The collision theory explains that gas-phase chemical reactions occur when molecules collide with sufficient kinetic energy. The collision theory is based on the kinetic theory of gases; therefore we are only dealing with gas-phase chemical reactions. Ideal gas assumptions are applied. Furthermore, we also are assuming:
Ultimately the collision theory of gases gives us the rate constant for bimolecular gas-phase reactions; it is equal to the rate of successful collisions. The rate of successful collisions is proportional to the fraction of successful collisions multiplied by the overall collision frequency.
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