Several conditions must be met for chemical reactions to occur . Molecules must collide. The geometry of the molecules must be proper for forming new bonds. The molecules must have enough energy to break old bonds and form new bonds.

Reaction rates increase as the number of molecules colliding with sufficient energy to break old bonds and form new ones increases. The energy required to break the old bonds and form new ones is called the activation energy. Only a fraction of the molecules with sufficient energy react because the orientation of molecule during collision is not always favorable for breaking old bonds and forming new ones.

The number of collisions increases with concentration of reactants, and the number of molecules with enough energy for reaction increases with temperature.