Chemistry, Third edition

266 15 · DYNAMIC CHEMICAL EQUILIBRIA

often confusing to beginners in the subject and is worthy of further explanation. To

do this, we consider the simplest kind of chemical reaction in which there is only one

reactant (A) and one product (B). The reaction is then represented as

A\===\B

Figure 15.3 represents the reaction as consisting of a store of A and B molecules on

either side of a barrier. The volume of the container is fixed, and so the numbers of

molecules represents the concentrations of those molecules. In our model, reaction

has taken place when a molecule passes over the barrier, and we show this by chang-

ing the colour of the circles (  ) used to represent the molecules. We shall assume

that the temperature is fixed.

In Fig. 15.3(a), we start with pure A. As the reaction proceeds, some of the A

molecules change into B molecules, and soon after this some B molecules revert to

A. At equilibrium (Fig. 15.3(b)) the composition of the mixture is fixed, but A

molecules are still changing into B and vice versa. The key point though, is that at

equilibrium the number of A molecules jumping over the barrier per second equals

the number of B molecules jumping over the barrier per second. In our example, the

equilibrium mixture contains 75% of A (12 out of 16 molecules) and only 25% of B.

In chemical reactions, the composition of the reactant–product mixture at equilib-

rium depends upon the precise concentration of reactants and products (if any) at

the start, and upon the temperature of the reaction mixture.

Our simple model may be used to illustrate another important feature of dynamic

equilibria. Figure 15.3(c) shows the reaction starting with pure B with an initial con-

centration equal to that of A in (a). As reaction starts in Fig. 15.3(c), some B changes

into A. Some A then reverts to B, and Fig. 15.3(d) shows the composition of the

equilibrium mixture. It is evident that it is the same as Fig. 15.3(b), i.e. as was arrived

at starting from pure A at the same temperature. We summarize this by saying that

at the same temperature the same equilibrium composition may be achieved from both

forward and reverse directions.

All reactions are, in principle, equilibrium reactions. However, for many reactions

there is such a low concentration of limiting reactantsat equilibrium that we may

safely assume that the reactants have been completely converted to products, i.e. that

the reaction ‘has gone to completion’. We do not use the\===\sign for reactions that go

to completion, and the ordinary ‘produces’ sign ( ) is used instead.

t

Concentration

Time

Reactants

Products

0

0

Fig. 15.2Concentration–time
profile for an equilibrium
reaction.

(a) (b)

(c) (d)

Fig. 15.3The chemical equilibrium A \===\B: represents A molecules; represents B molecules.