Catalysts
Revise Catalysts for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A catalyst speeds up a reaction by providing a pathway with a lower activation energy, and is not used up in the reaction.
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Topic overview
A catalyst speeds up the rate of a reaction without being used up itself, and without changing the products of the reaction.
It works by providing a different reaction pathway with a lower activation energy. Because the activation energy is lower, a greater proportion of collisions have enough energy to be successful, so the reaction proceeds faster.
Because catalysts are not used up, only a small amount is needed and it can be used repeatedly. Different reactions need different catalysts, and enzymes are the biological catalysts that control reactions in living cells.
Revision notes
How catalysts work
A catalyst provides a different reaction pathway with a lower activation energy.
A greater proportion of the colliding particles therefore have enough energy to react, so more collisions are successful and the rate increases.
Key properties
A catalyst is not used up during the reaction and is chemically unchanged at the end.
Only a small amount is needed, and it can be used over and over. It does not change the products of the reaction, only the speed at which they form.
On a reaction profile
A catalyst lowers the height of the activation energy hump.
The positions of the reactants and products are unchanged, so the overall energy change is the same. Enzymes are biological catalysts.
Key points
- A catalyst speeds up a reaction.
- It is not used up in the reaction.
- It provides a pathway with lower activation energy.
- More collisions are therefore successful.
- It does not change the products.
- Enzymes are biological catalysts.
Worked examples
Example 1
Explain how a catalyst increases the rate of a reaction. [2 marks]
Model answer
It provides a different reaction pathway with a lower activation energy (1 mark), so a greater proportion of collisions have enough energy to be successful (1 mark).
Example 2
Explain why only a small amount of catalyst is needed. [2 marks]
Model answer
The catalyst is not used up during the reaction (1 mark), so the same catalyst can be used repeatedly (1 mark).
Example 3
State the effect of a catalyst on the overall energy change of a reaction. [2 marks]
Model answer
The overall energy change is unaffected (1 mark), because the catalyst only lowers the activation energy and does not change the energies of the reactants or products (1 mark).
Common mistakes
Saying a catalyst is used up.
It is chemically unchanged at the end of the reaction.
Saying a catalyst changes the products.
It changes only the rate, not what is formed.
Saying it increases the activation energy.
It lowers it, which is why more collisions succeed.
Saying it changes the overall energy change.
Only the activation energy changes.
Exam tips
- Say lower activation energy and different pathway.
- State that the catalyst is not used up.
- Note that products and overall energy change are unaffected.
- Mention enzymes as biological catalysts if relevant.
Key terms
- Catalyst
- A substance speeding up a reaction without being used up.
- Activation energy
- The minimum energy needed for a successful collision.
- Reaction pathway
- The route a reaction takes from reactants to products.
- Enzyme
- A biological catalyst.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.