Chemistry Practical Manual 2012

7/31/2019 Chemistry Practical Manual 2012

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ST. HILDAS SECONDARY SCHOOL

SC(CHEMISTRY)5116/5118 PAPER 5

PRACTICAL MANUAL

NAME: ________________________

CLASS: ________________

DATE: January November 2012


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SCHEME OF ASSESSMENT 2012

Candidates are required to enter for Paper 1, Paper 5 and two of Papers 2, 3 and 4.

Science (Physics, Chemistry), Syllabus 5116

Paper 1 will be based on the Physics and Chemistry sections of the syllabus.

Paper 2 will be based on the Physics section of the syllabus.Paper 3 will be based on the Chemistry section of the syllabus.

Paper 5 will be based on the Physics and Chemistry sections of the syllabus.

Science (Chemistry, Biology), Syllabus 5118

Paper 1 will be based on the Chemistry and Biology sections of the syllabus.

Paper 3 will be based on the Chemistry section of the syllabus.

Paper 4 will be based on the Biology section of the syllabus.

Paper 5 will be based on the Chemistry and Biology sections of the syllabus.

Practical Assessment (Paper 5)

Paper 5 is designed to test appropriate skills in C,Experimental Skills and Investigations.

In one or more of the questions in Paper 5, candidates will be expected to suggest a

modification or an extension, which does not need to be executed.

Depending on the context in which the modification/extension element is set, the number

of marks associated with this element will be in the range of 10% to 20% of the total marks

available for the practical test.

SHSS Science Department Page 2 of 16


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What is Qualitative Analysis?

Qualitative Analysis is an experimental method used to find out information about on one

or more unknown samples after carrying out some basic tests on them. These samples may

be pure substances or mixtures of substances.

There are usually two things that you usually do when you do qualitative analysis(a) Determine the identity of an unknown sample(b) Determine the nature of a sample

(a) In the determination of the identity of an unknown sample, you are expected to do thefollowing things:

Determine the name of the sample

Determine the ions present in the sample

(b) In the determination of the nature of an unknown sample, you are expected to do the

following things:

Determine if the sample is acidic, basic, amphoteric or neutral Determine if the sample is oxidizing or reducing

Determine if the sample is organic or inorganic

Determine if the sample is saturated or non-saturated

Some basic tests that you are required to perform include:

Test Descriptions Reference

Appearance colour and texture of substance Table 1

Solubility

whether they can dissolve in hot water,

cold water or other

solvents

Table 2

Heatingheating the solid or liquid and use

chemical test for the gases evolvedTable 3 & 3.1

Test for gases

use chemical test for the gases evolved to

make inference on the identity of

unknown sample

Table 4

Test for cations

use chemical test to identify the positive

ion (cation) present in a substance. Eg inNaCl, Na+ is the cation.

Table 5

Test for Anionsuse chemical test to identify the negativeion (anion) present in a substance. Eg in

NaCl, Cl- is the anion.

Table 6

Flame Testuse flame colours to make inference on

the identity of some cationsTable 7

Test for reducingagents

use oxidising and reducing agents todetermine the nature of the substance in a

redox reaction

Table 8

Test for oxidisingagents

Table 9

1. Appearance of Unknown Substance



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When you are required to describe a sample, you must describe the following:

(a) state (solid or solution)

(b) colour of sample

Table 1

Colour of

Unknown

Substance

Inference

Black oxides of copper(II), iron(II) or manganese(IV) or iodine crystals or charcoal

Blue copper(II) salts eg CuSO4 or Cu(NO3)2

Green copper(II) or iron(II) salts eg CuCO3 or FeSO4

Grey powdered metals eg iron, aluminium, lead or zinc

Note: shiny grey aluminium or zinc granules

dark grey iron filingsdull grey lead

Orange K 2Cr2O7 ( potassium dichromate(VI) )

Purple KMnO4 ( potassium manganate(VII) )

White salts of potassium, sodium, ammonium, zinc, aluminium or calcium

Yellow orBrown

iron(III) salts eg FeCl3

Colourless

solution

dilute acids or alkalis or H2O2 ( hydrogen peroxide)



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2. Solubility of Salts

The following table describes the solubility of some common substances in water. Note

that precipitate is produced when one of the products of a reaction is insoluble in water.

Table 2

Type of salts Soluble in water Insoluble in water

sodium, potassium and

ammonium compounds

(ie. SPA compounds)

All SPA compounds are

soluble in water

None

nitrate All nitrates are soluble inwater

None

chloride All chlorides are soluble inwaterexcept those listed on

the right

silver chloridelead chloride (soluble only

in hot water)

iodide All iodides are soluble in

waterexcept those listed on

the right

silver iodide

lead iodide (soluble only

in hot water)

sulphate All sulphates are soluble in

waterexcept those listed onthe right

lead sulphate

barium sulphatecalcium sulphate (slightly

soluble only)

carbonate sodium carbonatepotassium carbonate

ammonium carbonate

All carbonates are insolublein waterexcept those listed

on the left

hydrogencarbonate All hydrogencarbonates aresoluble in water

None

hydroxide sodium hydroxide

potassium hydroxide

ammonium hydroxide(ammonia solution)

All hydroxides are insoluble

in waterexcept those listed

on the left



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3. Action of Heat

Background

When heat is applied to a solid, thermal decomposition takes place. A gas may be releasedfrom anions such as carbonates, nitrates, hydroxides or water vapour from hydrated crystal.

Heat however, will only melt the compounds if they are from Group I metals like sodiumor potassium as their anions are held strongly by the Group I metal ions.

Tests must be conducted for the gases released upon heating and any colour changes must

be observed and recorded down. Residue left behind later heating may be used for furthertesting such as adding acids or dissolving it with water.

Some notes on heating the unknown substance

Normally when we heat an unknown, we have to test for the gas. Follow the instructions

given in the question paper.

There are the fourcommon gases:

(i) carbon dioxide

(ii) ammonia

(iii) oxygen

(iv) hydrogen

Based on the gas given off, we can determine what are present in the unknown

(i) if carbon dioxide is given off, that means that a carbonate (CO32-) is present and the

unknown may be a metal carbonate

(ii) If ammonia is given off, the unknown may be an ammonium salt. However, if youhave added aqueous ammonia before heating, the ammonia gas may come form the

aqueous ammonia itself. So be careful

(iii) If ammonia is produce when we add sodium hydroxide (NaOH) and aluminium

powder, then a metal nitrate is present

(iv) If hydrogen gas is given off when an acid is added to the unknown, then unknownsubstance is a metal

You can refer to table 3 and 3.1 for more details.



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Table 3

Gas Evolved Inference

substance sublimes ammonium salts such as NH4Cl, I2

oxygen evolved (relights

glowing splint)

some oxides or nitrates

carbon dioxide gas evolved(forms white ppt in limewater)

carbonates or hydrogencarbonates such as CaCO3, Al2(CO3)3,except sodium carbonate and potassium carbonates

*Note: Group I metal carbonates are normally not affected by

heat. These carbonates will just melt when heated, with nogases evolved.

ammonia gas evolved (pungent

gas that turns damp red litmus

paper blue)

ammonium salts

water vapour evolved (water

droplets formed on the upperpart of test-tube)

hydrated salts such as CuSO4.5H2O, some hydroxides such as

Cu(OH)2, NaOH, or hydrogencarbonates

nitrogen dioxide gas evolved

(brown gas with irritating smell)

some nitrates such as Cu(NO3)2, NaNO3

*Note: nitrate of potassium and sodium will only yield one gas(oxygen) while all of the other metals from Group II onwards

will yield two gases (oxygen and nitrogen dioxide)

sulphur dioxide gas evolved

(choking gas which turnsacidified potassium dichromate

(VI) solution green)

