2Cr2072" + 16H* > 3CH 3 C00H +4Cr3* + 1IH2O

(e) (i) Al readily oxidises to Al2O3. Oxide layer is tightly held and electrolysis is necessary to

remove it.

(ii) Alumina/bauxite from Australia.

(iv) Refer to above diagram.

(v) One of the following points is required:

• cans;

• aluminium window frames.

(vi) Lightweight.

(vii) One of the following points is required:

• conducts;

• malleable;

• ductile;

• shiny.

QUESTION THIRTEEN (Questionp57) (SFEE)


• nitrogen dioxide;

NO 2 .

(b) Copper (Il) ion/cupric.

Cu2*.

(c) Cu * Cu2* + 2e"

(d) N0 3 ' + 2H+ + e > N O 2 + H2O


(a) The solution will turn blue.

(b) Oxidation - Cu * Cu2* +2e"

Reduction - Ag*+e" * Ag

(c) Cu + 2Ag"(aq) > 2Ag + Cu2"(aq)


(d) A g - + 1

N - + 5

0 - - 2

(e) Oxidant - Ag+

Reductant - Cu

(f) Iodine.

QUESTION FIFTEEN (Question p58)

(b) Reaction One: 2 H , 0 + 2e" - » 2 0 H +H2

Reaction Two: 2СГ -> Cl2 + 2e"

(c) Gas A is chlorine. The negative chloride ions are attracted to the positive electrode (the anode). Oxidation occurs at the anode; the chloride ions are oxidised to form chlorine gas.

QUESTION SIXTEEN (Question p58)

(a)

ft

graphite electrode

mo*en magnesium chloride

(SFEE)


(b) Left Hand.

(c) Right Hand.

(d) 2СГ » a + 2 e -

(e) In the molten state the ions are able to move and conduct an electric current.

(f) They are inert.

QUESTION SEVENTEEN (Question p59) (SFEE)

(a) Fe2O3 + 3 C 0 * 2Fe + 3C0 2

(b) Iron - +3.

Oxygen - -2.


• endothermic;

• high activation energy.

(d) The following two points are required:

Fe2O3 + 3 H2SO4 * Fe2(SO4)3 + 3 H2O;

• Fe + H, SO j » FeSO + H, . 2 4 4 2

(e) (i) Mg > Mg2' + 2e"

(ii) Fe2++2e" • Fe

(iii) Mg + Fe2" * Mg2" + Fe

(iv) Magnesium.

(v) The following two points are required: • expense of magnesium - would need a lot of magnesium and a reactive metal is

difficult to make.

• expense of sulfuric acid - would need to use large amounts of sulfuric acid.

(f) H2+Cl2 > 2HC1

(g) One of the following points is required:

• ZnO;

• CuO.

(h) SiO2 or SO, .

Acidic oxide will not react with an acid.



(a) Purple.

(b) The H+ from the acid is necessary to remove the O atoms and form water.

(c) Fe24 * Fe3*+e"

QUESTION NINETEEN (Question p60) (SFEE)

(a)

(c)

(i)

(ii)

(b) (i)

B. One of the following points is required:

• KCl;

• CuSO

4

(iii) A.

(iv) Electrode A - Anode

Electrode B - Cathode 2СГ •Cl+2e"

(ii) 2H20+2e" > H + 2 0 H

Cu anode

h

6 spoon (cathode)

, CuSO4laql

-•beaker


(a) Unreacted Hydrogen gas.

(b) Oxidant - oxygen

Reductant - hydrogen.

(c) The following points are required:

• H2 + 20H" —> 2 H 2 0 + 2e";

• O2 + 2 H 2 0 + 4e" —• 4 0 H ' .


(d) 2H2 +O 2 2 H 2 0

(e) One of the following points is required:

• chemical energy is converted directly into electrical energy, not chemical to heat to mechanical to electrical;

• little energy is lost as heat or noise.

(f) Lead accumulator batteries are very heavy and weight is restricted on space missions.

QUESTION TWENTY-ONE (Question p61) (SFEE)

(a) Two of the following points are required: • potassium permanganate; • potassium dichromate;

• hydrogen peroxide.

(b) Dilute sulfuric acid.

