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.
Some cards in this deck are on other syllabus topics: 28.1 General physical and chemical properties of the first row of transition elements, titanium to copper and 28.2 General characteristic chemical properties of the first set of transition elements, titanium to copper. Each card is labelled with its topic.
A homogeneous catalyst and provide an example
A homogeneous catalyst is in the same phase as the reactants.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat 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 that a catalyst can be either homogeneous or heterogeneous.
- Describe how a heterogeneous catalyst works, covering adsorption of reactants, weakening of bonds and desorption of products, with examples such as: (a) iron in the Haber process; (b) palladium, platinum and rhodium catalysing the removal of nitrogen oxides from car engine exhaust gases.
- Describe how a homogeneous catalyst works by taking part in one step and being regenerated in a later step, with examples such as: (a) nitrogen oxides in the atmosphere in the oxidation of atmospheric sulfur dioxide; (b) Fe²⁺ or Fe³⁺ in the reaction between I⁻ and S₂O₈²⁻.
Also in this deck: 28.1 General physical and chemical properties of the first row of transition elements, titanium to copper
- Define a transition element: an element of the d block that forms at least one stable ion with a partly filled set of d orbitals.
- Sketch the shapes of the 3dxy and 3dz² orbitals.
- Understand that transition elements: (a) show variable oxidation states; (b) act as catalysts; (c) form complex ions; (d) form coloured compounds.
- Explain the variable oxidation states of transition elements using the similar energies of the 3d and 4s sub-shells.
- Explain the catalytic behaviour of transition elements in terms of their having more than one stable oxidation state and having empty d orbitals at accessible energies, which can accept dative bonds from ligands.
- Use their empty, energetically accessible d orbitals to explain why transition elements are able to form complex ions.
Also in this deck: 28.2 General characteristic chemical properties of the first set of transition elements, titanium to copper
- Describe and explain the formation of complexes when transition elements react with ligands, covering the complexes of cobalt(II) and copper(II) ions with H₂O, NH₃, OH⁻ and Cl⁻.
- Define a ligand as a species that donates an electron lone pair to the metal atom or ion at the centre, making a dative covalent bond.
- Understand and use these terms, and know the named ligands for each: (a) monodentate ligand: H₂O, NH₃, Cl⁻ and CN⁻; (b) bidentate ligand: 1,2-diaminoethane (en, H₂NCH₂CH₂NH₂) and the ethanedioate ion, C₂O₄²⁻; (c) polydentate ligand: EDTA⁴⁻.
- Define a complex as a molecule or ion in which one or more ligands surround a central metal atom or ion.
- For linear, square planar, tetrahedral and octahedral transition element complexes, describe their geometry, meaning the shape and bond angles.
- (a) State what coordination number means. (b) Predict a complex ion's formula and charge from the metal ion, the metal's charge or oxidation state, the ligand, and the coordination number or geometry.
- Explain qualitatively that ligands can be exchanged, including in the complexes that copper(II) and cobalt(II) ions form with water, ammonia, hydroxide ions and chloride ions.
- Use E⦵ values to predict whether a redox reaction is feasible when it involves a transition element or its ions.
- Describe these reactions and do calculations on them, given suitable data: (a) MnO₄⁻ with C₂O₄²⁻ in acid solution; (b) MnO₄⁻ with Fe²⁺ in acid solution; (c) Cu²⁺ with I⁻.
- Given suitable data, carry out calculations on other redox systems.
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 these topics
- › A full description of heterogeneous catalysis must include adsorption of reactants, bond weakening, reaction, and finally desorption of products.
- › Remember that electrons are removed from the 4s orbital before the 3d orbital when forming transition metal cations.
- › For vanadium (Group 5) the maximum oxidation state is +5, matching the group number; this pattern holds from Sc to Mn.
- › Define transition elements as d-block elements that form one or more stable ions with incomplete d-subshells.
- › Define a ligand as a species that uses a lone pair of electrons to form a dative covalent bond to a metal ion.
Topic 26.2 · Homogeneous and heterogeneous catalysts
Define a homogeneous catalyst and provide an example.
A homogeneous catalyst is in the same phase as the reactants.
Topic 26.2 · Homogeneous and heterogeneous catalysts
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.
Topic 28.1 · General physical and chemical properties of the first row of transition elements, titanium to copper
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.
Topic 28.2 · General characteristic chemical properties of the first set of transition elements, titanium to copper
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.
Topic 28.2 · General characteristic chemical properties of the first set of transition elements, titanium to copper
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°.
Topic 28.1 · General physical and chemical properties of the first row of transition elements, titanium to copper
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.
Topic 28.2 · General characteristic chemical properties of the first set of transition elements, titanium to copper
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.
Topic 28.2 · General characteristic chemical properties of the first set of transition elements, titanium to copper
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⁺.
Topic 28.1 · General physical and chemical properties of the first row of transition elements, titanium to copper
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.
Topic 28.2 · General characteristic chemical properties of the first set of transition elements, titanium to copper
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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