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Unit 4 Kinetics, equilibria and organic chemistry

1 Kinetics 7 Rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 9 Determining rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 10 The effect of temperature on rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 11 Rate equations and reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■2 Equilibria 13 The equilibrium constant, Kc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 15 The effect of temperature, concentration and

catalysts on Kc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■3 Acid–base equilibria 17 Brönsted–Lowry acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 17 Strong and weak acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 17 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 18 The ionic product of water, Kw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 20 The acid dissociation constant, Ka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 21 Titration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 24 Buffer solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■4 Isomerism in organic chemistry 27 Structural isomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 27 Stereoisomerism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■5 Compounds containing the carbonyl group 31 Aldehydes and ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 33 Carboxylic acids and esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 35 Acylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■6 Aromatic compounds 40 Structure of benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 41 Reactions of benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■7 Amines and amino acids 45 Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 48 Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 49 Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■8 Polymers 52 Addition polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 53 Condensation polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 54 Disposing of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

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180800_01_A2_Chem_1-39.indd 4 10/12/12 12:24 PM

7Unit 4 Kinetics, equilibria and organic chemistry

1 Kinetics

Rate equationsWhat is meant by ‘rate of a chemical reaction’?

For the reaction:

A + B → X + Y

the rate of reaction is equal to the rate at which the concentration of reactant A or B is decreasing, or the rate at which the concentration of product X or Y is increasing.

The rate of a reaction can be measured at any time during the reaction, but it is most conveniently done at the very start of the reaction, at time t = 0. This rate is called the initial rate. The initial rate of reaction is calculated by drawing the tangent to the concentration–time graph at t = 0.

Co

nce

ntr

atio

n o

f p

rod

uct

Time

∆t

0

∆cGradient

= ∆t∆c

Figure 1.1 Calculating the initial rate of reaction

It is possible to measure the rate at any time during the reaction by measuring the gradient of the tangent at that particular time.

Concentration and rate

For the general reaction between reactants A and B in solution:A(aq) + B(aq) → X + Yrate = k[A]a[B]b

where square brackets, [ ], represent the molar concentrations.

The variables a and b are called the orders of reaction, with respect to each reactant, and k is called the rate constant.

The rate constant is constant with time and varying concentration, but it does increase with increasing temperature.

Revised

Rate of reaction is defined as the rate of change of concentration of a reactant or a product with time. Rate of reaction always has units mol dm−3 s−1.

Revised

180800_00_A2_Chem_1-39.indd 7 04/02/13 8:45 PM

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Unit 4 Kinetics, equilibria and organic chemistry

1 Kinetics 7 Rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 9 Determining rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 10 The effect of temperature on rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 11 Rate equations and reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■2 Equilibria 13 The equilibrium constant, Kc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 15 The effect of temperature, concentration and

catalysts on Kc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■3 Acid–base equilibria 17 Brønsted–Lowry acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 17 Strong and weak acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 17 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 18 The ionic product of water, Kw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 20 The acid dissociation constant, Ka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 21 Titrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 24 Buffer solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■4 Isomerism in organic chemistry 27 Structural isomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 27 Stereoisomerism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■5 Compounds containing the carbonyl group 31 Aldehydes and ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 33 Carboxylic acids and esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 35 Acylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■6 Aromatic compounds 40 Structure of benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 41 Reactions of benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■7 Amines and amino acids 45 Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 48 Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 49 Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■8 Polymers 52 Addition polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 53 Condensation polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 54 Disposing of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

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57 Infrared spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 58 Mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 60 Nuclear magnetic resonance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 63 Chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

Unit 5 Energetics, redox and inorganic chemistry

10 Thermodynamics 68 Enthalpy change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 73 Entropy and Gibb’s free energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■11 Periodicity 78 Reactions of period 3 elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 79 The oxides of period 3 elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■12 Redox equilibria 83 Redox equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 84 Electrode potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 86 Electrochemical series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 89 Electrochemical cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■13 Transition metals 92 Electronic configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 92 General properties of transition metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 101 Reactions of hydrated transition metal ions . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 105 Ligand substitution reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

108 Now test yourself answers

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l Organise your notes, making sure you have covered everything on the specification. The revision planner will help you to group your notes into topics.

l Work out a realistic revision plan that will allow you time for relaxation. Set aside days and times for all the subjects that you need to study, and stick to your timetable.

l Set yourself sensible targets. Break your revision down into focused sessions of around 40 minutes, divided by breaks. These Revision Notes organise the basic facts into short, memorable sections to make revising easier.

