A catalyst speeds a reaction up without being used up
Definition
Catalyst
A substance that increases reaction rate without being used up by providing an alternative pathway with lower activation energy.
- A catalyst increases the rate of a reaction.
- It is not used up during the reaction, so its mass is the same at the end as at the start.
- Because it is not used up, a catalyst is not shown in the overall equation for the reaction.
- A small amount of catalyst can speed up the reaction of a large amount of reactant.
- Different reactions need different catalysts, so a catalyst is specific to the reaction it speeds up.
A catalyst works by lowering the activation energy
Definition
Activation energy
The minimum energy that reacting particles must have for a collision to result in a reaction.
Definition
Reaction profile
A diagram showing how the energy changes as reactants become products during a chemical reaction.
- A catalyst provides a different reaction pathway with a lower activation energy.
- More of the colliding particles now have enough energy to react.
- A larger fraction of collisions are therefore successful, so the reaction goes faster.
- On a reaction profile, the catalysed pathway has a lower peak than the uncatalysed one.
- The reactant and product energy levels are unchanged, so the overall energy change stays the same.
Key Idea
- A catalyst lowers the activation energy but does not change the overall energy change.
- It speeds up the reaction without altering the amount of product formed.
Enzymes are the catalysts found in living things
- Enzymes are biological catalysts that speed up the reactions in living organisms.
- They are also used in industry, for example in making foods and washing powders.
Common Mistake
- Do not write the catalyst as a reactant or a product in the equation.
- A catalyst changes only the rate, not the amount of product you can make.
Exam technique
- Say a catalyst gives a different pathway with a lower activation energy, not that it gives particles more energy.
- If asked about a reaction profile, show a lower peak but the same reactant and product levels.
- State that the catalyst is not used up, so it is left out of the overall equation.
Self review
- What is a catalyst?
- How does a catalyst increase the rate of a reaction?
- Why is a catalyst not included in the overall equation?
- What does a catalyst do to the activation energy and to the overall energy change?
- What name is given to biological catalysts?
