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:

• Higher reactive elements or compounds react faster than those of lower reactivity since more of the collision will have sufficient energy to react due to the more reactive particle having a lower Ea. Can use the activity series to determine which elements are more reactive. As a result, more reactive particles have a higher fraction of effective collisions.
• Simpler molecules (e.g. mono atomic ions) are more reactive than those of more complex molecules (e.g. poly atomic ions) dues to the collision geometry. Large molecules have higher difficulty reacting due to size interference and also bonds having to be broken – as a result complex molecules have a lower fraction of effective collisions.