PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 306

Tabulka9.1:Types of elementary reactions and their kinetic equations.

molecularity elementary reaction kinetic equation

1 A → products −

dcA

dτ

=k cA

2 2 A → products −

dcA

dτ

=k c^2 A

A + B → products −

dcA

dτ

=k cAcB

3 3 A → products −

dcA

dτ

=k c^3 A

2 A + B → products −

dcA

dτ

=k c^2 AcB

A + B + C → products −

dcA

dτ

=k cAcBcC

of elementary reactions) has a corresponding set of kinetic equations, i.e. first-order ordinary
differential equations. Solving the reaction mechanismis no less complex task. By this
we mean finding integral forms of a set of (differential) kinetic equations describing the given
mechanism. A solution in an analytic form is usually required, which is not always possible.
In the case of more complex mechanisms we have to put up either with a numerical solution of
kinetic equations, or with only anapproximateanalytic solution.

Another complication is the fact that radicals (denoted A·), “activated molecules” (denoted

A∗), etc. often act as intermediates in elementary reactions. The concentrations of these
reactive components of chemical reactions are usually small and experimentally difficult to
obtain or unobtainable. The numerical values of the rate constants of the reactions in which
they participate are not known, either. It is thus undesirable for these concentrations to occur
in the resulting integrated forms of kinetic equations.
In order to simplify the solution of reaction mechanisms, the following approximations are