Collision Theory and Transition State Theory

It has been recognized for a long time that the rate of reaction is markedly affected by temperature. In fact, it is generally found that by increasing the reaction temperature by 10K approximately doubles the rate of reaction. Two theories - the collision theory and the transition state theory have been developed to explain this temperature effect. The collision theory is based on the kinetic theory and assumes a collision between reactants before a reaction can take place. The transition state theory suggests that as reactant molecules approach each other closely they are momentarily in a less stable state than either the reactants or the products. Both of these classical theories provide upper limits of enzyme catalysis.

Like all catalysts, enzymes decrease the energy required to a reaction started. It does this by decreasing the activation energy needed to form the transition state.

The key points for these theories are:

  1. All reactions involving two reactants require collisions between particles to proceed.
  2. Not all collisions taking place between particles results in a reaction.
  3. In the middle of a reaction, there is a configuration of the particles, which is difficult to achieve - this is the Transition State.
  4. The total kinetic energy of reactant molecules must be at least as high as the activation energy in order to achieve the Transition State so the reaction can proceed.
  5. The Transition State does not always proceed to the products of the reaction - it can also return to reactant molecules.

Collision Theory
Transition State Theory