AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Catalysts: Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts. Enzymes act as catalysts in biological systems.
A catalyst is a substance added to a reaction that changes its rate without being consumed.
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A catalyst is a substance added to a reaction that changes its rate without being consumed. It is not a reactant used up in the reaction, so it can be recovered chemically unchanged at the end, even though it takes part in an intermediate step. For example, manganese(IV) oxide speeds up the decomposition of hydrogen peroxide to water and oxygen; filtering it off shows the same mass remains. Catalysts are specific: the catalyst that works for one reaction may not work for another, so different reactions need different catalysts. In living organisms, enzymes are biological catalysts that speed up metabolic reactions such as digestion and respiration at moderate temperatures.
What earns the marks
- A catalyst changes the rate of a chemical reaction, usually increasing it, and is not used up during the reaction.
- The catalyst is not a reactant consumed in the overall reaction; it can be recovered chemically unchanged at the end.
- Catalysts are specific, so different reactions need different catalysts.
- Enzymes are catalysts in biological systems and speed up reactions such as digestion.
- A named example, such as manganese(IV) oxide catalysing hydrogen peroxide decomposition, can support the explanation.
Common mistakes and how to put them right
Saying the catalyst is used up.
Put it right by stating it is recovered chemically unchanged at the end, even though it may take part in an intermediate step.
Thinking one catalyst works for every reaction.
Put it right by stating catalysts are specific, so different reactions need different catalysts.
Confusing enzymes with reactants or with energy.
Put it right by describing enzymes as biological catalysts that change rate without being consumed.
Exam tips
- State clearly that the catalyst is not used up and can be recovered chemically unchanged.
- Use a named example, such as manganese(IV) oxide for hydrogen peroxide, to show the catalyst is not consumed.
- When asked about enzymes, link them to biological systems and to changing the rate of metabolic reactions.
Try the lesson
The start of the Catalysts: Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts. Enzymes act as catalysts in biological systems. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes what a catalyst does in a chemical reaction?
Listen
A catalyst is a substance added to a reaction that changes its rate without being consumed. It is not a reactant used up in the reaction, so it can be recovered chemically unchanged at the end, even though it takes part in an intermediate step. For example, manganese(IV) oxide speeds up the decomposition of hydrogen peroxide to water and oxygen; filtering it off shows the same mass remains. Catalysts are specific: the catalyst that works for one reaction may not work for another, so different reactions need different catalysts. In living organisms, enzymes are biological catalysts that speed up metabolic reactions such as digestion and respiration at moderate temperatures. A strong answer earns marks for these points. A catalyst changes the rate of a chemical reaction, usually increasing it, and is not used up during the reaction. The catalyst is not a reactant consumed in the overall reaction; it can be recovered chemically unchanged at the end. Catalysts are specific, so different reactions need different catalysts. Enzymes are catalysts in biological systems and speed up reactions such as digestion. A common slip: Saying the catalyst is used up. Instead, stating it is recovered chemically unchanged at the end, even though it may take part in an intermediate step. Exam tip: State clearly that the catalyst is not used up and can be recovered chemically unchanged.
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Key words
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Catalyst specificity
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Biological catalysts
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See every definition
- Catalyst specificity
- Catalysts are specific, so different reactions need different catalysts.
- Biological catalysts
- Enzymes are catalysts in biological systems and speed up reactions such as digestion.
Label the diagram
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1Box 1
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3Box 3
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