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OCR A Level Chemistry A – Module 5 Physical Chemistry and Transition Elements

5.1.1
How Fast?

Section 5.1.1 covers rate, order, overall order, the rate constant, half-life and the rate-determining step; deducing orders and rate equations from experimental data; calculating k and its units; concentration–time graphs including the constant half-life of a first-order reaction and k = ln 2 / t½; rate–concentration graphs; the initial-rates and continuous monitoring techniques including colorimetry; predicting mechanisms from rate equations; and the Arrhenius equation with the graphical determination of Eₐ and A. PAG 10 supplies the rate data.

Exam Paper
Paper 1 & 3
H432/01 and H432/03
Learning Outcomes
5.1.1 (a) – (k)
11 learning outcomes
Topic Parts
7 pages
Revision notes available
Exam Board
OCR A
H432 (2015 onwards)

Revision Notes

Work through 5.1.1 How Fast? in a structured sequence.

rate = k[A]ᵐ[B]ⁿ
Rate
Equations
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
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
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, k from the half-life.

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Compare the runs
Initial-Rates
Method
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
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
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 haloalkanes hydrolysis.

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k = Ae^(−Eₐ/RT)
Arrhenius
Equation
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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Specification Coverage

5.1.1 How Fast? – OCR A Level Chemistry A

The following OCR A learning outcomes state what candidates should be able to do. Wording is taken from the OCR A Level Chemistry A (H432) specification.

5.1.1 Orders, rate equations and rate constants

5.1.1(a)
i
explanation and use of the term rate of reaction
ii
explanation and use of the term order
iii
explanation and use of the term overall order
iv
explanation and use of the term rate constant
v
explanation and use of the term half-life
vi
explanation and use of the term rate-determining step
5.1.1(b)
(i)
deduction of orders from experimental data
(ii)
deduction of a rate equation from orders of the form: rate = k[A]ᵐ[B]ⁿ, where m and n are 0, 1 or 2
5.1.1(c)
i
calculation of the rate constant, k, and related quantities, from a rate equation
ii
determination of the units of the rate constant, k, from a rate equation

5.1.1 Concentration–time and rate–concentration graphs, techniques

5.1.1(d)
from a concentration–time graph:
(i)
deduction of the order (0 or 1) with respect to a reactant from the shape of the graph
(ii)
calculation of reaction rates from the measurement of gradients (see also 3.2.2 b)
5.1.1(e)
from a concentration–time graph of a first order reaction, measurement of constant half-life, t½
5.1.1(f)
for a first order reaction, determination of the rate constant, k, from the constant half-life, t½, using the relationship: k = ln 2/t½
5.1.1(g)
from a rate–concentration graph:
(i)
deduction of the order (0, 1 or 2) with respect to a reactant from the shape of the graph
(ii)
determination of rate constant for a first order reaction from the gradient
5.1.1(h)
i
the techniques and procedures used to investigate reaction rates by the initial rates method (see also 3.2.2 e)
ii
the techniques and procedures used to investigate reaction rates by continuous monitoring, including use of colorimetry (see also 3.2.2 e)

5.1.1 Rate-determining step and the effect of temperature

5.1.1(i)
(i)
for a multi-step reaction, prediction of a rate equation that is consistent with the rate-determining step
(ii)
for a multi-step reaction, prediction of possible steps in a reaction mechanism from the rate equation and the balanced equation for the overall reaction
5.1.1(j)
a qualitative explanation of the effect of temperature change on the rate of a reaction and hence the rate constant (see 3.2.2 f–g)
5.1.1(k)
(i)
the exponential relationship between the rate constant, k and temperature, T given by the Arrhenius equation, k = Ae^(–Eₐ/RT)
(ii)
determination of Eₐ and A graphically using: ln k = –Eₐ/RT + ln A derived from the Arrhenius equation