Concise Physical Chemistry

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

154 CHEMICAL KINETICS

(beneficial) but they are also found in cancers undergoing uncontrolled reproduction.

The biological role of free radicals is complex and not completely understood.

10.4 EXPERIMENTAL DETERMINATION OF THE RATE EQUATION

For some reactions, determination of the rate law and the rate constant may be as

simple as mixing the reagents, titrating a sample of one of the reactants or antic-

ipated products, waiting a while, titrating another sample, and so on, to obtain a

series of concentrationsA, each at a timet. Treating this data set by an appropriate

mathematical procedure gives the rate law andk.

Some of the most interesting contemporary research in kinetics involves tracking

reaction components or intermediates by physical means, perhaps because they are

of fleeting existence, being present in the reacting system for a second or less.

A straightforward approach to fairly fast reactions involves use of astopped-flow

reactor. One experimental design consists of two syringes connected to a mixer.

To initiate the reaction, the syringes are driven simultaneously so that reactants A

and B flow into the mixer, are mixed thoroughly, and flow out of the mixer into a

tube where the reaction mixture is monitored by a spectrophotometer. The output

from the spectrophotometer is transmitted to a microcomputer at time intervals of,

say, a few milliseconds or, perhaps, microseconds. The problem is well within the

capabilities of contemporary spectrophotometers and computer interfacing hardware,

so the principal limiting factor is speed of mixing. Reactions with a half-time of a

millisecond or less have been successfully studied in this way.

For very fast reactions, the limitation has been only the ingenuity of the researcher

andflash photolysisstudies of reactions with the astonishing half-time oft 1 / 2 =

10 −^15 s indicate that, for some people, this hasn’t been much of a limitation (Zewail,

Nobel Prize, 1999, see Zewail, 1994).

10.5 REACTION MECHANISMS

Themechanismby which molecules actually combine is rarely as simple as the rate

law suggests. Complex reactions may involve equilibriums,

A←→B

consecutive reactions,

A→B→C

parallel reactions,

A→C



B→D

or combinations of any or all of them.