PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 312

• Branched-chain reactions, in which more reactive intermediates are formed than
  consumed in the course of the cycle.
  c)Terminationor end of reaction during which the reactive intermediates cease to exist
  in consequence of either their mutual reaction or their adsorption on the walls of the
  containing vessel.

Kinetic chain lengthis the number of cycles in the stage of propagation induced by one
reactive intermediate.

Note:In branched-chain reactions, the reaction in the stage of propagation permanently

accelerates. If its termination is not sufficiently fast, the reaction ends up in an explosion.

The reaction

H 2 +

1

2

O 2 →H 2 O

may serve as an example of an explosive reaction of this type.

Another type of explosion is thermal explosion in which the heat evolved by an exothermic

reaction increases temperature and consequently also the rate of reaction.

An example of an unbranched-chain reaction is hydrogen burning in chlorine with hydrogen
chloride formed.
H 2 + Cl 2 →2 HCl.

The reaction is initiated thermally
Cl 2 →2 Cl

.

.

(Analogous dissociation of the hydrogen molecule does not occur; it is substantially more
energy-consuming.) One propagation cycle is comprised of two reactions

Cl

.

+ H 2 →HCl + H

.

,

H.+ Cl 2 →HCl + Cl..

The number of these cycles, or the kinetic chain length is approx. 10^6. One chlorine radical
yields millions of hydrogen chloride molecules! The step

2 Cl.→Cl 2.

terminates this reaction.