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Edexcel A Level Chemistry

Topic 16
Kinetics II

Topic 16 turns rate into an equation: the rate equation, orders and the rate constant, choosing and justifying techniques for rate data, deducing orders from concentration–time and rate–concentration graphs and from initial-rate data, the iodine–propanone reaction, the rate-determining step and mechanisms including the evidence for Sₙ1 and Sₙ2, and activation energy from graphical data. Core Practicals 11 and 12 supply the rate data.

Exam Paper
Paper 1
9CH0/01 (and Paper 3)
Specification Points
16.1 – 16.12
12 points covered
Topic Parts
8 pages
Revision notes available
Exam Board
Edexcel
Pearson 9CH0

Revision Notes

rate = k[A]ᵐ[B]ⁿ
Rate
Equations
orders, k, units of k
1
Available

Rate Equations, Orders and the Rate Constant

Rate equations: orders of reaction, the rate constant k, overall order and working out the units of k.

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Choose a method
Measuring
Rates
quench, colorimeter, syringe, clock
2
Available

Techniques for Measuring Rates

Measuring rates: titration, colorimetry, mass and gas-volume methods, continuous monitoring and the initial-rates method with clock reactions.

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Curves and t½
Concentration–
Time Graphs
order from shape, constant half-life
3
Available

Concentration–Time Graphs and Half-Life

Concentration–time graphs: deducing order from the shape, tangents and rates, half-life and first-order reactions.

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Compare the runs
Initial-Rates
Method
double [A], what happens to rate?
4
Available

Rate–Concentration Graphs and the Initial-Rates Method

The initial-rates method: rate–concentration graphs, deducing orders from tables of data, writing the rate equation and calculating k.

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Worked example
Iodine and
Propanone
zero order in I₂, rate = k[propanone][H⁺]
5
Available

The Iodine–Propanone Reaction

The iodine–propanone reaction: collecting rate data, the orders with respect to iodine, propanone and H⁺, and deducing the mechanism.

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The slow step
Rate-Determining
Step
mechanisms, intermediates, Sₙ1 and Sₙ2
6
Available

Rate-Determining Step and Reaction Mechanisms

The rate-determining step: rate equations from mechanisms, mechanisms from rate equations, intermediates, and Sₙ1/Sₙ2 evidence from halogenoalkanes hydrolysis.

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k = Ae^(−Eₐ/RT)
Arrhenius
Equation
ln k against 1/T, gradient = −Eₐ/R
7
Available

Activation Energy and the Arrhenius Equation

The Arrhenius equation: temperature and the rate constant, ln k against 1/T graphs and finding the activation energy from the gradient.

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Surfaces and cycles
Catalysis
adsorb, weaken, react, desorb
8
Available

Heterogeneous and Homogeneous Catalysis

Catalysis: heterogeneous catalysts at surfaces (adsorption, desorption, the Haber process, catalytic converters) and homogeneous catalysts that are regenerated.

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Specification Coverage

Topic 16 Kinetics II – Edexcel A Level Chemistry

The following specification points outline the knowledge and skills students are expected to demonstrate. Wording follows the Edexcel A Level Chemistry (9CH0) specification.

16.1 – 16.7 Rate equations, orders and rate data

16.1
i
understand the terms: rate of reaction
ii
understand the terms: rate equation
iii
understand the terms: order with respect to a substance in a rate equation
iv
understand the terms: overall order of reaction
ix-a
understand the term: heterogeneous
ix-b
understand the term: homogenous catalyst
16.2
a
be able to determine rate equations of the form rate = k[A]^m[B]^n
b
be able to use rate equations of the form rate = k[A]^m[B]^n
c
know that m and n are 0, 1 or 2
16.3
be able to select and justify a suitable experimental technique to obtain rate data for a given reaction, including:
i
titration
ii
colorimetry
iii
mass change
iv
volume of gas evolved
16.4
understand experiments that can be used to investigate reaction rates by:
i
an initial-rate method, carrying out separate experiments where different initial concentrations of one reagent are used A ‘clock reaction’ is an acceptable approximation of this method
ii
a continuous monitoring method to generate data to enable concentration-time or volume-time graphs to be plotted
16.5
a
be able to calculate rate of reaction from a concentration-time graph
b
be able to calculate rate of reaction from a volume-time graph
c
be able to calculate half-life of a first-order reaction from a concentration-time graph
d
be able to calculate half-life of a first-order reaction from a volume-time graph
16.6
be able to deduce the order (0, 1 or 2) with respect to a substance in a rate equation using data from:
i
a concentration-time graph
ii
a rate-concentration graph
16.7
be able to deduce the order (0, 1 or 2) with respect to a substance in a rate equation using data from an initial-rate method

16.8 – 16.12 The iodine–propanone reaction, mechanisms and activation energy

16.8
understand how to:
i
obtain data to calculate the order with respect to the reactants (and the hydrogen ion) in the acid-catalysed iodination of propanone
ii
use these data to make predictions about species involved in the rate-determining step
iii
deduce a possible mechanism for the reaction
16.9
be able to deduce a rate-determining step from a rate equation and vice versa
16.10
be able to deduce a reaction mechanism, using knowledge from a rate equation and the stoichiometric equation for a reaction
16.11
understand that knowledge of the rate equations for the hydrolysis of halogenoalkanes can be used to provide evidence for SN₁ or SN₂ mechanisms for tertiary and primary halogenoalkane hydrolysis
16.12
be able to use graphical methods to find the activation energy for a reaction from experimental data The Arrhenius equation will be given if needed.