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Chemistry explainer

How catalysts work

Why some reactions need a push to get started — and how a catalyst makes that push smaller.

Draft explainer. Pending final review by Katalyst educators.

The energy barrier

For a chemical reaction to happen, particles must collide with enough energy to break existing bonds. The minimum energy needed is called the activation energy, written Ea. Even reactions that release energy overall usually need this initial input — like pushing a boulder over a hill before it can roll down the other side.

The higher the activation energy, the fewer collisions have enough energy at any moment, and the slower the reaction.

What a catalyst does

A catalyst is a substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy. More collisions now have enough energy to react, so the reaction goes faster.

Energy diagram: how a catalyst lowers the barrier Two paths from a starting point to a lower end point. The path without a catalyst rises to a tall peak; the catalysed path rises to a much lower peak, so less energy is needed to get started. Both paths start and end at the same levels. Energy needed Reaction progress Ea Ea Starting point End point Without a catalyst With a catalyst
The catalysed path (solid) has a lower peak than the uncatalysed path (dashed). The start and end energies are the same.

Key facts to remember

  • A catalyst is not used up. It takes part in the reaction but is regenerated at the end, so a small amount can be used again and again.
  • A catalyst does not change the overall energy change of a reaction — the starting and ending energy levels stay the same.
  • In a reversible reaction, a catalyst speeds up the forward and reverse reactions equally. It helps equilibrium be reached faster but does not change the position of equilibrium.

Catalysts in real life

  • Enzymes are biological catalysts. For example, the enzyme catalase speeds up the breakdown of hydrogen peroxide into water and oxygen in living cells.
  • Catalytic converters in cars use metals such as platinum, palladium and rhodium to convert harmful exhaust gases into less harmful ones.
  • The Haber process uses an iron catalyst to make ammonia from nitrogen and hydrogen on an industrial scale.

Two types

Homogeneous catalysts are in the same phase as the reactants (for example, all dissolved in solution). Heterogeneous catalysts are in a different phase — usually a solid surface with gaseous or liquid reactants, as in a catalytic converter.

Check your understanding

1. Is a catalyst consumed in a reaction?

No. It participates in the reaction pathway but is regenerated, so it is not used up overall.

2. How does a catalyst increase the reaction rate?

It provides an alternative pathway with a lower activation energy, so a greater proportion of collisions have enough energy to react.

3. Does a catalyst change how much product forms at equilibrium?

No. It speeds up both forward and reverse reactions equally, so equilibrium is reached sooner but its position is unchanged.

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