Sulfate ions are stable or not oxidised.

QUESTION TWENTY-TWO (Question p62) (SFEE)

(a) One of the following points is required: • aluminium oxide is an amphoteric oxide; • aluminium oxide dissolves in aqueous sodium hydroxide.

(b) Cathode - Al3*+3e" * Al Anode - 202" > 0,+4e"

(c) One of the following points is required: • cryolite lowers the temperature needed for the process; • acts as a solvent;

• lowers melting point.

QUESTION TWENTY-THREE (Question p62)

(a) Reduces. The oxidation number of Mn in MnO4" is +7 and in Mn2+ it is +2. This reduction in oxidation number means that it has been reduced.

(b) MnO4" is the oxidant. Fe2+ is the reductant.

(c) Oxidation: F e 2 * ( a q ) —> F e 3 + ( a 4 ) + e~

Reduction: M n 0 4 ~ ( a q , + 8H* + 5e" M n 2 tw + 4 H , 0

(d) 5Fe2* + M n O 4 " +8H~ -> 5Fe3" + Mn2* + 4 H ; 0


(e) The original solution is purple due to the MnO4" present. As the reaction occurs Mn2* is produced which is colourless, so the solution turns colourless. It may turn slightly orange due to Fe3* formation. Fe2+ is very pale green so this hardly affects the observation.

Species to be mixed Does a reaction occur?

(yes/no)

If a reaction occurs, describe what would be

observed.

If a reaction occurs, write a balanced equation for the reaction.

( i ) C l 2 ( aq ) + B r (aq) Yes Colourless solution turns brown

Cl2 +2e" -» 2СГ

2Br" —> Br2 +2e" C 1 2 ( a q ) +2Br' (aq) -» Br2(]) + 2Cl" (aq)

( i i ) C l ' ( a q ) + I 2 ( a q ) No

QUESTION TWENTY-FOUR (Question p63)

(a) (i) +4

(ii) +6

(b) S 0 2 ( a q ) + 2 H 2 0 ( 1 ) - > S O 42 ^ 4 ) +4H* + 2 e -

^(aq) + 2 e ' - > 2 Г ( а ч )

( C ) S0 2 ( a q ) + 2H20 ( 1 ) + I2(aq) S O 42 ^ a , , + 4H+ + 2I (aq)

(d) Turn from orange (due to Cr2O72") to green (due to Cr3* produced). The alcohol causes the

reduction of the orange Cr2O?2" to produce green Cr3 ' .

QUESTION TWENTY-FIVE (Question p63)

(a) (i)


(ii) Refer to diagram above (i.e. movement is from +ve to -ve). Show anions to anode; cations to cathode.

(b) (i) 202 ' ( a q ) —> 02 („| + 4e~ (aq) ^ 2(g»

\ s )

(c) C ( s ) + 0 2 ( c l - > C 0

(ii) А13т(аЧ) +3e" -» Alls

The carbon and oxygen react to produce carbon dioxide gas, so the carbon electrode gradually erodes away and must therefore be eventually replaced.

QUESTION TWENTY-SIX (Question p64)

Ag —> Ag+ + e (oxidation occurring at the silver metal electrode - the anode.)

Ag" + e" -> Ag (reduction occurring at the spoon - the cathode.)

The cathode (spoon) attracts the cations (Ag+) which accept an electron from the cathode to form silver which plates the spoon.

QUESTION TWENTY-SEVEN (Question p64)

(a) (0 + 1

(") -1

(iii) 0

(b) (i) 2Cl"(aq) - » C l 2 ( g ) +2e"

(ii) 20СГ + 4H+ + 2e_ Cl2 + 2 H 2 0

(c) Reaction (i) is an oxidation reaction: the oxidation number of chlorine increases from - 1 to 0; two electrons are produced.

(d) Oxygen is being oxidised because its oxidation number increases from - 2 (in the NaOCl) to 0 (in O2). Chlorine is being reduced since its oxidation number decreases from +1 (in NaOCl) to -1 (in NaCl).

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FEEDBACK FORM

We invite you to give us feedback about this book. We want the revision guide to be as useful as possible so your constructive comments are very welcome.

