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Entropy and Free Energy 257
Quick Tip
NH 4Cl(s) 203.9 kJ/mol
CaO(s) 604.2 kJ/mol
CaCl 2(s) 750.2 kJ/mol
H 2O(l) 237.2 kJ/mol
NH 3(g) 16.6 kJ/mol
All sources may not contain identical values for thermodynamic values.
Thermodynamic values are revised from time to time. You should use whatever
values your instructor or textbook provides.
To finish this problem we need to use the following relationship:
∆G ∆G products G reactants
∆G [(1 mol CaCl 2)(750.2 kJ/mol) (1 mol H 2O)(237.2 kJ/mol)
(2 mol NH 3)(16.6 kJ/mol)] [(1 mol CaO)(604.2 kJ/mol)
(2 mol NH 4Cl)(203.9 kJ/mol)]
∆G 8.6 kJ
Using the relationship: ∆G° ∆H° rxn – T∆S° rxn, calculate ∆G° for the following
reaction.
2 Al(s) 3 S(s) 6 O 2(g) l Al 2(SO 4) 3(s)
To begin the problem, we need the standard enthalpy and entropy values for
each of the reactants and products. These are the values we’ll be using to complete
this problem:
H f(kJ/mol) S(J/mol K)
Al(s) 0 28.32
Al 2(SO 4) 3(s) 3441 239
O 2(g) 0 205.0
S(s) 0 31.88