Reactions take place through collisions between reactant molecules. However, under ordinary conditions, only a small fraction of collisions result in reaction. The rate of collision molecules can be calculated from the kinetic theory of gases. If every collision led to reaction, the reaction would be over in a fraction of a second. Stable molecules are held together by strong chemical bonds, These bonds must be broken if products are to form. Only those molecules which are moving very rapidly will collide with enough force to break apart. Collisions must involve a certain minimum amount of energy if they are to result in reaction. The energy which colliding molecules must have if they are to react is called the activation energy. It is given the symbol Ea and expressed in kilocalories per mole of reacting molecules. For every reaction, Ea, has a definite value. .
Activation energy comes from the kinetic energy of colliding molecules. Ordinarily, the activation energy corresponds to a high kinetic energy. It is possessed by only a small fraction of the molecules. Only those molecules having kinetic energies equal to or greater than Ea will react when they collide. If, for a particular reaction, Ea were zero, all collisions would be effective. Reaction would take place in a fraction of a second. Reactions with large activation energies should occur relatively slowly. Reactions with small activation energies should occur relatively rapidly.A catalyst is a substance which changes the rate of a reaction without being consumed by it. Often the simplest, least expensive way to spend up a reaction is to find the right catalyst. Catalyst are used in the production of such common materials as gasoline, synthetic rubber, and plastics. Solid catalyst are used in the petroleum industry for many purposes. Other catalysts are used to produce branched-chain from straight-chain hydrocarbons. Many important inorganic chemicals are made by processes that require a catalysts.