sulphate(IV) except K2SO3 and Na2SO3

Table 3.1

Colour of residue/Sublimate

after heating

Inference

white residue Na+, K+, Ca2+, Zn2+, Al3+

yellow residue when hot, white

when cold

Zn2+

reddish brown residue Fe3+

black residue when hot, reddishbrown when cold

Fe2+

black residue Cu2+

violet fumes, black sublimate I-

white sublimate NH4+

yellow sublimate Pb2+

4. Tests for Gases



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One of the skills you need to acquire is to know when to test for gases. You must be able toanticipate the production of a gas in certain experiments.

Some common experiments where the production of gases is likely are listed below:

(a) When a substance, especially a solid is heated(b) When an acid is added to a substance.

If acid is added to a carbonate, then you would expect carbon dioxide gas to be

produced.

If acid is added to a metal, then you would expect hydrogen gas to be produced.(c) When an alkali is being added to a substance.

If alkali is added to a substance that has NH4+, then you would expect ammonia

gas to be produced.

Table 4

Gas Colour Odour Litmus Test Confirmatory Test

ammonia gas (NH3) colourless pungent red to blue -

carbon dioxide gas

(CO2)

colourless odourless blue to red forms white ppt inlimewater

chlorine gas (Cl2) yellowish

green

irritating blue to red to

white(bleached)

-

hydrogen gas (H2) colourless odourless neutralextinguished a lighted

splint with a pop

sound

oxygen gas (O2) colourless odourless neutralrelights a glowingsplint

sulphur dioxide gas

(SO2)

colourless choking blue to red turns acidified

potassium

dichromate(VI) from

orange to green

water vapour colourless odourless neutral water droplets on theupper parts of the test

tube

5. Test for Cations



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Background

Ionic substance consists of a positive ion and negative ion. We therefore have to find outthe identity of these two types of ions in our experiments.

To determine the identity of the cation present in a substance, a solution containing thecation is required. For substances that dissolve in water, the solution is easily prepared. For

substances that do not dissolve in water, an acid is usually used to produce a solutioncontaining the cation.

An alkali (sodium hydroxide and ammonia solution) is added to the solution containing the

cation to precipitate out the metal in the form of insoluble hydroxide. This hydroxide that is

precipitated will be able to provide clues to allow you to identify the cation present in thesubstance.The precipitates formed are insoluble metal hydroxides form from the combination of thecation in the solution and the hydroxide ions (OH -) from sodium hydroxide and aqueous

ammonia (ammonium hydroxide)

Eg. ZnSO4(aq) + 2NaOH Zn(OH)2(s) + Na2SO4(aq)

(unknown)

White ppt

Procedure


Add sodium hydroxide or aqueous

ammonia in excess.

(Add in excess means adding excesssodium hydroxide or aqueous

ammonia to the same test-tube toabout full). Then observe if the pptdissolves in this excess amount of

reagents. (may need to use a stirrer to

stir the mixture)

Add sodium hydroxide or

aqueous ammonia drop by

drop slowly (about 3 5

drops) into the test-tube.See if there is any ppt and

observe the colour of theppt.

The colour of the ppt when a few drops ofNaOH/NH

3is added in, together with solubility

of ppt in excess of these alkalis will help you

determine the cation present in the sample.Refer to Table 5.


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Table 5

Cation Effect of aqueous sodium

hydroxide

Effect of aqueous ammonia

(ammonium hydroxide)

ammonium

(NH4+)

No ppt, ammonia gas produced -

calcium (Ca2+) White ppt, insoluble in excess No pptor very slight white ppt

copper (Cu2+) Light blue ppt, insoluble in excess Light blue ppt, soluble in excess to

produce a dark blue solution

iron(II) (Fe2+) Green ppt, insoluble in excess Green ppt, insoluble in excess

iron (III) (Fe3+) Red-brown ppt, insoluble in excess Red-brown ppt, insoluble in excess

lead(II) (Pb2+) White ppt, soluble in excess, giving

a colourless solution

White ppt, insoluble in excess

zinc (Zn2+) White ppt, soluble in excessgiving a colourless solution

White ppt, soluble in excessgiving a colourless solution

aluminum (Al3+) White ppt, soluble in excess, givinga colourless solution

White ppt, insoluble in excess

*[Lead(II) ions can be distinguished from aluminium ions by the insolubility of lead(II) chloride.]

