Homogeneous and heterogeneous catalysts
Cambridge A-Level Chemistry (9701) · Unit 26: Reaction kinetics · 10 flashcards
Homogeneous and heterogeneous catalysts is topic 26.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 26 — Reaction kinetics , alongside Simple rate equations, orders of reaction and rate constants. In one line: A homogeneous catalyst is in the same phase as the reactants.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 10 flashcards — 4 definitions and 6 key concepts — covering the precise wording mark schemes reward. Use the 4 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.
A homogeneous catalyst and provide an example
A homogeneous catalyst is in the same phase as the reactants.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 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 that catalysts can be homogeneous or heterogeneous
- describe the mode of action of a heterogeneous catalyst to include adsorption of reactants, bond weakening and desorption of products, for example: (a) iron in the Haber process (b) palladium, platinum and rhodium in the catalytic removal of oxides of nitrogen from the exhaust gases of car engines
- describe the mode of action of a homogeneous catalyst by being used in one step and reformed in a later step, for example: (a) atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide (b) Fe2+ or Fe3+ in the I–/S2O8 2– reaction
- describe and explain qualitatively the trend in the thermal stability of the nitrates and carbonates including the effect of ionic radius on the polarisation of the large anion
- describe and explain qualitatively the variation in solubility and of enthalpy change of solution, ΔH ⦵ sol, of the hydroxides and sulfates in terms of relative magnitudes of the enthalpy change of hydration and the lattice energy
- define a transition element as a d-block element which forms one or more stable ions with incomplete d orbitals
- sketch the shape of a 3dxy orbital and 3dz² orbital
- understand that transition elements have the following properties: (a) they have variable oxidation states (b) they behave as catalysts (c) they form complex ions (d) they form coloured compounds
- explain why transition elements have variable oxidation states in terms of the similarity in energy of the 3d and the 4s sub-shells
- explain why transition elements behave as catalysts in terms of having more than one stable oxidation state, and vacant d orbitals that are energetically accessible and can form dative bonds with ligands
- explain why transition elements form complex ions in terms of vacant d orbitals that are energetically accessible
- describe and explain the reactions of transition elements with ligands to form complexes, including the complexes of copper(II) and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions
- define the term ligand as a species that contains a lone pair of electrons that forms a dative covalent bond to a central metal atom / ion
- understand and use the terms: (a) monodentate ligand including as examples H2O, NH3, Cl – and CN– (b) bidentate ligand including as examples 1,2-diaminoethane, en, H2NCH2CH2NH2 and the ethanedioate ion, C2O4 2– (c) polydentate ligand including as an example EDTA4–
- define the term complex as a molecule or ion formed by a central metal atom / ion surrounded by one or more ligands
- describe the geometry (shape and bond angles) of transition element complexes which are linear, square planar, tetrahedral or octahedral
- explain qualitatively that ligand exchange can occur, including the complexes of copper(II) ions and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions
- predict, using E ⦵ values, the feasibility of redox reactions involving transition elements and their ions
- describe the reactions of, and perform calculations involving: (a) MnO4 – / C2O4 2– in acid solution given suitable data (b) MnO4 – / Fe2+ in acid solution given suitable data (c) Cu2+ / I– given suitable data
- perform calculations involving other redox systems given suitable data
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 a homogeneous catalyst and provide an example.
A homogeneous catalyst is in the same phase as the reactants.
Explain the mode of action of a heterogeneous catalyst, including the key steps.
Heterogeneous catalysts work via adsorption of reactants onto the catalyst surface, weakening of reactant bonds, reaction on the surface, and desorption of products. An example is iron in the Haber process.
Why do transition elements exhibit variable oxidation states?
Transition elements have variable oxidation states due to the small energy difference between the 3d and 4s sub-shells. This allows for the loss of varying numbers of electrons from these sub-shells.
Define a ligand, and give two examples of monodentate ligands.
A ligand is a species that contains a lone pair of electrons that forms a dative covalent bond to a central metal atom/ion. Examples of monodentate ligands include H₂O and NH₃.
Describe the geometry of a [Cu(H₂O)₆]²⁺ complex ion.
The [Cu(H₂O)₆]²⁺ complex ion has an octahedral geometry. This means the central copper(II) ion is surrounded by six water ligands, with bond angles of 90°.
Explain why transition elements can act as catalysts.
Transition elements act as catalysts because they have more than one stable oxidation state and vacant d orbitals. These orbitals can form dative bonds with reactants, facilitating bond formation and/or weakening.
What is a complex ion, and how is it formed?
A complex ion is formed by a central metal atom/ion surrounded by one or more ligands. These ligands attach via dative covalent bonds utilizing the metal ion's vacant d orbitals.
Describe the colour change when excess ammonia is added to a solution of copper(II) ions.
Initially, adding ammonia to copper(II) ions forms a pale blue precipitate of copper(II) hydroxide. Adding excess ammonia dissolves the precipitate, forming a deep blue solution of the tetraamminecopper(II) complex, [Cu(NH₃)₄(H₂O)₂]²⁺.
Define a transition element.
A transition element is defined as a d-block element which forms one or more stable ions with incomplete d orbitals.
What is ligand exchange and give an example with copper(II) ions.
Ligand exchange is the replacement of one ligand by another in a complex.
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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.
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