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Lessons / Rates and equilibrium / GCSE and A-level

Why do reactions go faster when it’s hot?

The GCSE picture: collisions, activation energy and why heat matters so much.

  1. A glow stick15 cm
  2. The liquid inside1 mm
  3. A crowd of molecules10 nm
  4. Two molecules meeting1 nm
rate ∝ collisions per second × share with E ≥ Ea
What the simulation shows, in words

Black dots are molecules of A and crimson dots are molecules of B, flying about in a box. When an A hits a B, the two react only if they carry enough energy between them, at least the activation energy. A reaction is shown by a flash, and the two molecules become hollow grey products. At 300 K reactions are rare. Raise the temperature and the molecules speed up, they collide a little more often, and many more of the collisions have enough energy, so the reactants are used up much sooner.

A reaction is a collision

For two substances to react, their particles have to hit each other. Most of the time they just bounce off. A collision only leads to a reaction if the particles bring enough energy to start breaking bonds. That minimum is the activation energy, Eₐ. A collision with at least that much energy is a successful collision.

So the rate of a reaction depends on two things: how often particles collide, and what share of those collisions are successful.

Why heat speeds things up so much

When you heat a substance, its particles move faster. That does two things.

  1. They collide more often. Faster particles cover more ground and meet each other more frequently.
  2. More of the collisions are successful. A larger share of particles now carries at least the activation energy.

The second effect is far bigger than the first. Warming a gas from 25 °C to 35 °C makes its molecules collide less than 2% more often. Yet for many reactions near room temperature, that same 10 °C rise roughly doubles the rate. Almost all of the speed-up comes from more collisions having enough energy.

Try it in the simulation. At 300 K, A and B meet often but almost nothing happens. Push the temperature up and watch the flashes become common.

Where you have seen this

  • Food lasts longer in a fridge, because the reactions that spoil it go more slowly in the cold.
  • A glow stick glows brighter but runs out sooner in warm water, and dims in the cold. The light comes from a chemical reaction, so its brightness shows you the rate.
  • Cooking is faster at higher temperatures for the same reason.

The other ways to speed up a reaction

Temperature is one of four things you need to know. The others work on the same idea, collisions:

  • Concentration or pressure: more particles in the same space means more frequent collisions.
  • Surface area: a powder has far more surface than a lump, so more particles are exposed to collide.
  • A catalyst: gives the reaction a different route with a lower activation energy, so more collisions are successful at the same temperature. The catalyst is not used up.
Safety

Faster reactions can mean more heat, gas or splashing. Never heat or mix chemicals at home to see this for yourself. Practical work on rates belongs in a school or college lab with a teacher.