PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 311

Example

The above mechanism is used to explain the oxidation of nitrogen oxide to nitrogen dioxide

2 NO+O 2 → 2 NO 2 ,

which is a third-order reaction. Write the mechanism of the reaction, find out which substance

is the intermediate, and what has to apply for the rate constants of the partial reactions in order

that we could calculate the concentration of the intermediate from relation (9.152).

Solution

The oxidation mechanism is

2 NO ⇀↽N 2 O 2 ,

N 2 O 2 +O 2 → 2 NO 2.

The intermediate of the reaction is nitrogen oxide dimer N 2 O 2. Relation (9.152) is based on

the assumption of pre-equilibrium, see the preceding section. In this case it is assumed that the

dimerization of nitrogen oxide quickly reaches the state of thermodynamic equilibrium, and that

the dimer oxidation to nitrogen dioxide is relative slow, i.e.k 3 k 1 andk 3 k 2.

9.5.9 Chain reactions

Chain reactions are complex consecutive reactions characterized by a cyclic consumption and
formation of reactive intermediates, most often radicals. We distinguish three stages of a chain
reaction:

a) Initiationor formation of reactive intermediates. Usually it occurs when the reactants

are heated (thermal initiation) or irradiated (photoinitiation).

b) Propagation. This is a cyclically proceeding reaction or a sequence of reactions (cycle).

We distinguish:

• Unbranched-chain reactions, in which the same number of reactive intermediates is
  formed as consumed in the course of the cycle.