Collisions and reactions

You will be expected to explain, in terms of particles and their collisions, why changing the conditions of a reaction changes its rate.

Collisions

For a chemical reaction to occur, the reactant particles must collide. Collisions with too little energy do not produce a reaction.

The collision must have enough energy for the particles to react. The minimum energy needed for particles to react is called the activation energy.

Changing concentration or pressure

If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased:

• There are more reactant particles in the same volume
• There is a greater chance of the particles colliding
• The rate of reaction increases

Changing particle size

If a solid reactant is broken into small pieces or ground into a powder:

• Its surface area is increased
• More particles are exposed to the other reactant
• There is a greater chance of the particles colliding
• The rate of reaction increases

Changing the temperature

If the temperature is increased:

• The reactant particles move more quickly
• More particles have the activation energy or greater
• The particles collide more often, and more of the collisions result in a reaction
• The rate of reaction increases

Using a catalyst

Catalysts increase the rate of reaction without being used up. They do this by lowering the activation energy needed. With a catalyst, more collisions result in a reaction, so the rate of reaction increases. Different reactions need different catalysts.

Catalysts are important in industry because they reduce costs.