Chemistry / Unit 11 Chem 2121 worked example, 60 to 90 seconds
Calculating ΔG°rxn from ΔG°f Values
The idea
Calculating a reaction's standard free energy change means summing products' ΔG°f values, each multiplied by its stoichiometric coefficient, and subtracting the same sum for reactants, the identical pattern already used for ΔH°rxn and, at Chem 2099, for ΔS°rxn.
In practice
For CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l), with ΔG°f values −50.5, 0, −394.4, and −237.1 kJ/mol respectively, ΔG°rxn = [−394.4 + 2(−237.1)] − [−50.5 + 0] = −868.6 − (−50.5) = −818.1 kJ, strongly negative and spontaneous, consistent with methane combustion proceeding readily once ignited. Oxygen's ΔG°f of exactly zero, as an element in its standard state, contributes nothing to the reactant sum despite appearing in the balanced equation.
Where people slip
Including an element in its standard state as anything but zero in this sum, the same slip possible with ΔH°f, throws off the whole calculation and is easy to miss when an equation lists several elements at once.
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