Page 162 - Physical chemistry understanding our chemical world

4

Reaction spontaneity and

the direction of

thermodynamic change

Introduction

We start by introducing the concept of entropy S to explain why some reactions
occur spontaneously, without needing additional energy, yet others do not. The sign
of S for a thermodynamic universe must be positive for spontaneity. We explore
the temperature dependence of S.
In the following sections, we introduce the concept of a thermodynamic universe
(i.e. a system plus its surroundings). For a reaction to occur spontaneously in a
system, we require the change in Gibbs function G to be negative. We then explore
the thermodynamic behaviour of G as a function of pressure, temperature and reaction
composition.
Finally, we investigate the relationship between G and the respective equilibrium
constant K, and outline the temperature interdependence of G and K.

4.1 The direction of physicochemical
change: entropy

Why does the colour spread when placing a drop
of dye in a saucer of clean water?
Reaction spontaneity and the direction of change

However gently a drop of dye solution is added to a saucer of clean, pure water,
the colour of the dye soon spreads into uncoloured regions of the water. This mixing
occurs inevitably without warming or any kind of external agitation – the painter with
watercolour would find his art impossible without this effect. Such mixing continues