*Note: About Al3+ and Pb2+ ions

Both Al3+ and Pb2+ ions give the same results when testing using both sodium hydroxide

and aqueous ammonia. This is because both Al(OH)3 and Pb(OH)2 that were formed are

both soluble in excess sodium hydroxide and aqueous ammonia because they are bothamphoteric in nature.

One way to distinguish these two cations is to use dilute hydrochloric acid. The test-tubecontainingPb2+ ions will produce a white ppt of PbCl2, an insoluble salt while the test-tube

containing Al3+ will produce a soluble AlCl3.



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The flow chart below shows how to identify a cation from its reaction with aqueous

sodium hydroxide.



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The flow chart below shows how to identify a cation from its reaction with aqueous

ammonia.



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6. Test for Anions

Background

Two types of experiments can be used to identify anions:

(a) Adding chemical reagent to form aprecipitate. The colour of precipitate will helpyou to identify the anion.

(b) Chemical reagent(s) are added to produce agas. The identity of gas will help you toidentify the anion.

*Note: For the identification of chloride, iodide and sulphate ions, a chemical is added to

form a precipitate. For the identification of carbonate, nitrate and hydroxide ions,chemicals are added in to produce a gas.

Carbonates

This is the only anion in the syllabus that can be tested in the solid state or in solution

form. An acid, usually nitric acid is added to the unknown (either in solid state or

solution form). Effervescence (bubbles) will be observed. The identity of this gas canbe confirmed by using limewater where a white ppt will be produced in limewater.

This white ppt is calcium hydroxide.

Chlorides, Iodides, Nitrates and Sulphates

For all these 4 anions in the syllabus, solutions of the sample that contain these anions

must be used for testing. If you are given sample in the solid state, you must first

obtain a solution by dissolving with distilled water. If the sample is insoluble in water,you will have to use acid to obtain a salt solution.

For chloride ions, you must first acidify the solution with dilute nitric acid first, to

remove other anions. This is then followed by the addition of silver nitrate solution.The presence of a white ppt confirms the presence of the chloride ions.

Ag+ (aq) + Cl- (aq) AgCl (s)

White ppt

For iodide ions, you must first acidify the solution with dilute nitric acid first, to

remove other anions. This is then followed by the addition of lead(II) nitrate solution.

The presence of a yellow ppt confirms the presence of the iodide ions.

Pb2+ (aq) + I- (aq) PbI2 (s)

Yellow ppt

For sulphate ions, you must first acidify the solution with dilute nitric acid first, to

remove other anions. This is then followed by the addition of barium nitrate solution.The presence of a white ppt confirms the presence of the sulphate ions.

Ba2+ (aq) + SO42- (aq) BaSO4 (s)



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White ppt



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For nitrate ions, aqueous sodium hydroxide is first added, followed by a few pieces ofaluminium foil (also known as Devardas alloy). The mixture is then warmed. The gas

produced is tested with a piece ofdamp red litmus paper. If the red litmus paper turns

blue and a pungent smell of ammonia gas is produced, then nitrate ions are confirmedto be present.

*Note: For chloride, iodide and sulphate ions test, if acid is to be added in after the otherchemical reagent, do observe if the precipitate formed dissolves when the acid is

added in. If the ppt dissolves, most likely the anion present is carbonate ions, and

not the rest. Confirmatory test for this is to test for carbon dioxide gas.

