10.05.3 Catalysts

Syllabus

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.
Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
A reaction profile for a catalysed reaction can be drawn as right:

Students should be able to

identify catalysts in reactions from their effect on the rate of reaction and because they are not included in the chemical equation for the reaction.
explain catalytic action in terms of activation energy.

Students do not need to name catalysts other than those specified in the subject content.

What does this mean?

What is a catalyst?

If an exam question asks "What is a catalyst? [2]"

Then a perfectly good two mark answer is "a substance that speeds up a reaction [1] without being used up [1]."

But is there are more marks available then you'll have to add to the answer.

For instance, "a substance that speeds up a reaction [1] without being used up [1] by providing an alternative way to react [1] with a lower Activation Energy [1]."

Or, "a substance that speeds up a reaction [1] without being used up [1] by providing an alternative reaction pathway [1] with a lower Activation Energy [1]."

There are many ways that catalysts can work but the only theory we need to be able to explain is the same one you'll get in Biology (yawn!).

Enzymes

An enzyme is a biological catalyst - a piece of protein of a specific shape.

In the diagram above the purple and yellow circles represent reactants.

These may react very slowly because their activation energy is high (the minimum energy needed for them to react).

This means that even when they collide the collision is likely to be unsuccessful.

The enzyme has spaces exactly the right shape for the two reactants to fit into.

The Activation Energy for a reactant sticking to the enzyme is low - so almost every time the enzyme collides with a reactant they will stick together.

Sticking to the enzyme weakens the bonds in the reactants, so less energy is now needed for the two reactants to react.

When they do, they move away from the enzyme, leaving it able to catalyse the reaction again and again.

But because the enzyme has to be a specific shape, enzymes are usually specific to one reaction - only catalyse that one.

The chart right shows that an alternative pathway which involves two reactions instead of one, but that both have smaller Activation Energies than the uncatalysed reaction.

At GCSE we can just say "lowers the activation energy", however.

Recognising a catalyst.

A common exam question asks you to pick out the best catalyst from a list.

For example, a reaction is carried out without a catalyst and found to take 230 seconds exactly.

The same experiment was then carried out 5 more times, each time with a different possible catalyst A,B,C,D and E

A - not a catalyst because although the reaction was faster some of the catalyst was used up (mass decreased)

B - not a catalyst because although the catalyst was unchanged it didn't speed up the reaction

C - not a catalyst because although the reaction was faster and the mass of the catalyst stayed the same it must have changed because it turned from blue to orange.

D - is a catalyst because the reaction was faster, the mass and the colour remained the same so it was unchanged.

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