Higher and foundation tier
Before any chemical reaction can happen between two substances the particles present in each of them must collide. Consider the reaction between hydrogen and chlorine to make hydrogen chloride gas, the image below shows a molecule of hydrogen and a molecule of chlorine reacting to make 2 new molecules of hydrogen chloride gas.
Before the hydrogen and chlorine can react you need to think about what must happen during the reaction:
If the molecules in the reactants collide with each other with enough force to break the bonds holding the molecules
together it is called a successful collision. If the molecules collide with less force and the bonds in the reactants
do not break, this is called an unsuccessful collision.
Any factor that increases the number of successful collisions in a reaction will increase the rate of that reaction. There are four main factors that affect the rate of a chemical reaction, these are: