Ch 2 · Homogeneous and heterogeneous catalysts · AS & A Level Chemistry 9701
Topic 2 of 2 in Reaction kinetics — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
At AS Level you learned that a catalyst speeds up a reaction by providing a pathway of lower activation energy without being used up overall. At A2, that single idea is developed into two distinct mechanisms depending on whether the catalyst shares a phase with the reactants: homogeneous catalysis, which proceeds through a genuine chemical intermediate, and heterogeneous catalysis, which proceeds at a surface.
Key ideas — Homogeneous and heterogeneous catalysis
A homogeneous catalyst is in the same phase as the reactants, most often all species dissolved in aqueous solution or all present as gases. A heterogeneous catalyst is in a different phase, almost always a solid catalysing reactants that are gases or in solution. Both lower the overall activation energy by opening an alternative reaction pathway, but they do so by different means: a homogeneous catalyst forms an intermediate compound with a reactant, while a heterogeneous catalyst provides a surface on which reactant molecules are held and their bonds weakened.
🧑🏫 Teacher's explanation
Imagine you are trying to assemble a complex LEGO set. A homogeneous catalyst is like having a specialized tool that you hold in your hand while you work; you use it to snap two pieces together, and then you are ready to use it again for the next piece. Because the tool is in your hand, it is part of the same 'environment' as you are.
📖 Definitions
🔍 Worked example — Easy
Identify if the following reaction involves homogeneous or heterogeneous catalysis: 2H_2(g) + O_2(g) xrightarrowPt(s) 2H_2O(g)
🔍 Worked example — Standard
In the reaction between S_2O_8^2-(aq) and I^-(aq), Fe^2+(aq) is used as a catalyst. Explain the role of Fe^2+ in terms of reaction intermediates.
🔍 Worked example — Exam-style
A heterogeneous catalyst works by providing a surface for adsorption. Describe the three-step mechanism for a reactant molecule on a solid surface.
⚠️ Common errors
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🎯 Exam question
(a) State the type of catalysis used. (b) Describe how the iron catalyst increases the rate of reaction in terms of adsorption and bond weakening. [3 marks]
Key ideas — Mode of action of a homogeneous catalyst
A homogeneous catalyst is used up in one step of the mechanism, forming an intermediate species, and is then reformed in a later step, so it does not appear in the overall equation despite being essential to the pathway. This replaces one high-activation-energy step with two lower-activation-energy steps. A classic example is mathrmFe^2+ (or mathrmFe^3+) catalysing the reaction between iodide and peroxodisulfate(VI) ions, which is otherwise slow because both reacting ions are negatively charged and repel each other electrostatically:
🧑🏫 Teacher's explanation
Imagine you are trying to move a heavy sofa through a narrow doorway. It is difficult and slow because the sofa is too wide for the opening (this is like the high activation energy of the direct reaction). A homogeneous catalyst acts like a professional mover who breaks the sofa down into smaller cushions, carries them through one by one, and then puts them back together on the other side. The mover is 'used' to move the pieces, but they end up exactly as they started once the job is done.
📖 Definitions
🔍 Worked example — Easy
In the reaction between S_2O_8^2-(aq) and I^-(aq), Fe^2+(aq) is used as a catalyst. If the first step is S_2O_8^2-(aq) + 2Fe^2+(aq) ightarrow 2SO_4^2-(aq) + 2Fe^3+(aq), what is the second step required to regenerate the catalyst?
🔍 Worked example — Standard
Calculate the total change in the concentration of Fe^2+ ions if 0.050 mol of S_2O_8^2- reacts with I^- in the presence of excess Fe^2+ catalyst.
🔍 Worked example — Exam-style
A reaction has an activation energy E_a of 100 kJ/mol. A homogeneous catalyst is added, and the new activation energy is 75 kJ/mol. Calculate the percentage decrease in activation energy.
⚠️ Common errors
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🎯 Exam question
(a) Write a possible two-step mechanism for this reaction involving Fe^2+ and Fe^3+. [2] (b) Explain why the overall rate increases when Fe^2+ is added. [2] [4 marks]
Key ideas — Mode of action of a heterogeneous catalyst
A heterogeneous catalyst acts through a three-stage surface mechanism. Reactant molecules are first adsorbed onto active sites on the catalyst surface, forming weak bonds (often involving the catalyst's partially filled d orbitals). This adsorption weakens the bonds within the reactant molecules, sometimes breaking them, and holds the reactant molecules close together in a favourable orientation. The weakened, adjacent species then react with a much lower activation energy than in the gas phase, and the product molecules desorb, leaving the active sites free for further reactant molecules.
