Reaction rates
The rate of a reaction is how fast reactants turn into products.
Collision theory says particles react only when they collide with enough energy and in the right orientation. The minimum energy needed is the activation energy, Ea. Most collisions fall short of it and the particles just bounce apart.
Anything that causes more collisions, or more energetic ones, speeds up a reaction.
| Factor | Effect | Why |
|---|---|---|
| Higher temperature | Faster | Particles move faster, so more collisions exceed Ea |
| Higher concentration | Faster | More particles in the same space collide more often |
| More surface area | Faster | More of a solid is exposed to collisions |
| Catalyst | Faster | Provides a path with a lower Ea |
A catalyst speeds up a reaction without being used up. It does not change ΔH or the amount of product. Enzymes are the catalysts of living things.
Worked example: why does powdered zinc react faster than a lump of zinc?
The powder has far more surface area. More zinc atoms are exposed at once, so there are more collisions per second with the other reactant.
Common mistake
Thinking a catalyst adds energy or makes more product. It only lowers the energy barrier, so the same products form sooner.