Page 171 - Physical chemistry understanding our chemical world

138 REACTION SPONTANEITY AND THE DIRECTION OF THERMODYNAMIC CHANGE

After their release from solvating this chloride ion, each water
The word ‘universe’ in
this context is com- molecule has as much energetic disorder as did the whole chlo-
pletely different from ride ion complex. Therefore, we expect a sizeable increase in the
a ‘universe’ in astron- entropy of the solvent during crystallization because many water
omy, so the two should molecules are released.
not be confused. A When we look at the spontaneity of the crystallization pro-
thermodynamic ‘uni- cess, we need to consider two entropy terms: (i) the solute (which
verse’ comprises both decreases during crystallization) and (ii) the concurrent increase as
a ‘system’ and its ‘sur- solvent is freed. In summary, the entropy of the solute decreases
roundings’. while the entropy of the solvent increases.
The crystallization process involves a system (which we are inter-
ested in) and the surroundings. In terms of the component entropies
in this example, we say S (system) is the entropy of the solute crys-
We define a thermo- tallizing and that S (surroundings) represents the entropy change of
dynamic universe as the solvent molecules released.
‘that volume large We call the sum of the system and its surroundings the ther-
enough to enclose modynamic universe (see Figure 4.3). A thermodynamic universe
all the changes’; the is described as ‘that volume large enough to enclose all the ther-
size of the surround- modynamic changes’. The entropy change of the thermodynamic
ings depends on the universe during crystallization is S (total) , which equates to
example.
S (total) = S (system) + S (surroundings) (4.8)

The value of S (system) is negative in the example of crystallization. Accordingly, the
value of S (surroundings) must be so much larger than S (system) that S (total) becomes
positive. The crystallization is therefore spontaneous.

Thermodynamic
universe

Surroundings

System

Figure 4.3 We call the sum of the system and its surroundings the ‘thermodynamic universe’.
Energy is exchanged between the system and its surroundings; no energy is exchanged beyond
the surrounds, i.e. outside the boundaries of the thermodynamic universe. Hence, the definition ‘a
universe is that volume large enough to enclose all the thermodynamic changes’