🧑🏫 Teacher's explanation
Imagine a busy airport check-in desk. If people (reactants) just wander around the terminal, they rarely meet or interact efficiently. A heterogeneous catalyst is like the check-in counter: it provides a specific, physical surface where travelers are forced to stop (adsorption). Once they are standing there, they are positioned perfectly to talk to one another (favourable orientation), and the stress of the interaction makes it much easier for them to exchange information or documents (bond weakening/lowering activation energy). Once the transaction is done, they walk away (desorption), leaving the desk free for the next person.
📖 Definitions
🔍 Worked example — Easy
Identify the three main stages of heterogeneous catalysis in order.
🔍 Worked example — Standard
In the Haber process, N_2(g) + 3H_2(g) leftharpoons 2NH_3(g), an iron catalyst is used. Explain why the reaction rate increases on the iron surface.
🔍 Worked example — Exam-style
A student claims that if the adsorption of products is too strong, the rate of reaction will decrease. Explain why using the concept of active sites.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe the mode of action of this heterogeneous catalyst in terms of adsorption, bond weakening, and desorption. [3] (b) Explain why the catalyst is described as 'heterogeneous'. [1] [4 marks]
Key ideas — Industrial and environmental examples
In the Haber process, N_2(g) + 3H_2(g) leftharpoons 2NH_3(g), finely divided iron adsorbs both N_2 and H_2; adsorption weakens the very strong N≡ N triple bond and the H-H bond, allowing the atoms to combine as ammonia, which then desorbs. In a car's catalytic converter, exhaust gases pass over a ceramic honeycomb coated with platinum-group metals (platinum, palladium and rhodium), which adsorb carbon monoxide and oxides of nitrogen, allowing the toxic gases to react at the surface, for example 2CO(g) + 2NO(g) arrow 2CO_2(g) + N_2(g), before the harmless products desorb. Homogeneous catalysis also occurs in the atmosphere: oxides of nitrogen catalyse the breakdown of ozone in the stratosphere, since gaseous NO and gaseous O_3/O are in the same phase:
🧑🏫 Teacher's explanation
Think of a catalyst like a professional matchmaker. In a heterogeneous reaction, the matchmaker provides a physical meeting room (the surface) where people (reactants) can sit down, hold hands, and bond more easily. In a homogeneous reaction, the matchmaker actually joins the group, dances with them to facilitate the connection, and then steps back out, unchanged, to do it all over again.
📖 Definitions
🔍 Worked example — Easy
In the Haber process, N_2(g) + 3H_2(g) leftharpoons 2NH_3(g), iron acts as a heterogeneous catalyst. If the surface area of the iron is doubled, how does this affect the rate of reaction?
🔍 Worked example — Standard
A catalytic converter uses platinum-group metals to convert 2CO_(g) + 2NO_(g) arrow 2CO_2(g) + N_2(g). Describe the four-step mechanism of the heterogeneous catalyst in this reaction.
🔍 Worked example — Exam-style
In the atmospheric destruction of ozone, NO_(g) acts as a homogeneous catalyst. The steps are:
⚠️ Common errors
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🎯 Exam question
Step 1: S_2O_8^2(aq) + Fe^2(aq) arrow SO_4^2(aq) + SO_4^2(aq) + Fe^3(aq) Step 2: Fe^3(aq) + I_(aq)^- arrow Fe^2(aq) + I_(aq) (a) State whether the Fe^2(aq) ion acts as a homogeneous or heterogeneous catalyst. [1] (b) Identify the intermediate produced in this reaction. [1] (c) Write the overall balanced equation for the reaction. [1] [3 marks]
Worked example — Model answer
Question. (a) Explain why the reaction between I^- and mathrmS_2O_8^2- ions has a high activation energy in the absence of a catalyst. [1] (b) Using equations, explain how mathrmFe^2+ ions catalyse this reaction, and state why this is an example of homogeneous catalysis. [4] (c) State one difference between how this catalyst acts and how the iron catalyst in the Haber process acts. [1]
🔍 Worked example — Easy
Identify whether the following is homogeneous or heterogeneous: A solid platinum surface used in a car's catalytic converter to react gaseous CO and O_2.
🔍 Worked example — Standard
In the reaction between I^- and S_2 extO_8^2-, the Fe^2+ ion is used in the first step and regenerated in the second. Write the two intermediate steps as chemical equations.
🔍 Worked example — Exam-style
Explain the role of the surface in heterogeneous catalysis (e.g., the Haber process) using the terms: adsorption, bond weakening, and desorption.
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Practice
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