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Cambridge International AS & A Level Chemistry – Physical Chemistry

Topic 7
Equilibria

Topic 7 covers reversible reactions and dynamic equilibrium in a closed system, Le Chatelier’s principle, the equilibrium constants Kc and Kp with mole fractions and partial pressures, the Haber and Contact processes, and the AS acids-and-bases section: Brønsted-Lowry theory, strong and weak acids and bases, pH, neutralisation, pH titration curves and the choice of indicator. Ionic equilibria (Ka, pH calculations, buffers) follow in Topic 25.

Exam Papers
Papers 1 & 2
9701 AS Level
Learning Outcomes
7.1 – 7.2
20 outcomes covered
Topic Parts
6 pages
Revision notes available
Exam Board
Cambridge
9701 (2025 onwards)

Revision Notes

Work through Topic 7 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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Gases
Kp and
Partial Pressures
5
Available

Kp and Partial Pressures

Kp: mole fraction, partial pressure, Kp expressions, units and calculations for gas equilibria.

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Proton transfer
Acids, Bases
and pH Curves
6
Available

Acids, Bases and pH Curves

Acids and bases: Brønsted–Lowry theory, strong and weak acids, pH, neutralisation, titration curves and indicators.

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Specification Coverage

Topic 7 Equilibria – Cambridge International AS Level Chemistry

The following Cambridge learning outcomes state what candidates should be able to do. Wording is taken from the Cambridge International AS & A Level Chemistry (9701) syllabus.

7.1 Chemical equilibria: reversible reactions, dynamic equilibrium

7.1.1
understand what is meant by a reversible reaction and by dynamic equilibrium (equal rates, constant concentrations), and the need for a closed system
7.1.2
define Le Chatelier’s principle
7.1.3
use Le Chatelier’s principle to deduce qualitatively the effects of changes in temperature, concentration, pressure or the presence of a catalyst on a system at equilibrium
7.1.4
deduce expressions for equilibrium constants in terms of concentrations, Kc
7.1.5
use the terms mole fraction and partial pressure
7.1.6
deduce expressions for equilibrium constants in terms of partial pressures, Kp
7.1.7
use the Kc and Kp expressions to carry out calculations (no quadratic equations)
7.1.8
calculate the quantities present at equilibrium, given appropriate data
7.1.9
state whether changes in temperature, concentration, pressure or the presence of a catalyst affect the value of the equilibrium constant
7.1.10
describe and explain the conditions used in the Haber process and the Contact process, as examples of the importance of an understanding of dynamic equilibrium in the chemical industry and the application of Le Chatelier’s principle

7.2 Brønsted–Lowry theory of acids and bases

7.2.1
state the names and formulae of the common acids (HCl, H₂SO₄, HNO₃, CH₃COOH) and alkalis (NaOH, KOH, NH₃)
7.2.3
describe the Brønsted–Lowry theory of acids and bases
7.2.4
describe strong acids and strong bases as fully dissociated, and weak acids and weak bases as partially dissociated, in aqueous solution
7.2.5
appreciate that water has pH 7, acid solutions pH below 7 and alkaline solutions pH above 7
7.2.6
explain qualitatively the differences in behaviour between strong and weak acids, including the reaction with a reactive metal and the difference in pH values (pH meter, universal indicator or conductivity)
7.2.7
understand that neutralisation reactions occur when H⁺(aq) and OH⁻(aq) form H₂O(l), and that salts are formed in neutralisation reactions
7.2.9
sketch the pH titration curves of titrations using combinations of strong and weak acids with strong and weak alkalis
7.2.10
select suitable indicators for acid–alkali titrations, given appropriate data