The oxidation of SO₂ to SO₃ is one of the reactions involved in the formation of acid rain. If you want to predict the spontaneous direction of the reaction for a specific mixture of the gases, you need to calculate the reaction quotient under those conditions. The standard Gibbs free energy of reaction for 2 SO₂ (g) + O₂ (g) → 2 SO₃ (g) is ΔG_r° 5 –141.74 kJ · mol^–1 at 25.00°C.

(a) What is the Gibbs free energy of reaction when the partial pressure of each gas is 100. bar?

(b) What is the spontaneous direction of the reaction under these conditions?

PLAN Calculate the reaction quotient and substitute it and the standard Gibbs free energy of reaction into Eq. 5. If ΔG_r r > 0, the reverse reaction is spontaneous at the given temperature and composition. If ΔG_r = 0, there is no tendency to react in either direction; the reaction is at equilibrium. At 298.15 K, RT = 2.479 kJ · mol^–1.

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AG₁ = AG,+ RT In Q (5)