AS Chemistry

Enthalpy Changes

Learning ObjectivesCandidates should be able to:• Explain that some chemical reactions are

accompanied by energy changes, principally in the form of heat energy.

• Construct (and interpret) a reaction pathway diagram, in terms of the enthalpy change of the reaction.

• Calculate enthalpy changes from appropriate experimental results, including the use of the relationship: q=mcT.

• Explain and use the terms enthalpy change of reaction and standard conditions, with particular reference to formation and combustion.

Starter activityCan you complete tasks 1 and 2 on your notes.

Exothermic

Endothermic

Simple calorimeter – flammable liquids

Bomb calorimeter

Simple calorimeter – chemicals in solution

AS Chemistry

Hess’ Law and Enthalpy Cycles

Learning ObjectivesCandidates should be able to:• apply Hess’ Law to construct simple energy

cycles

• carry out calculations involving such cycles

Starter activity

Can you write out definitions for ΔHθc and

ΔHθf ?

Standard enthalpy change of combustion, ΔHθ

c, 298 is the enthalpy change when 1 mole of a substance is burned completely in oxygen under standard conditions (100kPa and 298K), all reactants and products being in their standard states. Standard enthalpy change of formation, ΔHθ

f,

298 is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions (100kPa and 298K), all reactants and products being in their standard states.

Hess’ Law The enthalpy change of a reaction depends only on the initial and final state of the reaction and is independent of the route by which the reaction may occur.

H = (Hc reactants) - (H

c products)‘CRAP rule’

H = (Hf products) - (H

f reactants)

AS Chemistry

Bond Enthalpies

Learning ObjectivesCandidates should be able to apply Hess’ Law to construct simple energy cycles, and carry out calculations involving such cycles and relevant energy terms, with particular reference to average bond energies.

Starter activityCan you write equations for the ΔHθ

c and ΔHθ

f of glucose (C6H12O6)?

Bond breaking and bond makingChemical reactions involve bond breaking and bond making.

The quantity of energy needed to break a particular bond in a molecule is called the bond dissociation enthalpy (H

diss), or bond enthalpy for short. It refers to the enthalpy change when one mole of bonds of the same type are broken in gaseous molecules under standard conditions.

Bond energy

H – H (g) H(g) + H(g)

Bond energy

Mean Bond Enthalpy

The mean bond enthalpy is the amount of energy needed to break a covalent bond.They are average values taken from many different molecules

Bond Numberbroken

Numberformed

Average MolarBond Enthalpy

kJ mol-1

C-C 1 0 +347

C-H 5 0 +413

C-O 1 0 +358

O-H 1 6 +464

O=O 3 0 +498

C=O 0 4 +805

Here bond enthalpies are defined endothermically.

Bond breaking:Total endothermic value = (+347 x 1) + (+413 x 5) + (+358 x

1) + (+464 x 1) + (+498 x 3) = +4728 kJ

Bond making:Total exothermic value = (-464 x 6) + (-805 x 4) = -6004 kJ

Sum total of bond breaking and bond making:Hc = +4728 + - 6004 = -1276 kJ mol-1

AS Chemistry

Kinetics

Learning ObjectivesCandidates should be able to:

• Explain and use the terms rate of reaction and activation energy.

• Show understanding, including reference to the Boltzmann distribution, of what is meant by the term activation energy.

Starter activityWorking in groups of 3, complete task 1.

SecondsMinutesHoursDaysWeeksMonthsYearsDecadesCenturiesMillennia

Different rates of reactiondynamite explodingmagnesium and acid

cake baking

fruit ripening

plants growingrusting of iron

erosion of rock

crude oil forming

The rate of a reaction is found by measuring the amount in moles of a reactant which is used up, or the amount of product produced, in a given time.

The units are often mol dm-3 s-1.

Rate of Reaction

There are five factors which can affect the rate of a reaction:

1. Surface Area2. Concentration3. Temperature4. Use of a catalyst5. Intensity of light

Changing the rate of a reaction

Reactions occur when the particles of reactants collide, provided they collide with a certain minimum amount of kinetic energy (and in the correct orientation).

Collision Theory

Activation Energy - the minimum energy required for a reaction to occur.

Enthalpy

Progress of Reaction

Enthalpy

Progress of Reaction

Enthalpy

Progress of Reaction

Enthalpy

Progress of Reaction

Reactants

Reactants

Products

Reactants

Reactants

Products

Products

Products

A

C D

B

Maxwell-Boltzmann Distribution

AS Chemistry

Effect of Temperature

Learning ObjectivesCandidates should be able to:

• Explain qualitatively, in terms of both of the Boltzmann distribution and of collision frequency, the effect of temperature change on the rate of reaction.

