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AQA A Level Chemistry

3.1.6
Chemical Equilibria,
Le Chatelier and Kc

Section 3.1.6 covers reversible reactions and dynamic equilibrium, Le Chatelier’s principle for temperature, pressure and concentration, why a catalyst does not move the position of equilibrium, the compromise conditions used in industry, and the equilibrium constant Kc: writing the expression, calculating a value from equilibrium concentrations and predicting how temperature changes it.

Exam Paper
Paper 1 & 2
7405/1 and 7405/2
Specification Points
3.1.6.1 – 3.1.6.2
2 sections covered
Topic Parts
4 pages
Revision notes available
Exam Board
AQA
7405 (2015 onwards)

Revision Notes

Work through 3.1.6 Chemical Equilibria in a structured sequence.

Reversible
Dynamic
Equilibrium
1
Available

Dynamic Equilibrium

Dynamic equilibrium: reversible reactions, equal forward and reverse rates, constant concentrations, closed systems and the position of equilibrium.

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Shifts
Le Chatelier’s
Principle
2
Available

Le Chatelier’s Principle

Le Chatelier’s principle: temperature, pressure and concentration changes, catalysts, and the ammonia and chromate equilibria as examples.

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Yield versus rate
Equilibria in
Industry
3
Available

Equilibria in Industry

Equilibria in industry: the Haber and Contact processes, compromise temperature and pressure, catalysts and evaluating yield data.

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Products over reactants
Equilibrium
Constant Kc
4
Available

The Equilibrium Constant Kc

Kc: expressions, units, calculations from equilibrium amounts, the magnitude of Kc and the effect of temperature.

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Specification Coverage

3.1.6 Chemical Equilibria, Le Chatelier’s Principle and Kc – 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.

3.1.6.1 Chemical equilibria and Le Chatelier’s principle

i
Many chemical reactions are reversible.
ii
In a reversible reaction at equilibrium, forward and reverse reactions proceed at equal rates and the concentrations of reactants and products remain constant.
iii
Apply Le Chatelier’s principle to predict the effect of changes in temperature, pressure and concentration on the position of equilibrium in homogeneous reactions.
iv
A catalyst does not affect the position of equilibrium.
v
Explain why, for a reversible reaction used in an industrial process, a compromise temperature and pressure may be used.

3.1.6.2 Equilibrium constant Kc for homogeneous systems

i
Construct an expression for Kc for a homogeneous system in equilibrium; the concentration of a species X is represented by [X] in mol dm⁻³.
ii
The value of Kc is not affected by changes in concentration or by addition of a catalyst.
iii
Calculate a value for Kc from the equilibrium concentrations for a homogeneous system at constant temperature, and perform calculations involving Kc.
iv
Predict the qualitative effects of changes of temperature on the value of Kc.