One week to go

l Try to fit in at least one more timed practice of an entire past paper and seek feedback from your teacher, comparing your work closely with the mark scheme.

l Check the revision planner to make sure you haven’t missed out any topics. Brush up on any areas of difficulty by talking them over with a friend or getting help from your teacher.

l Attend any revision classes put on by your teacher. Remember, he or she is an expert at preparing people for examinations.

The day before the examination

l Flick through these Revision Notes for useful reminders, for example the Examiners’ tips, examiners’ summaries, typical mistakes and key terms.

l Check the time and place of your examination.

l Make sure you have everything you need — extra pens and pencils, tissues, a watch, bottled water, sweets.

l Allow some time to relax and have an early night to ensure you are fresh and alert for the examination.

4–6 weeks to go

l Read through the relevant sections of this book and refer to the examiners’ tips, examiners’ summaries, typical mistakes and key terms. Tick off the topics as you feel confident about them. Highlight those topics you find difficult and look at them again in detail.

l Test your understanding of each topic by working through the ‘Now test yourself’ questions in the book. Look up the answers at the back of the book.

l Make a note of any problem areas as you revise, and ask your teacher to go over these in class.

l Look at past papers. They are one of the best ways to revise and practise your exam skills. Write or prepare planned answers to the exam practice questions provided in this book. Check your answers online and try out the extra quick quizzes at www.therevisionbutton.co.uk/myrevisionnotes

l Use the revision activities to try out different revision methods. For example, you can make notes using mind maps, spider diagrams or flash cards.

l Track your progress using the revision planner and give yourself a reward when you have achieved your target.

My exams

A2 Chemistry Unit 4

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Location: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A2 Chemistry Unit 5

Date: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Time: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Location: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Revised

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Countdown to my exams



1 Kinetics

Rate equationsWhat is meant by ‘rate of a chemical reaction’?

For the reaction:

A + B → X + Y

the rate of reaction is equal to the rate at which the concentration of reactant A or B is decreasing, or the rate at which the concentration of product X or Y is increasing.

The rate of a reaction can be measured at any time during the reaction, but it is most conveniently done at the very start of the reaction, at time t = 0. This rate is called the initial rate. The initial rate of reaction is calculated by drawing the tangent to the concentration–time graph at t = 0.

Co

nce

ntr

atio

n o

f p

rod

uct

Time

∆t

0

∆cGradient

= ∆t∆c

Figure 1.1 Calculating the initial rate of reaction

It is possible to measure the rate at any time during the reaction by measuring the gradient of the tangent at that particular time.

Concentration and rate

For the general reaction between reactants A and B in solution:A(aq) + B(aq) → X + Yrate = k[A]a[B]b

where square brackets, [ ], represent the molar concentrations.

The variables a and b are called the orders of reaction, with respect to each reactant, and k is called the rate constant.

The rate constant is constant with time and varying concentration, but it does increase with increasing temperature.

Revised

Rate of reaction is defined as the rate of change of concentration of a reactant or a product with time. Rate of reaction always has units mol dm−3 s−1.

Revised



1 K

inet

ics In the equation above, the order with respect to A is a, and the order with

respect to B is b. Orders will only have the (integer) values 0, 1 and 2 in examples at this level.

For the reaction in aqueous solution between propanone, CH3COCH

3(aq),

and iodine, I2(aq), in the presence of an acid catalyst:

CH3COCH

3 + I

2 → CH

3COCH

2I + HI

the rate equation is found by experiment to be:

rate = k[CH3COCH

3]1[I

2]0[H+]1

So the rate of this reaction is first order with respect to propanone, first order with respect to hydrogen ions, and zero order with respect to iodine.

Example

The overall order of a reaction is simply the sum of all of the individual orders. If the rate equation is:

rate = k[CH3COCH

3]1[I

2]0[H+]1

the overall order is 1 + 0 + 1 = 2, or second order overall.

The units of the rate constant can be found by rearranging the rate equation. For the propanone/iodine reaction above:

rate = k[CH3COCH

3]1[I

2]0[H+]1

Making k the subject of this equation gives:

k =rate

[CH COCH (aq)] [I (aq)] [H (aq)]3 3 1 2 0 1+

kSo the units of aremol dm s(mol dm )

3 1

3 2

− −

−

= mol−1 dm3 s−1

Examiners’ tipRemember the power rules from your maths lessons:

a b a ba

ba b a b ab(y ) (y ) = y and

yy

= y and (y ) = y( ) ( )× + −

Order of reaction and rate

When the concentration of a reactant in a chemical reaction is monitored with time, it may vary in one of three main ways, as indicated by the graphs shown in Figure 1.2.

