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S-7
IV. Collision Theory
In the Collision Theory, one assumes that, in order for a reaction to occur, molecules of reactants must
collide with an energy greater than some minimum value. This minimum energy is called the activation
energy, E
a
, and is the threshold energy that must be overcome to produce a chemical reaction. The
activation energy of a reaction is very dependent on the nature of the reacting species. If two negatively
charged ions must collide in order for a reaction to take place, the activation energy is expected to be
relatively large. From your high school physics you will remember that like-charges repel while unlike
charges attract. To overcome this repulsion, the molecules must collide with sufficient energy (E
a
) to
overcome electrostatic repulsion. On the other hand, when two oppositely charged ions react, little
energy is required for these species to react. As all atoms, molecules, and ions have electrons
(negatively charged) surrounding their nuclei, electrostatic interactions must always be overcome when
two reacting species collide. Therefore, activation energy is almost always a positive quantity, which
has a lower limit very near zero and an upper limit of infinity.
There are numerous factors that can help determine whether one step in a reaction mechanism is fast or
slow. These include charges on the reacting species in the elementary reaction (as described above:
oppositely charged species tend to react faster than neutral species which tend to react faster than
reactants with the same charge - all things being equal). Another factor that helps one to determine the
rate limiting step is the number of reactant molecules colliding. The more molecules that must
simultaneously collide, the slower the rate for that elementary reaction. The stability of the product also
affects the rate of reaction. Formation of a reactive product or intermediate usually requires more
energy and a longer reaction time than the formation of a stable product or intermediate. Finally, if the
reactant molecules and/or ions in one elementary step of the mechanism also appear in the
experimentally determined rate law in the correct proportions, it is likely that this step is the rate
determining step. There is no way of determining, with absolutely certainty, the mechanism of a
reaction or for that matter the rate determining step in a reaction. Experiments and observations can
only suggest whether a mechanism is plausible and whether one mechanism is better than another
mechanism.
