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8

Chemical kinetics

Introduction

In previous chapters, we considered questions like: ‘How much energy does a reaction
liberate or consume?’ and ‘In which direction will a reaction proceed?’ We then asked
questions like: ‘To what extent will a reaction proceed in that direction, before it
stops?’ and even ‘Why do reactions occur at all?’ In this chapter, we look at a different
question: ‘How fast does a reaction proceed?’ Straightaway, we make assumptions.
Firstly, we need to know whether the reaction under study can occur: there is no
point in looking at how fast it is not going if a reaction is not thermodynamically
feasible! So we first assume the reaction can and does occur.
Secondly, we assume that reactions can be treated according to their type, so ‘reac-
tion order’ is introduced and discussed in terms of the way in which concentrations
vary with time in a manner that characterizes that order.
Finally, the associated energy changes of reaction are discussed in terms of the
thermodynamic laws learnt from previous chapters. Catalysis is discussed briefly
from within this latter context.

8.1 Kinetic definitions

Why does a ‘strong’ bleach clean faster
than a weaker one does?
Care:a supermarket
Introduction to kinetics: rate laws uses the word ‘strong’
in a different way from
We often clean away the grime and dirt in a kitchen with bleach, chemists: remember
from Chapter 6 that the
−
the active ingredient of which is the hypochlorite ion ClO .The
cleaning process we see by eye (‘the bleaching reaction’) occurs everyday word ‘strong’
has the specific chem-
between an aqueous solution of ClO − ion and coloured species ical meaning ‘a large
stuck to the kitchen surfaces, which explains why the dirt or grease,
equilibrium constant of
etc., appears to vanish during the reaction. The reaction proceeds dissociation’.
concurrently with colour loss in this example.