Page 168 - Physical chemistry understanding our chemical world

THE DIRECTION OF PHYSICOCHEMICAL CHANGE: ENTROPY 135

SAQ 4.1 By inspection alone, decide whether the condensation of water,
H 2 O (g) → H 2 O (l) is spontaneous or not.

Worked Example 4.3 Now consider the chemical process

SOCl 2(l) + H 2 O (g) −−−→ 2HCl (g) + SO 2(g) (4.5)

The reaction occurs spontaneously in the laboratory without recourse
to heating or catalysis. The sight of ‘smoke’ above a beaker of SOCl 2(l) This increase in the
is ample proof of reaction spontaneity. entropy S of a gas
We see that the reaction in Equation (4.5) consumes 1 mol of gas explains why an open
(i.e. water vapour) and 1 mol of liquid, and generates 3 mol of gas. beaker of thionyl chlo-
There is a small change in the number of moles: principally, the ride SOCl 2 in the lab-
amount of gas increases. As was seen above, the entropy of a gas oratory appears to be
is greater than its respective liquid, so we see a net increase in the ‘smoking’.
entropy of the reaction, making S positive.

Worked Example 4.4 By inspection alone, decide whether the formation of ammonia
by the Haber process (Equation (4.6)) is spontaneous or not.

(4.6)
N 2(g) + 3H 2(g) −−−→ 2NH 3(g)
All the species in Equation (4.6) are gases, so we cannot use the simple method of looking
to see the respective phases of reactants and products (cf. Equation (4.4)).
But we notice the consumption of 4 mol of reactant to form 2 mol of product. As a
crude generalization, then, we start by saying, ‘4 mol of energetic disorder are consumed
during the process and 2 mol of energetic disorder are formed’. Next, with Equation (4.1)
before us, we suggest the overall, crude entropy change S is roughly −2 mol of disorder
per mole of reaction, so the amount of disorder decreases. We suspect the process will
not be spontaneous, because S is negative.
In fact, we require heating to produce ammonia by the Haber pro-
cess, so the reaction is definitely not spontaneous.
The word ‘stoichiom-
SAQ 4.2 By inspection alone, decide whether the oxi- etry’ comes from the
dation of sulphur dioxide is thermodynamically sponta- Greek stoicheion,mean-
1
neous or not. The stoichiometry of the reaction is / 2O 2(g) ing ‘a part’.
+ SO 2(g) → SO 3(g) .

Worked Example 4.5 By inspection alone, decide whether the reaction Cl 2(g) + F 2(g) →
2FCl (g) should occur spontaneously.

Occasionally we need to be far subtler when we look at reaction spontaneity. The
reaction here involves two molecules of diatomic gas reacting to form two molecules of
a different diatomic gas. Also, there is no phase change during reaction, nor any change
in the numbers of molecules, so any change in the overall entropy is likely to be slight.