In a chemical reaction, the reactant particles can only react with each other when they bump into one another. According to collision theory when molecules collide bonds between their atoms can break, and then new bonds can form to make new molecules.
The molecules in gases and liquids are moving constantly, and millions of collisions take place every second. But only a small number of these collisions lead to the formation of product. For a collision to be 'successful', the particles involved must possess enough energy, called the activation energy, to break some of the existing bonds.
Any change that increases the number of collisions per second, or increases the energy of the particles that are colliding, will increase the rate of reaction.
If the temperature is increased, there will be more energy in the collisions. More collisions will have the activation energy, resulting in an increase in the rate of reaction.
If the concentration of one or more of the reactants is increased, there will be more collisions per second, resulting in an increase in the rate of reaction.
The rate of a reaction increases if:
The temperature is increased
The concentration of a dissolved reactant is increased
Solid reactants are broken into smaller pieces
A catalyst is used
Graph showing rates of reaction under changing conditions. At a lower temperature, lower concentration, or with larger pieces, the rate of reaction is slower than at higher temperatures,