Probably the easiest way to do this is to go to our website www.rur.co.nz and complete a feedback survey form. Alternatively, this form can be posted back to PO Box 19-939, Woolston, Christchurch or faxed to (03) 376 4545. Emailed comments are also welcomed at [email protected].

1. Please identify the level and title of this book, e.g. Level 2 Physics.

2. How did you obtain this book? [e.g. supplied by school, purchased through school, Whitcoulls]

3. What did you like about the book?

4. In what ways could the presentation of the book be improved? [e.g. layout, language used, print quality, binding]

5. In what ways could the content of the book be improved? [e.g. please note apparent mistakes with a page reference, material no longer assessed]

Many thanks for taking the time to provide this feedback. All the best with NCEA exams this year.



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email: Website: http://www.rur.co.nz

"I wish to order the following 2005 Revision Guides." Please y^ the appropriate title. The price to schools is $12.50 per CODV. Price to individuals is $15.00 per copy. These prices include the cost of freight and handling. Please allow 5 - 7 days for delivery.

NCEA Level 3 Accounting NCEA Level 2 English

NCEA Level 3 Economics NCEA Level 2 Mathematics

NCEA Level 3 Calculus NCEA Level 2 Biology

NCEA Level 3 Statistics and Modelling NCEA Level 2 Chemistry

NCEA Level 3 English NCEA Level 2 Physics

NCEA Level 3 Biology NCEA Level 2 Geography

NCEA Level 3 Chemistry _ _ NCEA Level 2 History

NCEA Level 3 Physics _ _ NCEA Level 1 English

NCEA Level 3 Geography NCEA Level 1 Mathematics

NCEA Level 3 History (New Zealand Topics) NCEA Level 1 Science

NCEA Level 3 History (England Topics) NCEA Level 1 Accounting

NCEA Level 3 Classical Studies (Roman Topics) NCEA Level 1 Economics

NCEA Level 3 Classical Studies (Greek Topics) NCEA Level 1 Geography

NCEA Level 2 Accounting NCEA Level 1 History

NCEA Level 2 Economics

Check out www.rur.co.nz. Orders and inquiries can now be made through the website.

Schools should use a purchase order number, include name, card number and expiry date).

VISA/Mastercard is also acceptable (please

These titles are also available through Whitcoulls, Dymocks, Paper Plus, some Books and More stores and other good booksellers.




ORDER FORM Date:

Name:

Address:.

Phone:

email:

Please make cheques payable to: Really Useful Resources P.O. Box 19-939 Woolston CHRISTCHURCH

Ph (03) 377 4545 Fax (03) 376 4545

E-mail: [email protected]

Website: http://www.rur.co.nz

"I wish to order the following 2005 Revision Guides." Please ^ the appropriate title. The price to schools is $12.50 per copy. Price to individuals is $15.00 per copy. Theseprices include the cost of freight and handling. Please allow 5 - 7 days for delivery.

NCEA Level 3 Accounting


NCEA Level 3 Calculus

_ NCEA Level 3 Statistics and Modelling

NCEA Level 3 English

NCEA Level 3 Biology

NCEA Level 3 Chemistry

NCEA Level 3 Physics

NCEA Level 3 Geography

NCEA Level 3 History (New Zealand Topics)

NCEA Level 3 History (England Topics)

NCEA Level 3 Classical Studies (Roman Topics)


NCEA Level 2 Mathematics

NCEA Level 2 Biology

NCEA Level 2 Chemistry

NCEA Level 2 Physics

NCEA Level 2 Geography

NCEA Level 2 History


NCEA Level 1 Mathematics

NCEA Level 1 Science

NCEA Level 1 Accounting


NCEA Level 3 Classical Studies (Greek Topics) NCEA Level 1 Geography

NCEA Level 2 Accounting NCEA Level 1 History


Check out www.rur.co.nz. Orders and inquiries can now be made through the website. Schools should use a purchase order number. VISA/Mastercard is also acceptable (please include name, card number and expiry date).

These titles are also available through Whitcoulls, Dymocks, Paper Plus, some Books and More stores and other good booksellers.




Ncea Level Two Chemistry Revision Guide 2005

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Transcript of Ncea Level Two Chemistry Revision Guide 2005