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.
'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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- explain and use the terms catalyst and catalysis: (a) explain that, in the presence of a catalyst, a reaction has a different mechanism, i.e. one of lower activation energy (b) explain this catalytic effect in terms of the Boltzmann distribution (c) construct and interpret a reaction pathway diagram, for a reaction in the presence and absence of an effective catalyst
- describe qualitatively (and indicate the periodicity in) the variations in atomic radius, ionic radius, melting point and electrical conductivity of the elements
- explain the variation in melting point and electrical conductivity in terms of the structure and bonding of the elements 9.2 Periodicity of chemical properties of the elements in Period 3 Learning outcomes
- describe, and write equations for, the reactions of the elements with oxygen (to give Na2O, MgO, Al 2O3, P4O10, SO2), chlorine (to give NaCl, MgCl 2, Al Cl3, SiCl 4, PCl 5) and water (Na and Mg only)
- state and explain the variation in the oxidation number of the oxides (Na2O, MgO, Al 2O3, P4O10, SO2 and SO3 only) and chlorides (NaCl, MgCl 2, Al Cl3, SiCl4, PCl 5 only) in terms of their outer shell (valence shell) electrons
- describe, and write equations for, the reactions, if any, of the oxides Na2O, MgO, Al 2O3, SiO2, P4O10, SO2 and SO3 with water including the likely pHs of the solutions obtained
- describe, explain, and write equations for, the acid / base behaviour of the oxides Na2O, MgO, Al 2O3, P4O10, SO2 and SO3 and the hydroxides NaOH, Mg(OH)2 and Al (OH)3 including, where relevant, amphoteric behaviour in reactions with acids and bases (sodium hydroxide only)
- describe, explain, and write equations for, the reactions of the chlorides NaCl, MgCl 2, Al Cl 3, SiCl 4, PCl 5 with water including the likely pHs of the solutions obtained
- explain the variations and trends in 9.2.2, 9.2.3, 9.2.4 and 9.2.5 in terms of bonding and electronegativity
- suggest the types of chemical bonding present in the chlorides and oxides from observations of their chemical and physical properties 9.3 Chemical periodicity of other elements Learning outcomes
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.
Tips to avoid common mistakes in Homogeneous and heterogeneous catalysts
- › Link the overall order and individual reactant orders to the mechanism; SN2 is suggested by first-order kinetics for both reactants.
- › A full description of heterogeneous catalysis must include adsorption of reactants, bond weakening, reaction, and finally desorption of products.
- › Use a sharp pencil or fine pen to plot points as small crosses (x) for better precision.
- › When explaining rate using collision theory, you must use the phrase 'frequency of collisions' rather than just 'number of collisions'.
- › Always check the units specified in the answer options and convert your time values accordingly before final calculations.
Define '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.
Explain how a catalyst affects the Boltzmann distribution and reaction rate.
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.
Draw a reaction pathway diagram showing the effect of a catalyst on the activation energy of a reaction.
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).
Give an example of a homogeneous catalyst and a heterogeneous catalyst, including the reaction they catalyze.
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.
What is the oxidation number of sulfur in SO3?
The oxidation number of oxygen is usually -2. Since SO3 is neutral, the oxidation number of sulfur must be +6 to balance the three oxygen atoms (3 x -2 = -6).
Write the equation for the reaction of phosphorus(V) oxide (P4O10) with water.
P4O10(s) + 6H2O(l) → 4H3PO4(aq). This reaction produces phosphoric acid.
Describe the acid-base behavior of aluminium oxide (Al2O3).
Aluminium oxide is amphoteric, meaning it can react with both acids and bases. With acids, it acts as a base: Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l). With strong bases, it acts as an acid: Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq).
Write the equation for the reaction of silicon tetrachloride (SiCl4) with water, including the state symbols.
SiCl4(l) + 2H2O(l) → SiO2(s) + 4HCl(g). This reaction produces silicon dioxide (a solid) and hydrogen chloride gas.
Explain why aluminium chloride (AlCl3) fumes in moist air.
Aluminium chloride is hydrolyzed by water vapor in the air, producing hydrogen chloride (HCl) gas. The HCl gas appears as white fumes as it reacts with the moisture in the air.
Explain the trend in the pH of the solutions formed when sodium oxide, magnesium oxide, and aluminium oxide react with water.
Na2O reacts to form strongly alkaline NaOH (high pH). MgO reacts to form weakly alkaline Mg(OH)2 (lower pH). Al2O3 is nearly insoluble and produces a neutral solution (pH close to 7). The pH decreases due to decreasing ionic character and increasing covalent character of the oxide.
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Homogeneous and heterogeneous catalysts sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Homogeneous and heterogeneous catalysts deck
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