Co

nce

ntr

atio

n

Zero order

0 Time

Co

nce

ntr

atio

n

First order

0 Time

Co

nce

ntr

atio

n

Second order

0 Time

Figure 1.2 Concentration–time graphs

Typical mistakeMany candidates think that a rate equation can be deduced from the balanced symbol equation. This is not true. It has to be determined experimentally.

Now test yourself

1 The rate equation for the reaction:

(CH3)3CBr + OH− → (CH3)3COH + Br−

is found to be rate = k[(CH3)3CBr]. The rate is measured in mol dm−3 s−1.(a) What are the orders with

respect to (CH3)3CBr and OH−?

(b) What is the overall order of the reaction?

(c) Give the units of the rate constant.

Answers on p. 108Tested

Revised



1 K

inet

icsWhen the rates of reaction are monitored against concentration, the

graphs shown in Figure 1.3 are obtained.R

ate

Zero order

Concentration

rate = k [reactant]0

Rat

e

First order

Concentration


Rat

e

Second order

Concentration


Figure 1.3 Rate–concentration graphs

l Zero order — when the concentration is doubled, the rate is unaffected.l First order — when the concentration is doubled, the rate increases by

a factor of 2.l Second order — when the concentration is doubled, the rate increases

by a factor of 4.

Determining rate equationsIn a reaction between reactants A, B and C, the following initial rates were obtained using the initial concentrations shown in four different experiments.

Experiment Initial concentration/mol dm−3 Initial rate/mol dm−3 s−1

[A] [B] [C]

1 0.05 0.03 0.12 1.20 × 10−4

2 0.20 0.06 0.24 3.84 × 10−3

3 0.05 0.03 0.24 2.40 × 10−4

4 0.20 0.03 0.12 1.92 × 10−3

Comparing experiments 1 and 3:[A] and [B] remain the same but [C] doubles. This doubles the rate so the order with respect to C must be 1.

Comparing experiments 1 and 4:[A] is multiplied by 4 and [B] and [C] stay constant. This multiplies the rate by a factor of 16. The order with respect to A must therefore be 2, because 42 = 16.

Comparing experiments 3 and 2:[A] is multiplied by 4, [B] is multiplied by 2 and [C] stays constant. This multiplies the rate by a factor of 16. We know that the order with respect to A is 2, so multiplying [A] by 4 should multiply the rate by 16 — and it does. Therefore the change in [B] has no effect on the rate. So the order with respect to B must be 0.

The overall rate equation is therefore:rate = k[A]2[B]0[C]1

and the reaction is third order overall.

Examiners’ tipRemember — ‘first order’ means that when the concentration of the reactant is multiplied by a factor x, the rate is also multiplied by the factor x.

Examiners’ tipQuestions asking you to deduce a rate equation using concentration rate data are very common in the exam.



1 K

inet

ics The value for the rate constant can be calculated using any of

experiments 1, 2, 3 or 4. Using the data from experiment 1 and that rate = k[A]2[C] gives:

1.2 × 10−4 = k × 0.052 × 0.121

kThis gives = 0.4, with unitsmol dm s(mol dm )

3 1

3 3

− −

−= mol−2 dm6 s−1

Now test yourself

2 The following data were measured for the reaction between nitrogen(ii) oxide and hydrogen:2NO(g) + 2H2(g) → N2(g) + 2H2O(g)

Experiment number

Initial concentrations/mol dm−3 Initial rate/mol dm−3 s−1

[NO(g)] [H2(g)]

1 0.100 0.100 1.11 × 10−3

2 0.100 0.200 2.24 × 10−3

3 0.200 0.100 4.45 × 10−3

(a) Determine the order with respect to NO and the order with respect to H2.(b) What is the overall order of reaction?(c) Write the rate equation for the reaction.(d) Determine a value for the rate constant and deduce its units.

Answers on p. 108

Tested

The effect of temperature on rateIn all chemical reactions, as the temperature is increased the rate will increase.

A typical Maxwell–Boltzmann distribution is shown in Figure 1.4.

T1T2

Ea Energy

Nu

mb

er o

f m

ole

cule

s

Figure 1.4 Graph of a Maxwell–Boltzmann distribution at two different temperatures, where T2 > T1

The rate increases with increasing temperature because the proportion of particles having energy greater than the activation energy increases. This increases the number of successful collisions taking place per unit time.

