A comment on the result: a decrease in entropy suggests a transition state that
is more organized than the reactants. When you consider that this reaction is
forming a single triatomic complex from two individual species, it makes
sense that the entropy decreases.20.11 Summary
Exactly how a chemical reaction proceeds may have very little to do with the
balanced chemical equation. Kinetics is the study of exactly how a reaction
proceeds. Kinetics usually focuses on how the rate of a chemical reaction varies
with the concentration of the individual reactants, and tries to determine a
simple mathematical model, a rate law, that describes how fast a reaction will
proceed. Kinetics can also relate the rate of a reaction with the absolute tem-
peratures, using the Arrhenius equation or modifications of this equation.
Also, the more intimate details of a chemical reaction are given by the ele-
mentary processes of a chemical reaction. The combination of all of the ele-
mentary processes, called the mechanism of a reaction, shows us exactly how
a chemical reaction does indeed proceed. Understanding how a reaction pro-
ceeds gives us enormous insight into how chemicals react and gives us handles
on how we might be able to control that reaction. Finally, we are able to apply
some basic theoretical ideas to elementary processes and, using our knowledge
of statistical thermodynamics, estimate the rate constants of simple reactions.
20.11 Summary 725