Starter activityCan you complete task 1?

Activation Energy

Increasing temperature

Increasing temperature

Temperature and Activation Energy

Temperature and Activation Energy

AS Chemistry

Catalysis

Learning ObjectivesCandidates should be able to:

• explain that, in the presence of a catalyst, a reaction has a different mechanism, i.e. one of lower activation energy, and interpret this catalytic effect in terms of the Boltzmann distribution.

• describe enzymes as biological catalysts (proteins) which may have specific activity.

Starter activityAnswer past paper question.

Activation Energy

Effect of a catalyst

"A catalyst provides an alternative route for the reaction with a lower activation energy."

Effect of a catalyst

Alternative Pathway

Alternative Pathway

Heterogeneous catalysis: where the reactants and catalyst are in different physical states. Common in industrial processes.Homogeneous catalysis: where the reactants and catalyst are in the same physical state. E.g. enzyme-catalysed reactions in cells.

Types of catalysis

Heterogeneous catalysis

Homogeneous catalysis

Peroxodisulphate and iodide ions

Enzymes are proteins that act as biological catalysts. Without them the reactions that make life possible would be too slow for life to exist.

Enzymes

‘Lock and Key’ mechanism

Enzymes are proteins

Active sites

AS Chemistry

Effect of concentration

Learning ObjectivesCandidates should be able to:

explain qualitatively, in terms of collisions, the effect of concentration changes on the rate of a reaction.

Starter activityIn your pairs, can you complete the activity ‘Kinetics starter’?

1 4 0

1 2 0

1 0 0

8 0

6 0

4 0

2 0

0 T im e

Volu

me

of C

O /

cm3

2

Am

ount

of p

rodu

ct2H2O2 (aq) 2H2O (l) + O2(g)

Amou

nt o

f pr

oduc

t

Concentration – time graph

Concentration – time graph

Amou

nt o

f rea

ctan

t

Time

Effect of concentration

Zero-order reaction

Effect of Pressure

Collecting gas e.g. Zn(s) + 2HCl(aq) ZnCl2 (aq) + H2(g)

Colorimeter

e.g. Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + SO2(g) + S(s) + H2O(l)

Precipitation or ‘clock experiment’

Loss in mass

Redox

AS Chemistry

Candidates should be able to:

• describe and explain redox processes in terms of electron transfer and/or of changes in oxidation number (oxidation state)

Learning Objectives

Starter activityIn groups can you select the most appropriate term/s to describe the following chemical reactions?

a.

Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)

b.

CuCO3(s) CuO(s) + CO2(g)

c.

2NaBr(aq) + Cl2(aq) 2NaCl(aq) + Br2(aq)

d.

2CuO(s)+ C(s) 2Cu(s) + CO2(g)

e.

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

f.

2NaCl(l) 2Na(l) + Cl2(g)

g.

NaOH(aq)+ HNO3(aq) NaNO3(aq) + H2O(l)

h.

CH4 + Cl2 CH3Cl + HCl

Learning Objectives

Zn + CuO ZnO + Cu

Definitions• Oxidation is gain of oxygen.

• Reduction is loss of oxygen.

Blast Furnace

An oxidising agent is a substance which oxidises something else. It gains electrons and is reduced.

A reducing agent reduces something else. It loses electrons and is oxidised.

•Oxidation is loss of hydrogen.•Reduction is gain of hydrogen.

Organic reactions

Consider the following reactions:SO2 + H2O + HgO H2SO4 + HgSO2 + 2H2O + Cl2 H2SO4 + HCl

Clearly in both reactions there is an oxidation of SO2(g) to SO3(g) i.e. H2SO4.

Yet the second reaction does not involve oxygen! We clearly need a more universal definition of oxidation and reduction.

Universal definition

Mg + ZnCl2 MgCl2 + Zn

Mg + CuCl2 MgCl2 + Cu

Definitions•Oxidation is loss of electrons.•Reduction is gain of electrons.

Electron transfer

•The oxidation state of an uncombined element is zero. •The sum of the oxidation states of all the atoms or ions in a neutral compound is zero.

•The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.

•The more electronegative element in a substance is given a negative oxidation state. The less electronegative one is given a positive oxidation state.

•Some elements almost always have the same oxidation states in their compounds:

elementusual oxidation state

exceptions

Group 1 metals always +1  

Group 2 metals always +2  

Oxygen usually -2 except in peroxides and F2O (see below)

Hydrogen usually +1 except in metal hydrides where it

is -1 (see below)Fluorine always -1  

Chlorine usually -1 except in compounds with O or F (see below

Redox reactions

AS Chemistry

Candidates should be able to:

• describe and explain redox processes in terms of electron transfer and/or of changes in oxidation number (oxidation state).

Learning Objectives

Can you work out the oxidation states of the transition metal elements in the following compounds?

• KMnO4

• K2Cr2O7

Starter activity

Naming compounds

•SnO•SnO2

•FeCl2•FeCl3•PbCl4•Cu2O•Mn(OH)2

•NO2-

•NO3-

•SO32-

•SO42-

•MnO4-

•CrO42-

•VO3-

1. 2ClO3- 2Cl- + 3O2

2. 2Br- + 2H+ + H2SO4 Br2 + SO2 + 2H2O

3. 8I- + 8H+ + H2SO4 4I2 + H2S + 4H2O

4. I2 + SO3- + H2O 2I- + SO4

2- + 2H+

What is oxidised, what’s reduced?

What is oxidised, what’s reduced?

Writing ionic equations

In a reaction chlorine gas oxidises iron(II) ions to iron(III) ions. In the process, the chlorine is reduced to chloride ions.

Write a balanced equation for this redox process.

Manganate(VII) ions, MnO4-, oxidise hydrogen

peroxide, H2O2, to oxygen gas. The reaction is done with potassium manganate(VII) solution and hydrogen peroxide solution acidified with dilute sulphuric acid.

The manganate(VII) is reduced to Mn2+.

Write a balanced equation for this redox process.

Writing ionic equations

Writing ionic equationsThis technique can be used just as well in examples involving organic chemicals.

Potassium dichromate(VI) solution acidified with dilute sulphuric acid is used to oxidise ethanol, CH3CH2OH, to ethanoic acid, CH3COOH.

The Cr2O72-

is reduced to Cr3+.

Write a balanced equation for this redox process.

Electrolysis

AS Chemistry

Manufacturing Chlorine using a Diaphragm Cell

• Candidates should be able to explain, including the electrode reactions, the industrial process of:

◦ the electrolysis of brine, using a diaphragm cell;

◦ the extraction of aluminium from molten aluminium oxide/cryolite; and

◦ the electrolytic purification of copper.

Learning Objectives

Starter activity

Complete task 1 on your worksheet.

Extraction of Aluminium

Environmental concerns

Fort William

Anglesey

Purification of Copper

Printed circuit board

Effect of impurities on conductivity

Purification of Copper

Purification of Copper

Dynamic Equilibrium

AS Chemistry

Candidates should be able explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium.

Learning Objectives

Starter activity

In pairs, consider the reaction given below. How many facts about this reaction can you write down? Try to use the correct scientific terminology.

N2(g) + 3H2(g) 2NH3(g)

H –ve

Forward reaction

Back reaction

Static equilibrium

Modelling dynamic equilibrium

Dynamic Equilibrium

Position of Equilibrium

Position of Equilibrium

Rate of forward reaction

Rate of back reaction

Dynamic equilibrium

Dynamic Equilibrium2 HI(g) H2(g) + I2(g)

Le Chatelier’s Principle

AS Chemistry

Candidates should be able to state Le Chatelier’s Principle and apply it to deduce qualitatively (from appropriate information) the effects of changes in temperature, concentration or pressure, on a system at equilibrium

Learning Objectives

Starter activity

Question 1 from worksheet ‘Problems for 7.1’

Put simply, Le Chatelier’s Principle states that:

If a system is at equilibrium, and a change is made in any of the conditions, then the system responds to counteract the change as much as possible.

Le Chatelier’s Principle

Effect of concentrationSuppose you have an equilibrium established between four substances A, B, C and D.

What would happen if you changed the conditions by increasing the concentration of A?

Effect of pressure

What would happen if you changed the conditions by increasing the pressure?

Effect of temperature

What would happen if you changed the conditions by increasing the temperature?

Equilibrium constants

AS Chemistry

Candidates should be able to deduce expressions for equilibrium constants in terms of

concentrations, Kc, and partial pressures, Kp.

deduce whether changes in concentration, pressure or temperature or the presence of a catalyst affect the value of the equilibrium constant for a reaction.

calculate the values of equilibrium constants in terms of concentrations or partial pressures from appropriate data.

calculate the quantities present at equilibrium, given appropriate data.

Learning Objectives

Starter activity

In pairs, consider the reaction given below. If you wanted to make as much ammonia as possible what conditions would you use?

