Concise Physical Chemistry

c10 JWBS043-Rogers September 13, 2010 11:26 Printer Name: Yet to Come

10 Chemical Kinetics

In general, chemical thermodynamics is concerned with where a chemical system

starts and where it ends up, whilechemical kineticsis concerned with how long

it takes it to get there. Many analytical techniques exist for following the rate of

change of the concentration of the reactants or products of a chemical reaction, but

single atom counting is not usually done. An exception is the class ofradiochemical

reactions for which counters exist to register, count, and record in computer memory

every decay particle produced. For example, the decay of radium givesαparticles

(helium nuclei) and radon^22286 Rn:

226

88 Ra→

222

86 Rn+

4

2 He

Each click of the counter registers oneαparticle, hence one decay. Here radium is

the radioactive element, and both radon and helium are products. Radon is referred to

as thedaughterof the parent element radium. Radon is itself a radioactive gas, and it

decays to polonium and further along a radioactive series ending eventually as lead

206

82 Pb. Radiochemical reactions show the statistical nature of rate laws very clearly,

For that reason, they are a good introduction to the entire field.

10.1 FIRST-ORDER KINETIC RATE LAWS

Suppose you have a sample consisting of a hundred billion atoms of radioactive

elementX. By means of a suitable counter, you observe an average of one radioactive

Concise Physical Chemistry,by Donald W. Rogers

Copyright©C2011 John Wiley & Sons, Inc.

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