What are catalysts and how do they work? Revision Notes | GCSE AQA Higher Chemistry

1. What Makes Reactions Go Faster?

Picture hydrogen peroxide slowly bubbling away in a bottle. Add a tiny amount of manganese dioxide powder and suddenly it fizzes violently, releasing oxygen gas. The manganese dioxide hasn't been used up - it's still there at the end. This is catalysis in action.

Catalysts are the speed-boosters of chemistry. They make reactions happen faster without getting consumed themselves. Think of them as the motorway that lets traffic (your reactants) reach their destination (products) much quicker than taking the slow country roads.

2. The Energy Barrier Problem

Every chemical reaction faces an energy barrier called activation energy. Picture trying to push a boulder over a hill - you need enough energy to get it to the top before it rolls down the other side. Particles need enough kinetic energy to overcome this barrier and react.

Here's where catalysts work their key scientific idea. Instead of helping particles jump over the high energy barrier, catalysts provide a completely different route - like building a tunnel through the mountain instead of climbing over it.

The catalyst creates an alternative reaction pathway with a lower activation energy. More particles now have enough energy to react, so the reaction rate increases dramatically. The overall energy change of the reaction stays the same - only the route changes.

3. How Catalysts Actually Work

Catalysts work by providing an alternative reaction mechanism. Think of it like this: instead of reactants A and B colliding directly (which needs high energy), the catalyst offers a two-step process.

Worked example: Catalytic Reaction Mechanism

Show how a catalyst C helps reactants A + B → AB

1. A + C → AC (intermediate) — The catalyst forms a temporary intermediate compound
2. AC + B → ABC — All three species are temporarily bonded together
3. ABC → AB + C — The product breaks away, regenerating the catalyst
4. A + B → AB (catalyst C unchanged) — The catalyst appears on both sides and cancels out

Answer: The catalyst provides a lower energy pathway but is regenerated unchanged

Each step in this catalytic pathway has a lower activation energy than the direct A + B collision. It's like taking two small steps up instead of one giant leap.

4. Biological Catalysts - Enzymes

Your body runs thousands of chemical reactions every second. Without catalysts, these reactions would be impossibly slow at body temperature. Biological catalysts are called enzymes.

Enzymes work exactly like industrial catalysts - they lower activation energy by providing alternative pathways. The difference is specificity. While iron can catalyse many different reactions, each enzyme is shaped to catalyse just one specific reaction.

For example, catalase in your liver breaks down toxic hydrogen peroxide into harmless water and oxygen. Amylase in your saliva breaks down starch into sugars. Each enzyme has evolved to do one job perfectly.

5. Real-World Catalysts in Action

Catalysts are everywhere in modern life. Car exhaust systems use platinum and rhodium catalysts to convert toxic carbon monoxide and nitrogen oxides into harmless carbon dioxide and nitrogen gas.

The Haber process uses iron catalysts to make ammonia for fertilisers. Without this catalyst, the reaction between nitrogen and hydrogen would be far too slow to be commercially viable. The catalyst doesn't change how much ammonia you get - it just helps you get there faster.

In your kitchen, biological washing powders contain enzymes that break down protein and fat stains at low temperatures. These enzyme catalysts work much better than traditional detergents for biological stains.

6. Exam Technique - Catalyst Questions

Catalyst questions often test whether you understand the key principles. Remember: catalysts speed up reactions without being used up, and they work by lowering activation energy.

Common exam mistake: Students often write that catalysts 'give particles more energy'. This is wrong - catalysts provide an easier route that needs less energy, not more energy to particles.