Factors affecting rate

Collision theory says that particles can only react when they collide with enough energy — at least the activation energy. A collision that leads to a reaction is a successful collision.

Anything that makes successful collisions more frequent (happen more often) increases the rate of reaction. The four factors are:

• concentration of solutions, or pressure of gases
• surface area of solids
• temperature
• a catalyst

Concentration (solutions) and pressure (gases): a higher concentration or pressure means more particles in the same volume. They collide more frequently, so the rate increases.

Surface area: cutting a solid into smaller pieces, or grinding it into a powder, increases its surface area to volume ratio. More particles are exposed on the surface for the other reactant to collide with, so collisions are more frequent and the rate increases. Powders react much faster than lumps.

Raising the temperature increases the rate for two reasons:

1. Particles move faster, so they collide more frequently.
2. Particles have more energy, so a greater proportion of collisions have at least the activation energy and are successful.

The second reason has the bigger effect. That's why a small rise in temperature can make a reaction much faster.

Catalysts also increase the rate, by lowering the activation energy (see Catalysts).