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Unit 4 Kinetics, equilibria and organic chemistry
1 Kinetics
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Determining rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The effect of temperature on rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rate equations and reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 Kinetics
2 Equilibria
Tick to track your progress
Use the revision planner on pages 4 and 5 to plan your
revision, topic by topic. Tick each box when you have:
l revised and understood a topic
l tested yourself
l practised the exam questions and gone online to
check your answers and complete the quick quizzes
13
15
The equilibrium constant, Kc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The effect of temperature, concentration and
catalysts on Kc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rate equations
3 Acid–base equilibria
17 Brönsted–Lowry acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17 Strong and weak acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What
is meant by ‘rate of a chemical reaction’?
18 The ionic product of water, Kw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
For
reaction:
20theThe
acid dissociation constant, Ka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21A +Titration
B → X + .Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24 Buffer solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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the rate of reaction is equal to the rate at which the concentration of
Rate of reaction always has units
4 Isomerism
in organic
chemistry
reactant
A or B is decreasing,
or the
rate at which the concentration of
mol dm−3 s−1.
27 Structural
isomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■
product
X or Y is increasing.
27
Stereoisomerism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Acylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Concentration of product
The rate of a reaction can be measured at any time during the reaction, but
group
it5is Compounds
most convenientlycontaining
done at the verythe
startcarbonyl
of the reaction,
at time t = 0.
This
is called theand
initial
rate. The
31 rate
Aldehydes
ketones
. . . . . .initial
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drawing
the
tangent
to
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graph
at
t
=
33 Carboxylic acids and esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.
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6 Aromatic compounds
40
41
Structure of benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∆t
Reactions
of benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Amines and amino acids
45
48
49
Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amino∆cacidsGradient
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= ∆c
Proteins . . . . . . . . . . . . .∆t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Features to help you succeed
8 Polymers
52
Addition polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
Time
53 Condensation polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 1.1 Calculating the initial rate of reaction
54 Disposing of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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It is possible to measure the rate at any time during the reaction by
measuring the gradient of the tangent at that particular time.
Examiners’ tips and summaries
Concentration and rate
Revised
For the general
reaction between
reactants A and B inin
solution:
Expert tips are given throughout the book to help you polish your
exam
technique
order to maximise your
A(aq) + B(aq) → X + Y
rate = k[A] [B]
chances in the exam.
where square brackets, [ ], represent the molar concentrations.
4
Exam practice answers and quick quizzes at www.therevisionbutton.co.uk/myrevisionnotes
a
b
180800_01_A2_Chem_1-39.indd 4
10/12/12 12:24 PM
The variables a and b are called the orders of reaction, with respect to
reactant, and k is called the rate constant.
The summaries provide a quick-check bullet list for each topic.each
The rate constant is constant with time and varying concentration, but it
does increase with increasing temperature.
Unit 4 Kinetics, equilibria and organic chemistry
Typical mistakes
The author identifies the typical mistakes
candidates make and explains how you can avoid
them.
Now test yourself
180800_00_A2_Chem_1-39.indd 7
7
04/02/13 8:45 PM
These short, knowledge-based questions provide the
first step in testing your learning. Answers are at the
back of the book.
Definitions and key words
Clear, concise definitions of essential key terms are
provided where they first appear.
Key words from the specification are highlighted in
bold throughout the book.
Exam practice
Practice exam questions are provided for each topic.
Use them to consolidate your revision and practise
your exam skills.
Book Interior Layout.indb 3
Revision activities
These activities will help you to understand each
topic in an interactive way.
Online
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05/02/13 10:01 AM
My revision planner
Unit 4 Kinetics, equilibria and organic chemistry
1 Kinetics
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Rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Determining rate equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The effect of temperature on rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rate equations and reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Equilibria
13 The equilibrium constant, Kc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15The 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18 The ionic product of water, Kw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20 The acid dissociation constant, Ka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21Titrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24 Buffer solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Isomerism in organic chemistry
27 Structural isomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27Stereoisomerism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Compounds containing the carbonyl group
31 Aldehydes and ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33 Carboxylic acids and esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35Acylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Aromatic compounds
40 Structure of benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41 Reactions of benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Amines and amino acids
45Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
48 Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Polymers
52 Addition polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
53 Condensation polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
54 Disposing of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Exam practice answers and quick quizzes at www.therevisionbutton.co.uk/myrevisionnotes
Book Interior Layout.indb 4
05/02/13 10:01 AM
9 Synthesis, analysis and structure determination
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57 Infrared spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
58 Mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60 Nuclear magnetic resonance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . .
63Chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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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
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Redox equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrode potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrochemical series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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108 Now test yourself answers
Exam practice answers and quick quizzes at www.therevisionbutton.co.uk/myrevisionnotes
My revision planner
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Countdown to my exams
6–8 weeks to go
One week to go
Start by looking at the specification — make sure
you know exactly what material you need to
revise and the style of the examination. Use the
revision planner on pages 4 and 5 to familiarise
yourself with the topics.
