With reference to eq14 of the previous article, where , we can define the standard reaction Gibbs energy as:
or in general, the reaction Gibbs energy as: .
Furthermore, just as the standard reaction enthalpy is , and the standard reaction entropy of a system is
, the standard reaction Gibbs energy of a system is:
where ΔGfo is the standard reaction Gibbs energy for the formation of a compound from its elements in their reference states, and vP and vR are the stoichiometric coefficients of the products and reactants respectively.
ΔGro, like ΔHro, is computed using energies of formation because we cannot determine the absolute Gibbs energy of a substance (unlike entropy). Even so, the terms ΔGro, ΔHro and ΔSro in eq15 are comparable, as the reference states in and
as well as those in
and
cancel out when we compute ΔGro and ΔHro respectively (see sections on standard enthalpy change of formation and Hess Law for details).
Combining eq15 and eq16,
Finally, ΔGro is used in the same way as ΔGsyso to predict the spontaneity of reactions, where ΔGro < 0 for a spontaneous reaction and ΔGro = 0 when the reaction attains equilibrium.
Question
Calculate the standard combustion Gibbs energy for the reaction given ΔGfo[C2H2(g)] = -32.9 kJmol-1, ΔGfo[CO2(g)] = -394.4 kJmol-1 and ΔGfo[H2O(l)] = -237.2 kJmol-1.
Answer
Using eq16 and noting that ΔGfo[O2(g)] = 0,
Since ΔGro < 0, the reaction is spontaneous.