Page 49 - A Working Method Approach For Introductory Physical Chemistry Calculations

Thermodynamics II 33

WORKING METHOD FOR THE CALCULATION OF AG TYPE
PROBLEMS
A common problem on examination papers is, when given a set of
AH" and So data, you are asked to evaluate AGR,. The following
working method describes a skeleton step-by-step outline on how to
approach such a problem.

1. Read the question carefully.
2. Identify the species involved (the reactants and the products)
and identify their states, i.e. (*) = (s), (1) or (g).
3. Write down a balanced chemical equation, with the states
indicated, i.e. vAA(+) + VBB(+) --+ v&+) + nDD(*), where VA,
vB, vc and vD, are the stoichiometry factors.
4. Determine AH;xn = [( vc x AH;(,,) + (VD x AH;(,,,)] -
[(VA xAH;(A)) + (vg x AH&,))], i.e. AH&, = C[AH;(Products)]
- C[AH;(Reactants)]. Remember AH;(element) = 0, and do
not forget the units.
5. Determine ASK,, in a similar fashion: ASR, = C[S"(Products)]
- C[S"(Reactants)], but S"(e1ement) is not equal to O! Write
down the units of AS".
6. Determine the temperature, T in K. Remember T(K) =
[T("C) + 2731 K.
7. Convert AH" and AS" to the same system of units, i.e. AH" in
J mol-' and AS" in J K-' mol-' or AH" in kJ mol-' and AS"
in kJ K-* mol-'.
8. Determine the value of AGO, using the equation
AGO = AH" - TAS", i.e. 'Gibbs HaTS'!
9. Answer any riders to the question. For example: AH" -ve,
exothermic reaction; AH" + ve, endothermic reaction; AS" - ve,
decrease in the entropy or the disorder of the reaction; AS' + ve,
increase in the entropy or the disorder of the reaction; AGO - ve,
spontaneous reaction; AGO + ve, non-spontaneous reaction;
AG = 0 reaction at equilibrium (explained in Chapter 4), where
K is defined as the equilibrium constant.
10. At equilibrium,

1 AG = AGO + RTlnK= 0 I + 1nK = -AG"/(RT).