Table 6

AnionTest

Test result

carbonate (CO32-)

add dilute acid (eg hydrochloric acid)Effervescence produed,Carbon dioxide produced

chloride (Cl-)[in solution)

acidify with dilute nitric acid, thenadd aqueous silver nitrate White ppt

iodide (I-)

[in solution]

acidify with dilute nitric acid, then

add aqueous lead(II) nitrate Yellow ppt

nitrate (NO3-)

[in solution]

add aqueous sodium hydroxide and

aluminium foil, warm carefully

Ammonia produced

sulphate (SO42-)

[in solution)

acidify with dilute hydrochloric acid

or nitric acid, then add bariumchloride/nitrate

White ppt



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7. Flame Tests

Background

Some metallic chlorides are volatile. When these metallic chlorides are heated in a bunsenflame, they give characteristic coloured flames.

Table 7

Flame Colour Inference

blue-green copper(II) ions

brick-red calcium ions

whitish blue lead(II) ions

lilac potassium ions

orange-yellow sodium ions



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8. Tests for Oxidising and Reducing Agents

Background

An oxidising agent:

Gives oxygen to another substance Takes away hydrogen from another substance

Increases oxidation state of another substance Removes negative electrons from another substance

In the process of oxidising another substance, the oxidising agent is reduced.

A reducing agent: Takes away oxygen to another substance

Gives hydrogen from another substance

Reduces oxidation state of another substance Gives negative electrons from another substance

In the process of reducing another substance, the reducing agent is reduced. Oxidation andreduction takes place simultaneously and such reactions are called redox reactions. These

reactions usually involve changes in colour which will allow you to interpret the nature of

the substance.

Table 8 (To test if substance is reducing in nature)

Oxidising Agent Colour of

Oxidising

Agent

Final colour of oxidising

agent after oxidising

another substance (itself

being reduced)

Reason for

colour change

acidified potassiummanganate(VII) solution

(KMnO4)

purplesolution

colourless solution due toMn2+

Mn7+ in MnO4-

reduced to Mn2+

acidified potassium

dichromate(VI) solution(K2Cr2O7)

orange

solution

green solution due to Cr3+ Cr6+ reduced to

Cr3+

fresh iron(III) solution

(Fe3+ solution)

yellow

solution

green solution due to Fe2+ Fe3+ reduced to

Fe2+

iodine solution (I2 solution) brown

solution

colourless solution due to I- I2 reduced to I-

chlorine water (Clorox, Cl2water)

light yellow

solution

colourless solution due to

Cl-Cl2 reduced to

Cl-

solid manganese(IV) oxide

(MnO2)

black powder colourless solution due to

Mn2+Mn4+ reduced to

Mn2+



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Table 9 (To test if substance is oxidising in nature)

Reducing Agent Colour of

Reducing

Agent

Final colour of reducing

agent after reducing

another substance

(itself being oxidised)

Reason for

colour change

acidified iron(II) sulphatesolution (Fe2+ solution) green solution yellow solution due to Fe

3+

Fe

2+

oxidised toFe3+

acidified potassium iodide

solution (I- solution)

colourless

solution

brown solution due to I2

Note: Iodine formed is

confirmed by adding

starch which turndark blue

I- oxidised to I2

solid iron filings (Fe) dark grey

powder

iron filings used up,

producing yellow solutiondue to Fe3+

Note: Fe3+ ion formed is

confirmed by adding

NaOH which forms areddish-brown ppt

Fe oxidised to

Fe

3+

Cr3+ salt greensolid/solution

orange solution due toCr2O7

2-

Cr3+ oxidised toCr6+ in Cr2O7

2-

concentrated hydrochloricacid (HCl) colourlesssolution colourless solutionremained, chlorine gas

produced

Note: Chlorine gas is

confirmed by moistblue litmus paper

which is bleached

Cl

-

oxidised toCl2 (gas)



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SHSS Science Department


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The following shows the data sheet you will be referring to during the practical exam.

Chemistry Practical Manual 2012

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