Question

Describe the salient features of the collision theory of reaction rates.

Answer 1

Collision theory states that the rate of a chemical reaction is proportional to the number of collisions between reactant molecules. The more often reactant molecules collides, the more often they react with one another, and the faster the reaction rate. In reality, only a small fraction of the collisions are effective collisions. Effective collisions are those that result in a chemical reaction.

In order to produce an effective collision, reactant particles must possess some minimum amount of energy. This energy, used to initiate the reaction, is called the activation energy. For every sample of reactant particles there will be some that possess this amount of energy.

The larger the sample, the greater the number of effective collisions, and the faster the rate of reaction. The number of particles possessing enough energy is dependent on the temperature of the reactants. If reactant particles do not possess the required activation energy when they collide, they bounce off each other without reacting.

Some chemical reactions also require that the reactant particles be in a particular orientation to produce an effective collision. Unless the reactant particles possess this orientation when they collide, the collision will not be an effective one. The reaction of ozone with nitrogen monoxide is an example of how orientation can be important.

To summarize, the requirements for an effective collision (for a chemical reaction to occur):

1. The reactants must collide with each other.

2. The molecules must have sufficient energy to initiate the reaction (called activation energy).

3. The molecules must have the proper orientation.