Modern inorganic chemistry

64 ENERGETICS

THE KINETIC FACTOR

Even given a favourable energy factor, a change may still not take

place or occur at a negligible rate if the kinetic factor is unfavourable.

The situation is somewhat analogous to an object on the ground.

First, the object can only move spontaneously if the ground slopes

downwards—it will not move spontaneously on level ground

or up a slope. If the object is, say, a smooth sphere, it will, given a

downward gradient, move spontaneously. However, if the object is

less regular in shape, say a lump of rock, it may be at rest on an

incline. This rock is energetically unstable but kinetically stable.

The rock can be made kinetically unstable by giving it a push to get

it over its energy barrier—adding initial energy (the energy of

activation). Similarly, many chemical systems are energetically

unstable but kinetically stable and need a 'push', usually in the form

of heat, to make them go (Figure 3.2). We should note that not all

.9-o

JT

E, energy barrier

, enthalpy of reaction

Products

Reaction coordinate

Figure 3.2

the molecules in a given system need to be given the additional

'activation energy' for the reaction to proceed. Each molecule that

reacts produces energy, in an exothermic reaction, and this can

activate more molecules. Hence, once a sufficiently large proportion

of the molecules reaches the activated state, the reaction proceeds

spontaneously. The burning of coal and wood are familiar examples

of this type of process.

FREE ENERGY AND EQUILIBRIA

Let us now consider two simple representative reactions: