Collision Theory and Chemical Nature

The chemical nature of reactants affects the threshold energy (and the fraction of collisions that are effective) in two possible ways.

  1. Some molecules have bonds that are relatively weak and small activation energy barriers, so the threshold energy is relatively low and a large fraction of molecules is capable of colliding effectively. Other molecules have strong bonds and high activation energy barriers, so most collisions are ineffective.

The physical states of reacting substances are important in determining their reactivities, with substances in the gas state being more reactive often than the same substance in the liquid state (remember the ethanol demo).

Chemical identities of elements and compounds affect reaction rates. Metallic sodium, with its low ionization energy, reacts rapidly with water at room temperature; metallic calcium has higher ionization energy and reacts only slowly with water at room temperature.

Since Ca has a higher Ea in the reaction between it and water, fewer particles have sufficient energy than a sample of sodium reacting under similar conditions.


A second factor is what is sometimes called collision geometry — some reactions involve complicated molecular substances or complex ions that are often less reactive because more bonds have to be broken and the molecules have to collide in the correct orientation relative to each other for a reaction to occur.


Therefore, collision theory explains the affect of chemical nature on the rate of a reaction by: