Increase in concentration causes an increase in the rate of a reaction because there will be more reactant particles in the same amount of space (volume) and so there is a greater chance of a collision occurring. The more collision there are per second, the faster a reaction can occur.
(Only works for reactions involving gases)
Increase of pressure causes an increase in the rate of a reaction because there will be more reactant particles in the same amount of volume and so there is a greater chance of collision occurring.
Increase in temperature causes an increase in the of a reaction. This is because the reactant particles are moving more quickly, there is a greater chance of collision occurring. Also, for collision to lead a reaction, it must have sufficient energy to overcome the activation energy barrier. On average, the particles move more quickly in a hot substance.
Stirring cause an increase in the rate of a reaction because there is a greater chance of collision.
Breaking a solid into small pieces exposes a greater surface area to attack by a liquid or gas.
Using a catalyst increases the rate of a reaction because it lowers the activation energy barrier. (It is a good idea to give a specific example if possible since only certain reactions have a suitable catalyst available). As an example, if 10% of the particles have sufficient energy to react without a catalyst present any reduction in the energy requirement means that a higher percentage of the particles will be able to react and do the reaction will be faster.
A substance that initiates or accelerates a chemical reaction without itself being affected.