5.5 Collision Model

The collision model explains reaction rate in terms of particle collisions. For a reaction to occur, particles must collide with enough energy and with the correct orientation. Not every collision produces products. Only effective collisions do. This means reaction rate depends not only on how often particles collide, but also on whether those collisions are energetic and properly aligned.

This model helps explain why temperature, concentration, and catalysts affect reaction rate. Higher concentration increases collision frequency. Higher temperature increases the fraction of particles with enough energy to react. Proper orientation matters because some molecules must collide in a very specific way for bonds to break and form correctly. The collision model connects microscopic particle behavior to the measurable macroscopic rate of reaction.

Example 1: If the concentration of reactants increases, the particles collide more often, so the reaction rate usually increases.

Example 2: If temperature increases, more particles have enough kinetic energy to overcome the activation barrier, so a larger fraction of collisions becomes effective.