AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
A catalyst is a substance that increases the rate of a chemical reaction…
Full specification wording
Catalysts: A catalyst is a substance that increases the rate of a chemical reaction without being changed in chemical composition or amount.
A catalyst speeds a reaction up and is present in the same chemical form and the same amount at the end as at the start.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A catalyst speeds a reaction up and is present in the same chemical form and the same amount at the end as at the start. That does not mean it is a spectator: it usually reacts, forming an intermediate, and is then regenerated in a later step, which is why the amount is unchanged rather than merely unaffected. Heterogeneous catalysts are in a different phase from the reactants, typically a solid surface with gases passing over it. Homogeneous catalysts share the phase. An autocatalyst is a product of the reaction that then catalyses it, so the rate rises as the reaction proceeds, as with the manganese(II) ions produced when manganate(VII) oxidises ethanedioate ions. A catalyst changes the rate only, never ∆H or the equilibrium yield.
What earns the marks
- Stating that a catalyst increases the rate of reaction.
- Adding that it is chemically unchanged, and unchanged in amount, at the end.
- Defining heterogeneous as being in a different phase or state from the reactants.
- Defining an autocatalyst as a product of the reaction which catalyses that reaction.
Common mistakes and how to put them right
Saying a catalyst takes no part in the reaction, when it takes part and is regenerated.
Defining an autocatalyst as a reactant that catalyses the reaction, when it must be a product.
Claiming a catalyst increases the yield of product rather than the rate at which the yield is reached.
Saying a catalyst is not used up but omitting that it is chemically unchanged in composition.
Describing a heterogeneous catalyst as being in a different phase from the products only.
Exam tips
- Give both halves of the definition, rate increased and chemically unchanged in composition and amount, for a two mark answer.
- For heterogeneous, say different phase or state from the reactants; different substance is not the point.
- If a reaction speeds up partway through with no change in conditions, suspect autocatalysis.
Try the lesson
The start of the Catalysts: A catalyst is a substance that increases the rate of a chemical reaction without being changed in chemical composition or amount. lesson, made from checked teaching notes for this specification point.
Have a go first
A student investigates the decomposition of hydrogen peroxide. They add 0.50 g of solid manganese(IV) oxide to the solution. The reaction fizzes rapidly. After the reaction is complete, the solid is filtered, washed, dried, and weighed. Which statement correctly describes the expected outcome and the role of the manganese(IV) oxide?
Listen
A catalyst speeds a reaction up and is present in the same chemical form and the same amount at the end as at the start. That does not mean it is a spectator: it usually reacts, forming an intermediate, and is then regenerated in a later step, which is why the amount is unchanged rather than merely unaffected. Heterogeneous catalysts are in a different phase from the reactants, typically a solid surface with gases passing over it. Homogeneous catalysts share the phase. An autocatalyst is a product of the reaction that then catalyses it, so the rate rises as the reaction proceeds, as with the manganese(II) ions produced when manganate(VII) oxidises ethanedioate ions. A catalyst changes the rate only, never ∆H or the equilibrium yield. A strong answer earns marks for these points. stating that a catalyst increases the rate of reaction. adding that it is chemically unchanged, and unchanged in amount, at the end. defining heterogeneous as being in a different phase or state from the reactants. defining an autocatalyst as a product of the reaction which catalyses that reaction. Exam tip: Give both halves of the definition, rate increased and chemically unchanged in composition and amount, for a two mark answer.
Key words
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catalyst
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heterogeneous and homogeneous
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autocatalysis
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regeneration of the catalyst
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See every definition
- catalyst
- A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount.
- heterogeneous and homogeneous
- Heterogeneous catalysts are in a different phase from the reactants, typically a solid surface with gases passing over it, while homogeneous catalysts share the phase.
- autocatalysis
- An autocatalyst is a product of the reaction that then catalyses it, so the rate rises as the reaction proceeds.
- regeneration of the catalyst
- It usually reacts, forming an intermediate, and is then regenerated in a later step, which is why the amount is unchanged.
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