Given a typical rate equation:rate = k[A]2[C]

as the temperature increases, the value of k will also increase.



1 K

inet

icsl The rate constant is related to the shaded area of the Maxwell–

Boltzmann distribution in Figure 1.4 to the right of the activation energy value. The larger this area, the more particles have energy E > Ea

so both the rate and the rate constant will be larger.

l The lower the temperature, the smaller will be the proportion of molecules having an energy greater than the activation energy. Therefore, the rate constant is smaller.

Rate equations and reaction mechanismA rate equation tells us how the concentrations of the reactants affect the rate of the reaction. However, it also indicates something about the mechanism of the reaction taking place.

For example, nitrogen(iv) oxide reacts with fluorine according to the equation:

2NO2(g) + F

2(g) → 2NO

2F(g)

The rate equation, found by experiment, is first order with respect to both reactants:

rate = k[NO2(g)][F

2(g)]

We can deduce the following about the mechanism of the reaction:l Since both NO

2 and F

2 are in the rate equation as non-zero orders, this

means that both their concentrations affect the rate.l Any species that is zero order in the mechanism must be involved in a

subsequent fast step.

Therefore, it can be deduced that:l NO

2 and F

2 are involved in the rate-determining step of the

mechanism — so the first step could be:NO2

(g) + F2(g) → NO

2F(g) + F(g) slow

l The next step — a fast one — may involve F(g) from the slow step reacting with another NO

2 molecule:

NO2(g) + F(g) → NO

2F(g) fast

Notice how the two individual steps in this mechanism add up to give the overall chemical equation:

Step 1: NO2(g) + F

2(g) → NO

2F(g) + F(g) slow

Step 2: NO2(g) + F(g) → NO

2F(g) fast

Overall: 2NO2(g) + F

2(g) → 2NO

2F(g)

The species appearing in the rate equation also occur in the rate-determining step of the mechanism.



1 K

inet

ics Exam practice

1 In a reaction taking place between substances A and B, the following results were obtained in three different experiments.

Assume that the concentrations given in the table are all initial concentrations.

[A]/mol dm−3 [B]/mol dm−3 Initial rate/mol dm−3 s−1

Experiment 1 1.0 × 10−2 4.0 × 10−3 3.20 × 10−3

Experiment 2 1.0 × 10−2 8.0 × 10−3 1.28 × 10−2

Experiment 3 2.0 × 10−2 8.0 × 10−3 2.56 × 10−2

(a) Deduce the order of the reaction with respect to A, and the order of the reaction with respect to B. [2](b) Hence, write the rate equation for the reaction and calculate the rate constant stating its units. [3](c) Calculate the rate of reaction when the initial concentrations of A and B are both 6.0 × 10−3 mol dm−3. [1](d) In another experiment in which the initial concentrations of A and B are both x mol dm–3, the initial rate

of reaction is found to be 9.22 × 10−2 mol dm−3 s−1. Calculate the value of x. [2]2 Propanone, in acidic solution, reacts with iodine according to:

CH3COCH

3 + I

2 → CH

3COCH

2I + HI

In an experiment, the time taken for the iodine to reach a certain concentration was measured. The concentration of hydrogen ions was kept constant in all four experiments described in the table below. The order with respect to the acid is known to be 1.

[CH3COCH3]/mol dm−3 [I2]/mol dm−3 Time/s

0.25 0.05 68

0.50 0.05 34

1.00 0.05 17

0.50 0.10 34

(a) Deduce the order of reaction with respect to propanone. [1](b) Deduce the order with respect to iodine. [1](c) Hence, write the rate equation for the reaction. [1](d) Comment, with a reason, whether or not the following mechanism is consistent with the rate equation

you have suggested in part (c). [2]Step 1: CH

3COCH

3 + H+ → [CH

3C(OH)CH

3]+ slow

Step 2: [CH3C(OH)CH

3]+ → CH

3C(OH)CH

2 + H+ fast

Step 3: CH3C(OH)CH

2 + I

2 → CH

3COCH

2I + HI fast

Answers and quick quizzes onlineOnline

Examiners’ summaryYou should now have an understanding of:

4 what is meant by rate of reaction and initial rate

4 the effect of changes in concentration on rate

4 the rate constant and how to determine its units

4 rate equations and how to determine them given initial rate information

4 concentration–time and rate–concentration graphs

4 the effect of temperature on reaction rate

4 the link between a rate equation and a reaction mechanism


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