Explaining reaction rates

Before a chemical reaction can occur two things must happen:

• Particles must collide

• The particles must have enough energy to react.

Changing Temperature

When you increase the temperature the particles move around faster and bump into each other more often.

The particles gain energy - more of the particles have enough energy to react.

When you decrease the temperature the opposite happens.

Changing Particle Size

When we cut a lump into smaller pieces or grind it into a powder we increase the surface area of the solid.

Making large particles smaller exposes more surfaces for collisions to take place on.

The larger surface area allows more collisions to take place as there are more available surfaces to react.

Decreasing particle size increases surface area, increasing the rate of reaction.

Changing Concentration

At higher concentrations there are more reactant particles to collide with each other so the reaction is faster.

The reaction rate increases when the concentration increases.

Fair tests and variables

Remember: To make sure that an experiment is a fair test only one variable can be changed. All other variables must be kept the same.

For example in an experiment investigating the effect of concentration on the rate of a reaction the same mass, volume and particle size of reactants should be used in each experiment and each experiment should be carried out at the same temperature.

Next section: Catalysts