20

O

NE MOLECULE OF A REACTANT SPECIES collides with another mol-

ecule, chemical bonds are rearranged all at once, and voilà—molecules

of product species are formed. This is how it works, right?

Not quite.

Whychemical reactions occur is mainly the focus of thermodynamics. Concepts

like energy and entropy are important in understanding whether processes are

spontaneous or nonspontaneous.Howthey occur is the main focus of kinetics. A

basic understanding of a process includes, foremost, how fast it goes. This is the rate

of the reaction. A deeper understanding of a chemical process includes knowing

why a particular chemical reaction proceeds as fast or as slow as it does: what are

the factors that influence the rate of the reaction? Are the factors controllable, like

concentrations or temperatures or available surface area or presence of catalysts? Or

are the factors inherent to the process, like the chemical identity of the reactants

and products or conditions dictated by thermodynamics? These are all factors that

must be considered in order to understand the kinetics of a chemical process.

First and foremost, it needs to be stressed how thermodynamics and ki-

netics relate to each other. Thermodynamics tries to determine if; kinetics

tries to determine how.Although they occasionally overlap, the two questions

ifand howask two different things. Thermodynamics will tell you if something

might occur, but will not tell you (by itself ) how long you might have to wait.

Kinetics will tell you how fast something might occur, but will not tell you (by

itself ) if it actually will happen. A proper understanding of any process re-

quires input from both kinetics and thermodynamics.

Kinetics is like classical thermodynamics in that much of it is phenomeno-

logical: it is based on observation. Experimental kinetics requires that a person

make measurements of a chemical process and then try to explain or general-

ize it. However, there has been some progress in theoretical kinetics, which we

will discuss briefly. In this chapter, we will go over some of the framework that

is used to generalize our understanding of chemical kinetics.

20.1 Synopsis

One of the central focuses of kinetics is the determination of rate laws: simple

mathematical expressions that tell us how fast a particular chemical reaction

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20.1 Synopsis

20.2 Rates and Rate Laws

20.3 Characteristics of Specific
Initial Rate Laws

20.4 Equilibrium for a
Simple Reaction

20.5 Parallel and Consecutive
Reactions

20.6 Temperature Dependence

20.7 Mechanisms and Elementary
Processes

20.8 The Steady-State
Approximation

20.9 Chain and Oscillating
Reactions

20.10 Transition-State Theory

20.11 Summary

Kinetics