Chemistry, Third edition

220 13 · ENERGY CHANGES IN CHEMICAL REACTIONS

4.If a reaction can occur by more than one route, the enthalpy change is the

same, whichever route is followed.

This is a statement of Hess’s law. As an example, we shall look at the production of

hydrogen iodide gas from iodine and hydrogen. The reaction can occur in one stage

in which hydrogen iodide gas is made directly:

H 2 (g)I 2 (s) 2HI(g) H—^ 53 kJ mol^1 (13.3)

We call this route A.

Alternatively, we can carry out this reaction in twostages:

I 2 (s) I 2 (g) H—^ 62 kJ mol^1 (13.4)

followed by

I 2 (g)H 2 (g) 2HI(g) H—^  9 kJ mol^1 (13.5)

We shall call reactions (13.4) and (13.5) route B.

Both routes are illustrated in Fig. 13.2(a), which shows the enthalpy changes for

the reaction in which 1 mol of reactants is converted into 2 mol of HI. The diagram

Fig. 13.2Hess’s law: (a) the manufacture of hydrogen iodide by two routes; (b) the general case.