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.
l
l
Revised
4–6 weeks to go
l
l
l
l
l
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.
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.
Make a note of any problem areas as you revise,
and ask your teacher to go over these in class.
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
Use the revision activities to try out different
revision methods. For example, you can make
notes using mind maps, spider diagrams or flash
cards.
Track your progress using the revision planner and
give yourself a reward when you have achieved
your target.
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.
Revised
The day before the examination
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.
l
Revised
My exams
A2 Chemistry Unit 4
Date: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Location: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A2 Chemistry Unit 5
Date: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Location: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Revised
6
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1 Kinetics
Rate equations
What 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.
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.
Concentration of product
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.
∆t
∆c
0
Gradient
= ∆c
∆t
Time
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
Revised
For the general reaction between reactants A and B in solution:
A(aq) + B(aq) → X + Y
rate = 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.
Unit 4 Kinetics, equilibria and organic chemistry
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1 Kinetics
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.
Example
Typical mistake
For the reaction in aqueous solution between propanone, CH3COCH3(aq),
and iodine, I2(aq), in the presence of an acid catalyst:
CH3COCH3 + I2 → CH3COCH2I + HI
the rate equation is found by experiment to be:
rate = k[CH3COCH3]1[I2]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.
Many candidates think that a rate
equation can be deduced from the
balanced symbol equation. This is
not true. It has to be determined
experimentally.
The overall order of a reaction is simply the sum of all of the individual
orders. If the rate equation is:
rate = k[CH3COCH3]1[I2]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[CH3COCH3]1[I2]0[H+]1
Making k the subject of this equation gives:
k=
rate
[CH3COCH3(aq)]1[I2(aq)]0 [H+ (aq)]1
So the units of k are
mol dm−3 s −1
(mol dm−3 )2
= mol−1 dm3 s−1
Examiners’ tip
Remember the power rules from your maths lessons:
(y a ) × (y b ) = y (a+ b) and
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. 108
ya
= y (a− b) and (y a )b = y ab
yb
Tested
Order of reaction and rate
Revised
0
Time
First order
0
Concentration
Zero order
Concentration
Concentration
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.
Time
0
Second order
Time
Figure 1.2 Concentration–time graphs
8
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Concentration
Rate
First order
Rate
Rate
Zero order
Concentration
rate = k [reactant]
rate = k [reactant]
0
1
1 Kinetics
When the rates of reaction are monitored against concentration, the
graphs shown in Figure 1.3 are obtained.
Second order
Concentration
rate = k [reactant]2
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.
Examiners’ tip
Remember — ‘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.
Determining rate equations
In 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.
Examiners’ tip
Questions asking you to deduce a
rate equation using concentration
rate data are very common in the
exam.
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.
Unit 4 Kinetics, equilibria and organic chemistry
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1 Kinetics
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
This gives k = 0.4, with units
mol dm−3 s −1
= mol−2 dm6 s−1
(mol dm−3 )3
Now test yourself
Tested
2 The following data were measured for the reaction between nitrogen(ii) oxide and hydrogen:
2NO(g) + 2H2(g) → N2(g) + 2H2O(g)
Experiment Initial concentrations/mol dm−3 Initial rate/
number
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
The effect of temperature on rate
In all chemical reactions, as the temperature is increased the rate will
increase.
Number of molecules
A typical Maxwell–Boltzmann distribution is shown in Figure 1.4.
T1
T2
Ea
Energy
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.
10
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1 Kinetics
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.
l
Rate equations and reaction
mechanism
A 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) + F2(g) → 2NO2F(g)
The rate equation, found by experiment, is first order with respect to both
reactants:
rate = k[NO2(g)][F2(g)]
We can deduce the following about the mechanism of the reaction:
l Since both NO2 and F2 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 NO2 and F2 are involved in the rate-determining step of the
mechanism — so the first step could be:
slow
NO2(g) + F2(g) → NO2F(g) + F(g)
l The next step — a fast one — may involve F(g) from the slow step
reacting with another NO2 molecule:
NO2(g) + F(g) → NO2F(g)fast
The species appearing in the
rate equation also occur in the
rate-determining step of the
mechanism.
Notice how the two individual steps in this mechanism add up to give the
overall chemical equation:
slow
Step 1: NO2(g) + F2(g) → NO2F(g) + F(g)
Step 2: NO2(g) + F(g) → NO2F(g) fast
Overall: 2NO2(g) + F2(g) → 2NO2F(g)
Unit 4 Kinetics, equilibria and organic chemistry
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1 Kinetics
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
−3
8.0 × 10
1.28 × 10−2
Experiment 3
2.0 × 10−2
8.0 × 10−3
2.56 × 10−2
−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:
CH3COCH3 + I2 → CH3COCH2I + 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: CH3COCH3 + H+ → [CH3C(OH)CH3]+slow
Step 2: [CH3C(OH)CH3]+ → CH3C(OH)CH2 + H+fast
fast
Step 3: CH3C(OH)CH2 + I2 → CH3COCH2I + HI
Answers and quick quizzes online
Online
Examiners’ summary
You 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
12
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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