Collision Theory: What Sets the Speed of a Reaction
- Chemistry
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Particles react only when they collide with at least the activation energy, in a suitable orientation.
- Temperature, concentration and surface area change the rate by changing successful collisions.
- A catalyst provides a pathway with a lower activation energy and is not used up.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Only some collisions count
To react, particles have to bump into each other. But in a liquid or gas, particles collide billions of times every second. If every bump caused a reaction, every reaction would finish instantly. In fact, most bumps do nothing. A collision only works if the particles hit hard enough. The smallest energy that works is called the activation energy. The rate of a reaction depends on how many collisions work each second. Heating makes a big difference. It gives particles more energy, so many more collisions are hard enough. For many reactions, warming by 10 °C roughly doubles the speed. That is why food lasts longer in a fridge. It is also why a lizard is slow on a cold morning. The reactions in its body slow down when it is cold.
Concentration, surface area and catalysts
Other changes work by making particles collide more often. A more concentrated solution has more particles packed into the same space, so they bump into each other more. A solid broken into small pieces has more surface, so more of its particles can be reached. You can see this with marble chips in dilute acid, which makes carbon dioxide gas. Marble powder makes gas much faster than the same mass of chips. Stronger acid speeds it up too. In the end, the same amount of gas is made either way; only the speed changes. A catalyst is different. It gives the reaction an easier path that needs less energy, so more collisions work. The catalyst is not used up. Enzymes in your body are catalysts, and factories use them too.
Curriculum links: NGSS HS-PS1-5