Page 47 - A Working Method Approach For Introductory Physical Chemistry Calculations

Thermodynamics II 31
**** (b) 1’> 0 K
(a)T=O K
****

****

***a@ *w**
***a@

Figure 3.3 (a) A perfect crystalline solid, AB, at 0 K (S = 0), and (b) at
above 0 K, when the molecules start to vibrate. The regular array
now becomes slightly disordered, i.e. S increases. This is an illustra-
tion of the Third Law of Thermodynamics

GIBBS FREE ENERGY, G, AND CHANGE IN GIBBS FREE
ENERGY, AG
AH and AS can be combined to give another state function, AG,
which is the change in Gibbs Free Energy. G, the Gibbs Free Energy,
is defined as:

I Gibbs Free Energy: G = H - TS (remembered by Gibbs HaTS) I

AG is a measure of the spontaneity of a reaction, i.e.

AG -ve for a spontaneous reaction;
AG + ve for a non-spontaneous reaction;
AG = 0 for a reaction at equilibrium (discussed in Chapter 4)

As AH and AS can assume both + ve and -ve values, this generates
four possible combinations: AG = AH - TAS

1. AH +ve and AS -ve =$ AG +ve, i.e. non-spontaneous at all
temperatures.
2. AH -ve and AS +ve j AG -ve, i.e. spontaneous at all
temperatures.
3. The other two combinations, AH +ve/AS +ve and AH
- ve/AS - ve respectively, are temperature dependent.
AH +ve and AS +ve (a) low T j TAS small =$ AG +ve, i.e.