Calculate the standard Gibbs free energy change at 298 K for the reaction FeO4 (s) + 4 CO (g) ⇌ 3 Fe (s) + 4 CO2 (g).

• A. -14.8 kJ/mol
• B. -757 kJ/mol
• C. 757 kJ/mol
• D. -757 kJ/mol

Answer: (A)

The concept being tested is how to get the standard Gibbs free energy change from standard Gibbs energies of formation. For a reaction, ΔG°rxn = ΣνpΔGf°(products) − ΣνrΔGf°(reactants). Elements in their standard state contribute zero, so Fe(s) has no effect here.

For this reaction, the products are 4 CO2, each with ΔGf° ≈ −394.4 kJ/mol, giving 4 × (−394.4) = −1577.6 kJ. The reactants are FeO4(s) with ΔGf° ≈ −1014.0 kJ/mol and 4 CO with ΔGf° ≈ −137.2 kJ/mol each, giving (−1014.0) + 4 × (−137.2) = −1562.8 kJ. Therefore, ΔG°rxn = (−1577.6) − (−1562.8) = −14.8 kJ/mol.

So the standard Gibbs free energy change at 298 K is negative, indicating the reaction is thermodynamically favorable under standard conditions.