will proceed. Rate laws that have similar mathematical forms imply that their
reactions behave a certain way as the reaction proceeds in time; we will con-
sider some of those behaviors. Using the tools of calculus, we will be able to
derive some simple expressions that will help us predict amounts of reactants
and products of reactions that have particular rate laws.
A central part of a rate law for any chemical reaction is its rate constant,
which (as its name implies) is a constant for a particular reaction at a partic-
ular temperature. This statement implies that the rate law constant does change
with temperature, and that’s correct. But we also have some simple models for
how the rate law constant changes with temperature.
Reactions don’t just occur singly; they occur sequentially or in parallel. We
will consider how several processes occurring simultaneously affect the
amounts of products and reactants. Finally, we recognize that most chemical
reactions occur in discrete steps. The overall combination of these steps, called
elementary processes, is what makes up the mechanism of a reaction. A pro-
posed mechanism must be consistent with the experimentally determined rate
law of a reaction. This requirement puts some restrictions on how we can ex-
pect a chemical reaction to occur on an atomic and molecular scale.
Near the end of the chapter, we will consider two interesting types of reac-
tions, branched reactions and oscillating reactions. Not only do such reactions
have interesting kinetics, but they have some fascinating applications. Finally,
we will discuss a little bit of theoretical kinetics, to leave you with the idea that
not all kinetics is phenomenological. More and more, basic physical chemical
principles are applied at the molecular level in attempts to describe adequate
models for chemical reactions—which are, after all, of fundamental interest to
chemists.
20.2 Rates and Rate Laws
One of the most basic descriptions of a chemical reaction is how fast it goes.
But when we speak of how fast a reaction goes, we are not thinking “fast” as in
a velocity in meters per second. Rather, we are thinking about how quickly
amounts (that is, moles) of reactants are converted into amounts (moles) of
products. The “quickness” implies that time (in units of seconds, minutes,
hours, days, and so on) will be a concern also. The rateof a reaction is an in-
dication of how many moles of a reactant or product are reacted or produced
over a period of time.
Rates of reactions are a central issue in kinetics. Understand that it is diffi-
cult to predict before the fact how fast a reaction will be (although we will
explore some of the factors that influence the rate of reactions). A lot of in-
formation about kinetics of reactions is experimentally determined. Reaction
rates also provide the fundamental information needed to deduce the indi-
vidual actions that reactant species take in order to make products. (We will
consider this near the end of this chapter.)
Furthermore, in a closed system, the rates of most reactions change over time.
Typically, amounts of reactants decrease over time. When discussing rates of re-
actions, it is important to indicate at what point along the extent of the reaction
we are. (Extents of reaction,, were discussed previously in Chapter 5.) It is con-
ventional to define rates of reactions as they would be at the very beginning of
a chemical process, in which only reactants are present, no products. The rate of
a reaction at this extent 0 is called the rate of initial reactionor the initial
reaction rate.In almost all cases, we will be dealing with initial reaction rates.
20.2 Rates and Rate Laws 681