Collision Theory

The main postulates of the collision theory are:

For molecules to react, they must collide.

Molecules must possess on collision sufficient energy to react.

Not all collisions result in reaction - only those which possess the required energy for reactions to occur will react.

This theory explains the factors which affect the rate of reactions (i.e., temperature, concentration/pressure if gases and surface area)

Temperature - when temperature is increased, the rate of reaction also increases. The reason is that, with increase in temperature, more molecules gain enough energy, more than the average energy of the system and collide more frequently with one another.

Subsequently, the number of collisions which can form reactions increases.

Concentration - increase in concentration of reactants leads to increase in the rate of reaction. Reason: high concentrations imply the crowding of reactant molecules in a relatively small area. This results in frequent collisions between them, which eventually leads to increase in reaction rate.

Surface area - the orientation of molecules, or the way they are positioned affect their collisions. Collisions which lead to reactions are known as effective collisions. For some reactions, collisions which are most effective in producing reactions are the head-on collisions.