N2(g) + 3H2(g) 2NH3(g)

H –ve

K – the equilibrium constant

Equilibrium constant

•to provide a quantitative measure of the extent of a reaction;

•to determine the position of equilibrium.

Kc

a A + b B c C + d D

ba

dc

c BADCK

][][][][

Calculating Kc

CH3CH2OH + CH3COOH CH3COOCH2CH3 + H2O

n(start) 1.0 1.0 0 0

n(equil.) 0.34 0.34 0.66 0.66

[ ] 3.4 3.4 6.6 6.6

Kc = 6.6 x 6.6 / 3.4 x 3.4 = 3.7 (no units)

KpPartial pressure

The total pressure exerted by a mixture of gases is the sum of the partial pressure of the gases.

pA = xA x ptot

KpPartial pressure and mole fraction

Partial pressure terms are expressed in SI units as Pa or kPa.

The equilibrium constant Kp

a A + b B c C + d D

Kp = ba

dc

BpApDpCp

)()()()(

Calculating Kp

PCl5(g) PCl3(g) + Cl2

n(start) 2.0 0 0

n(equil.) 0.8 1.21.2

x 0.25 0.375 0.375

P 166 249 249

Kp = 249 x 249 / 166 = 374 kPa

Change in temperature

∆H for reaction

Change in Temp.

Shift of Equilibriu

mYield of Product

Equilibrium

constantExo Increase Decrease Decrease

Exo Decrease Increase Increase

Endo Increase Increase Increase

Endo Decrease Decrease Decrease

Equilibria of importance

AS Chemistry

Candidates should be able to describe and explain the conditions used in the Haber process and the Contact process, as examples of the importance of an understanding of chemical equilibrium in the chemical industry.

Learning Objectives

Starter activityThe gases SO2, O2 and SO3 are allowed to reach equilibrium. The partial pressures of the gases are pSO2

= 0.050 atm, pO2 = 0.025 atm, pSO3

= 1.00 atm.

Find the values of Kp for the equilibria

a) SO2(g) + ½O2(g) SO3(g)

b) 2SO2(g) + O2(g) 2SO3(g)

Comment on your results!

Haber process

Process Contact HaberMain product Sulphuric acid Ammonia

Main uses of product

Paints, detergents and soaps, fertilisers and dyes.

Fertilisers, explosives, nitric acid, polymers.

Balanced equation for main reaction/s

1.S + O2 SO22. 2SO2 + O2 2SO33. SO3 + H2SO4 H2S2O74. H2S2O7 + H2O 2H2SO4

3H2 + N2 2NH3

Catalyst Vanadium(V) oxide Porous iron (with metal oxide promoter)

Is the equilibrium reaction exothermic or endothermic?

Exothermic Exothermic

Optimum conditions for highest yield

Low temperatureHigh pressure

Low temperatureHigh pressure

Actual conditions used

400 – 600oC1-2 atm

400 – 500oC200 atm

Why are these conditions chosen?

Catalyst ineffective at low T.High pressure uneconomical.

Reaction slow at low T.High pressure too costly.

Points of interest Reaction mixture cooled after each exothermic stage.Vast excess of air.

Ammonia removed as produced.N2 and H2 recycled.Waste heat re-used.

AS Chemistry

Acid and Base Equilibria

Starter Activity

A white solid is formed at X. Can you explain what is happening in this reaction? What words would you use to describe it?

Learning objectives:Candidates should be able to:

• show understanding of, and use the Bronsted-Lowry theory of acids and bases.

• explain qualitatively the differences in behaviour between strong and weak acids and bases and the pH values of their aqueous solutions in terms of the extent of dissociation.

Properties of Acids

React with metals to

produce H2

Taste sourTurn litmus

red

Have a pH < 7

React with carbonates to produce

CO2

Produce H+ in solution

Neutralise alkalis

In 1884 Arrhenius stated that:

•Acids are substances which produce hydrogen ions in solution.

•Bases are substances which produce hydroxide ions in solution.

Allowed an explanation for neutralisation:

Early theory of acid behaviour

Limitations of ArrheniusEasy to explain:

More of a challenge:

Limitations of Arrhenius

There is no solution!!!

Bronsted-Lowry

Acids are PROTON DONORS and bases are PROTON ACCEPTORS.

Conjugate acid-base pairs

A conjugate acid-base pair are related by the transfer of a proton.

H2O + H2O H3O+ + OH-

Water

Amphoteric behaviour

A strong acid is one which is virtually 100% ionised in solution.

A weak acid is only partially ionised in solution.

Strong or weak acids