The Foundations of Chemistry

Figure 16-1 (a) Blue dye is

reacting with bleach, which converts

it into a colorless product. The color

decreases and eventually disappears.

The rate of the reaction could be

determined by repeatedly measuring

both the color intensity and the

elapsed time. The concentration of

dye could be calculated from the

intensity of the blue color. (Solvent

molecules and reaction product

molecules have been omitted from

the diagram for clarity).

(b) A spectroscopic method for

determining reaction rates. Light of

a wavelength that is absorbed by

some substance whose concentration

is changing is passed through a

reaction chamber. Recording the

change in light intensity gives a

measure of the changing

concentration of a reactant or

product as the reaction progresses.

16-1 The Rate of a Reaction 649

This approach is suitable only if the reaction is sufficiently slow that the time elapsed

during withdrawal and analysis of the sample is negligible. Sometimes the sample is with-

drawn and then quickly cooled (“quenched”). This slows the reaction (Section 16-8) so

much that the desired concentration does not change significantly while the analysis is

performed.

It is more convenient, especially when a reaction is rapid, to use a technique that contin-

ually monitors the change in some physical property of the system. If one of the reactants

or products is colored, the increase (or decrease) in intensity of its color might be used

to measure a decrease or increase in its concentration. Such an experiment is a special

case of spectroscopicmethods. These methods involve passing light (visible, infrared, or

ultraviolet) through the sample. The light should have a wavelength that is absorbed by

some substance whose concentration is changing (Figure 16-1). An appropriate light-

sensing apparatus provides a signal that depends on the concentration of the absorbing

substance. Modern techniques that use computer-controlled pulsing and sensing of lasers

have enabled scientists to sample concentrations at intervals on the order of picoseconds

CH 3 C OCH 2 CH 3 (aq)

O

H 2 O(
)

H



 CH 3 C OH(aq) CH 3 CH 2 OH(aq)

O

ethyl acetate acetic acid ethanol

(a)

(b)

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 15.2, Rates of Chemical

Reactions.