0 0 Moodle
Home Revision Notes Courses For Schools Blog My Account Cart
Moodle
AQA A Level Chemistry

3.1.9
Rate Equations

Section 3.1.9 covers the rate equation, order and the rate constant, calculations with the rate equation and the units of k, the Arrhenius equation in both forms with calculations and the ln k against 1/T graph, and the determination of rate equations from concentration–time and rate–concentration graphs, initial rates and the rate-determining step, and what the orders reveal about the mechanism. Required practical 7 supplies the rate data.

Exam Paper
Paper 2
7405/2 (and Paper 3)
Specification Points
3.1.9.1 – 3.1.9.2
2 sections covered
Topic Parts
7 pages
Revision notes available
Exam Board
AQA
7405 (2015 onwards)

Revision Notes

Work through 3.1.9 Rate Equations 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.

Start revising
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.

Start revising
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.

Start revising
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.

Start revising
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.

Start revising
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 halogenoalkanes hydrolysis.

Start revising
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.

Start revising
Specification Coverage

3.1.9 Rate Equations – AQA A Level Chemistry

The following AQA specification points outline the knowledge and skills students are expected to demonstrate. Wording follows the AQA A Level Chemistry (7405) specification.

3.1.9.1 Rate equations

3.1.9.1
i
know that the rate of a chemical reaction is related to concentration by a rate equation of the form Rate = k[A]ᵐ[B]ⁿ
ii-a
define the term order of reaction
ii-b
define the term rate constant
iii-a
know that order zero is permitted in rate equations
iii-b
know that order one is permitted in rate equations
iii-c
know that order two is permitted in rate equations
iv
perform calculations using the rate equation
v
know that the rate constant k varies with temperature according to k = Ae^(−Eₐ/RT) where A is the Arrhenius constant, Eₐ is the activation energy and T is the temperature in K
vi
explain the qualitative effect of changes in temperature on the rate constant k
vii
perform calculations using the equation k = Ae^(−Eₐ/RT)
viii
understand that k = Ae^(−Eₐ/RT) can be rearranged to ln k = −Eₐ/RT + ln A and use this with experimental data to plot a straight line graph with slope -Eₐ/R

3.1.9.2 Determination of rate equation

3.1.9.2
i
know that the rate equation is an experimentally determined relationship
ii
use concentration-time graphs to deduce the rate of a reaction
iii
use initial concentration-time data to deduce the initial rate of a reaction
iv-a
use rate-concentration data to deduce the order with respect to a reactant
iv-b
use rate-concentration graphs to deduce the order with respect to a reactant
v
derive the rate equation for a reaction from the orders with respect to each of the reactants
vi
use the orders with respect to reactants to provide information about the rate determining step of a reaction
vii
understand that the orders with respect to reactants can provide information about the mechanism of a reaction