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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
Sub-topics
6 lessons
Revision notes available
Exam Board
Cambridge
9701 (2025 onwards)

Revision Notes

6 revision notes, in the order the specification teaches them.

Molecules leaving and returning to a liquid in a sealed jar
1

Dynamic Equilibrium

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

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A gas syringe with its plunger pushed in
2

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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A tall industrial reactor with a pipe and tanks
3

Equilibria in Industry

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

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A sealed flask holding reactant and product molecules
4

The Equilibrium Constant Kc

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

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A gas mixture beside a bar split into matching shares
5

Kp and Partial Pressures

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

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An S-shaped pH curve with a steep vertical section
6

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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Summary Map

Download, print and revise the key facts of this topic at a glance.

Summary maps

Download, print and revise the key facts of this topic at a glance.

Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Equilibria Map

Cambridge International AS & A Level Chemistry · Topic 7 Equilibria

The whole section on one A4 page: dynamic equilibrium, Kc and Kp, concentration, pressure, temperature and catalysts, and industrial conditions, with worked examples.

Download the map6 key ideas + a data table · A4 portrait · print ready
Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Acids and Titration Curves Map

Cambridge International AS & A Level Chemistry · Topic 7 Equilibria, 7.2

All of 7.2 on one A4 page: strong and weak acids and bases, pH, neutralisation, the four titration curves and choosing an indicator.

Download the map6 key ideas + worked examples · A4 portrait · print ready

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Specification

Every specification point in this topic, in the exam board's own words. Rate how confident you are with each one.

Official Cambridge documentsSyllabus (opens the Cambridge website in a new tab)

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7.1.1

  1. a

    understand what is meant by a reversible reaction

    Revise
  2. b i

    understand what is meant by dynamic equilibrium in terms of the rate of forward and reverse reactions being equal

    Revise
  3. b ii

    understand what is meant by dynamic equilibrium in terms of the concentration of reactants and products remaining constant

    Revise
  4. c

    understand the need for a closed system in order to establish dynamic equilibrium

    Revise

7.1.2

  1. 7.1.2

    define Le Chatelier's principle as: if a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise this change

    Revise

7.1.3

  1. a

    use Le Chatelier's principle to deduce qualitatively (from appropriate information) the effects of changes in temperature on a system at equilibrium

    Revise
  2. b

    use Le Chatelier's principle to deduce qualitatively (from appropriate information) the effects of changes in concentration on a system at equilibrium

    Revise
  3. c

    use Le Chatelier's principle to deduce qualitatively (from appropriate information) the effects of changes in pressure on a system at equilibrium

    Revise
  4. d

    use Le Chatelier's principle to deduce qualitatively (from appropriate information) the effects of the presence of a catalyst on a system at equilibrium

    Revise

7.1.4

  1. 7.1.4

    deduce expressions for equilibrium constants in terms of concentrations, Kc

    Revise

7.1.5

  1. a

    use the term mole fraction

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  2. b

    use the term partial pressure

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7.1.6

  1. 7.1.6

    deduce expressions for equilibrium constants in terms of partial pressures, Kp (use of the relationship between Kp and Kc is not required)

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7.1.7

  1. a

    use the Kc expression to carry out calculations (such calculations will not require the solving of quadratic equations)

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  2. b

    use the Kp expression to carry out calculations (such calculations will not require the solving of quadratic equations)

    Revise

7.1.8

  1. 7.1.8

    calculate the quantities present at equilibrium, given appropriate data

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7.1.9

  1. a

    state whether changes in temperature affect the value of the equilibrium constant for a reaction

    Revise
  2. b

    state whether changes in concentration affect the value of the equilibrium constant for a reaction

    Revise
  3. c

    state whether changes in pressure affect the value of the equilibrium constant for a reaction

    Revise
  4. d

    state whether the presence of a catalyst affects the value of the equilibrium constant for a reaction

    Revise

7.1.10

  1. a

    describe and explain the conditions used in the Haber process, as an example of the importance of an understanding of dynamic equilibrium in the chemical industry and the application of Le Chatelier's principle

    Revise
  2. b

    describe and explain the conditions used in the Contact process, as an example of the importance of an understanding of dynamic equilibrium in the chemical industry and the application of Le Chatelier's principle

    Revise

7.2.1

  1. a

    state the name and formula of the common acid hydrochloric acid, HCl

    Revise
  2. b

    state the name and formula of the common acid sulfuric acid, H2SO4

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  3. c

    state the name and formula of the common acid nitric acid, HNO3

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  4. d

    state the name and formula of the common acid ethanoic acid, CH3COOH

    Revise

7.2.2

  1. a

    state the name and formula of the common alkali sodium hydroxide, NaOH

    Revise
  2. b

    state the name and formula of the common alkali potassium hydroxide, KOH

    Revise
  3. c

    state the name and formula of the common alkali ammonia, NH3

    Revise

7.2.3

  1. 7.2.3

    describe the Brønsted–Lowry theory of acids and bases

    Revise

7.2.4

  1. a

    describe strong acids and strong bases as fully dissociated in aqueous solution

    Revise
  2. b

    describe weak acids and weak bases as partially dissociated in aqueous solution

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7.2.5

  1. 7.2.5

    appreciate that water has pH of 7, acid solutions pH of below 7 and alkaline solutions pH of above 7

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7.2.6

  1. a

    explain qualitatively the differences in behaviour between strong and weak acids including the reaction with a reactive metal

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  2. b

    explain qualitatively the differences in behaviour between strong and weak acids including the difference in pH values by use of a pH meter, universal indicator or conductivity

    Revise

7.2.7

  1. 7.2.7

    understand that neutralisation reactions occur when H+(aq) and OH–(aq) form H2O(l)

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7.2.8

  1. 7.2.8

    understand that salts are formed in neutralisation reactions

    Revise

7.2.9

  1. a

    sketch the pH titration curve of a titration using a strong acid with a strong alkali

    Revise
  2. b

    sketch the pH titration curve of a titration using a strong acid with a weak alkali

    Revise
  3. c

    sketch the pH titration curve of a titration using a weak acid with a strong alkali

    Revise
  4. d

    sketch the pH titration curve of a titration using a weak acid with a weak alkali

    Revise

7.2.10

  1. 7.2.10

    select suitable indicators for acid-alkali titrations, given appropriate data (pKa values will not be used)

    Revise