Multiple Choice
What is the entropy change (ΔS) of a reaction that has an enthalpy change (ΔH) of -70 kJ and a Gibbs free energy change (ΔG) of 18 kJ at a constant temperature of 76 degrees Celsius?
19. Chemical Thermodynamics
Gibbs Free Energy Calculations
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Fe2O3 (s) + 3 H2 (g) → 2 Fe (s) + 3 H2O (g) is a redox reaction. What would be its Gibbs Free energy change under standard conditions? Is the reaction spontaneous at 25 °C?
A
513.6 kJ; No, the reaction is not spontaneous at 25 °C.
B
56.4 kJ; No, the reaction is not spontaneous at 25 °C.
C
−513.6 kJ; Yes, the reaction is not spontaneous at 25 °C.
D
56.4 kJ; Yes, the reaction is spontaneous at 25 °C.
Verified step by step guidance1
Identify the substances involved in the reaction: Fe2O3 (s), H2 (g), Fe (s), and H2O (g).
Use the standard Gibbs free energy of formation (G°f) values from the table for each substance: Fe2O3 (s) = -742.2 kJ/mol, H2 (g) = 0 kJ/mol, Fe (s) = 0 kJ/mol, and H2O (g) = -228.6 kJ/mol.
Apply the formula for the standard Gibbs free energy change of the reaction: ΔG° = Σ(G°f products) - Σ(G°f reactants).
Calculate the Gibbs free energy for the products: 2 mol Fe (s) * 0 kJ/mol + 3 mol H2O (g) * (-228.6 kJ/mol).
Calculate the Gibbs free energy for the reactants: 1 mol Fe2O3 (s) * (-742.2 kJ/mol) + 3 mol H2 (g) * 0 kJ/mol. Subtract the reactants' total from the products' total to find ΔG° and determine if the reaction is spontaneous (ΔG° < 0) at 25 °C.
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