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August 18, 2010 11:36 9in x 6in b985-ch07 Elementary Physical Chemistry
Chapter 7
Chemical Kinetics
Much of physical chemistry is concerned with chemical reactions. Thermo-
dynamics enables us to determine whether reactions will proceed or will not
proceed. Thermodynamics will also determine the conditions that must be
satisfied to obtain equilibrium, but thermodynamics will not predict how
fast equilibrium will be reached. Chemical kinetics does this.
Several experimental methods are available for determining reaction
rates. They basically fall into two categories:
(1) Removal of successive samples from the reaction mixture — and
analyzing the mixture (hard to come by if the reaction is fast).
(2) Analyzing the reaction (using physical methods such as colorimetry,
spectral absorption etc.) while the reaction is proceeding. This method
is continuous.
7.1. The Rates of Reactions
Reaction rates were already studied quantitatively as far back as the 1850s.
For example, the rate of the reaction
H 2 O+C 12 H 22 O 11 → C 6 H 12 O 6 +C 6 H 12 O 6
sucrose glucose fructose (7.1)
was found to satisfy the relation
d[c]/dt = k[c] (7.2)
where [c] is the concentration of the unreacted sucrose, and k is a constant,
called the rate constant.
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