8.3 AS Level

Homogeneous and heterogeneous catalysts

Cambridge A-Level Chemistry (9701) · Unit 8: Reaction kinetics · 10 flashcards

Homogeneous and heterogeneous catalysts is topic 8.3 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 8 — Reaction kinetics , alongside Rate of reaction and Effect of temperature on reaction rates and the concept of activation energy. In one line: A catalyst is a substance that speeds up a chemical reaction without being consumed in the reaction itself. Catalysis is the process of speeding up a reaction with a catalyst. Catalysts provide an alternate reaction pathway with a lower activation energy.

Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).

The deck below contains 10 flashcards — 2 definitions, 7 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.

Some cards in this deck are on other syllabus topics: 9.2 Periodicity of chemical properties of the elements in Period 3. Each card is labelled with its topic.

Key definition

'catalyst' and 'catalysis'

A catalyst is a substance that speeds up a chemical reaction without being consumed in the reaction itself. Catalysis is the process of speeding up a reaction with a catalyst. Catalysts provide an alternate reaction pathway with a lower activation energy.

What the Cambridge 9701 syllabus says

2025-2027 syllabus, in our words · AS Level

What the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).

Explain what catalyst and catalysis mean and use both terms, including: (a) explaining that when a catalyst is present a reaction follows a different mechanism, one with a lower activation energy; (b) explaining this effect using the Boltzmann distribution; (c) constructing and interpreting reaction pathway diagrams for a reaction with and without an effective catalyst.

Also in this deck: 9.2 Periodicity of chemical properties of the elements in Period 3

Describe the reactions of the Period 3 elements with oxygen (forming Na₂O, MgO, Al₂O₃, P₄O₁₀, SO₂), with chlorine (forming NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅) and with water (only Na and Mg), and write equations for them.
State and explain how the oxidation number of the Period 3 element varies in the oxides (only Na₂O, MgO, Al₂O₃, P₄O₁₀, SO₂ and SO₃) and in the chlorides (only NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅), with reference to their valence (outer-shell) electrons.
Describe any reaction with water of the oxides Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ and SO₃, writing equations, and give the likely pH of each solution formed.
Describe and explain, with equations, how the oxides Na₂O, MgO, Al₂O₃, P₄O₁₀, SO₂ and SO₃ and the hydroxides of sodium, magnesium and aluminium (NaOH, Mg(OH)₂, Al(OH)₃) behave as acids or bases, including any amphoteric behaviour in reactions with acids and with bases (the only base required is sodium hydroxide).
Describe and explain how the chlorides NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅ react with water, giving equations, and state the likely pH of each solution formed.
Explain, using bonding and electronegativity, the trends and variations in oxidation number and in the reactions of the Period 3 oxides, hydroxides and chlorides (9.2.2 to 9.2.5).
Suggest what types of chemical bonding are present in the oxides and chlorides, based on observed physical and chemical properties.

Cambridge syllabus keywords to use in your answers

These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.

catalyst catalysis heterogeneous catalysts reaction mechanism reaction pathway diagram

Tips to avoid common mistakes in these topics

› Use P4O10 for the molecular formula of phosphorus(V) oxide as specified in the Cambridge syllabus; P2O5 is only the empirical formula.
› Remember that AlCl3, SiCl4 and PCl5 all hydrolyse in water to give acidic solutions; SiCl4 and PCl5 give more strongly acidic solutions than AlCl3.
› Recall that Period 3 chlorides from Si to P are simple molecular liquids or solids with low boiling points and covalent bonding.

Definition Flip

Topic 8.3 · Homogeneous and heterogeneous catalysts

Define 'catalyst' and 'catalysis'.

Answer Flip

Key Concept Flip

Topic 8.3 · Homogeneous and heterogeneous catalysts

Explain how a catalyst affects the Boltzmann distribution and reaction rate.

Answer Flip

A catalyst lowers the activation energy (Ea) of a reaction. This means a larger proportion of molecules in the Boltzmann distribution will have sufficient energy to overcome the activation barrier, leading to a faster reaction rate.

Key Concept Flip

Topic 8.3 · Homogeneous and heterogeneous catalysts

Draw a reaction pathway diagram showing the effect of a catalyst on the activation energy of a reaction.

Answer Flip

The diagram should show two curves: one representing the uncatalyzed reaction (higher Ea) and another representing the catalyzed reaction (lower Ea). Both curves should start and end at the same energy levels (reactants and products).

Definition Flip

Topic 8.3 · Homogeneous and heterogeneous catalysts

Give an example of a homogeneous catalyst and a heterogeneous catalyst, including the reaction they catalyze.

Answer Flip

Homogeneous: Acid (H+) in ester hydrolysis where the catalyst and reactants are in the same phase. Heterogeneous: Iron in the Haber process (N2 + 3H2 -> 2NH3) where iron is a solid and the reactants are gases.

Calculation Flip