Use the table below to answer the questions that follow.
Thermodynamic Quantities for Selected Substances at 298.15 K (25°C)
-The value of ΔG° at 373 °K for the oxidation of solid elemental sulfur to gaseous sulfur dioxide, is kJ/mol. At 298K, OH° for this reaction is - 269.9 kJ/mol, and OS° is +11.6 J/K.

Question

Use the table below to answer the questions that follow.
Thermodynamic Quantities for Selected Substances at 298.15 K (25°C) 
-The value of ΔG° at 373 °K for the oxidation of solid elemental sulfur to gaseous sulfur dioxide,  is kJ/mol. At 298K, OH° for this reaction is - 269.9 kJ/mol, and OS° is +11.6 J/K.

Quizplus · Accepted Answer

We can use the Gibbs-Helmholtz equation to find ΔG° at 373 K:
ΔG° = ΔH° - TΔS°
ΔH° = ΔU° + PΔV° = 0 (since solid S and gaseous SO2 have the same ΔU° and ΔV°)
ΔS° = 0 - R ln(1 bar/1 bar) = 0
ΔG° = -TΔS° = -(373 K - 298.15 K)(11.6 J/K/1000 J/kJ) = -2.38 kJ/mol
Now we use the relationship between ΔG° and the equilibrium constant K to find the answer:
ΔG° = -RT lnK
lnK = ΔG°/(-RT) = (2.38 kJ/mol)/[(8.314 J/mol-K)(373 K)] = -0.00074
K = e^(lnK) = e^(-0.00074) = 0.9993
The equilibrium constant is close to 1, which means the reaction is close to equilibrium at 373 K. Therefore, ΔG° for the reaction at 373 K is close to 0. We can estimate it to be around -2 kJ/mol. Since ΔG° becomes less negative (or more positive) as temperature increases, we choose the answer that is closest to -2 kJ/mol at 373 K, which is A) -274.2.

Use the Table Below to Answer the Questions That Follow