PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 305

Note:The reaction

H + Br 2 → HBr + Br

is elementary. It is a bimolecular reaction. Hydrogen burning

2 H 2 + O 2 →2 H 2 O

is not an elementary reaction. Its mechanism is given by the following sequence of ele-

mentary reactions:

H 2 + O 2 →HO 2

+ H,

H 2 + HO 2 .→H 2 O + OH.,

H 2 + OH

→H 2 O + H

,

H.+ O 2 →O.+ OH.,

H 2 + O.→H.+ OH..

All elementary reactions fall in the group of simple reactions, but far from all simple reactions
are elementary. The order of an elementary reaction is identical with its molecularity, the orders
with respect to individual components are equal to their stoichiometric coefficients and acquire
the value of 1, 2 or 3.

9.5.2 Kinetic equations for elementary reactions

For elementary reactions, the kinetic equation (9.8) has the following form:

−

dcA

dτ

=k caAcbB... , (9.138)

wherea,b,... are the stoichiometric coefficients of the reactants. There are six basic types of
elementary reactions. Their kinetic equations are given in Table9.1.

9.5.3 Solution of reaction mechanisms

Setting up the mechanism of a chemical reaction is a complex experimental and theoretical
task which we will not discuss here. A set-up mechanism of a chemical reaction (a sequence