A simple model that explains reaction rate well for many reactions is the collision model. If we consider a simple reaction such as

A(g) + B(g) -> products

Any study of reaction rate and the rate at which molecules collide quickly shows that not every collision between molecules creates products: in fact, the vast majority do not. There are two basic reasons why not every collision is effective.

First, the molecules have to be oriented in the correct way. A very simple gas phase reaction such as Br(g) + HI(g) -> HBr(g) + I(g) won't occur unless the molecules are oriented such that the hydrogen is easily accessible by the bromine. This is known as the steric factor. If you are curious, this page has five Chime animations showning various possible alignments of Br and HI and what happens when they collide.

The second factor is due to the activation energy. Molecules are held together with strong chemical bonds. In order to break these bonds, the colliding molecules have to have a large amount of kinetic energy from the collision. If they do not have enough energy, the reaction will not occur.

The rate constant for a reaction can be described in the collision model as

k = p*Z*f

• p is the steric factor: the fraction of collisions that have the proper geometry to react. This number is between 0 and 1, but the collision model does not give us any easy way to compute it.
• Z is the number of collision per unit time. For gases, this can be easily computed from the kinetic theory of gases
• f is the fraction of collisions that have more than the activation energy. It can be shown that f = e^-E_a/RT, where E_a is the activation energy, R is the gas constant and T is the temperature.

From this equation, we can understand a little about how reactions work. As the activation energy increases, f decreases, so reactions with large activation energies are slower at a given temperature than ones with small activation energies. (Many reactions do not occur at room temperature: for example, 2H₂ + O₂ -> 2H₂O is highly exothermic, but unless you heat the hydrogen and oxygen up, the reaction rate is effectively zero since the activation energy for this reaction is too high.) As T increases, the rate of the reaction increases.

Information provided by: http://learn.chem.vt.edu