The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed. Rate of reaction is affected by factors such as concentration, pressure, temperature and surface area to volume ratio of a solid reactant.
A model of colliding particles helps to explain why different factors affect rate of reaction.
For a chemical reaction to happen:
A collision that produces a reaction is called a successful collision. The activation energy is the minimum amount of energy needed for a collision to be successful. This is different for different reactions.