Question

Calculate the standard free energy change for the reaction,

4NH₃ (g) + 5O₂ (g) 4NO (g) + 6H₂O (l)

Given that the standard free energies of formation (Δ_fG°) for NH₃ (g), NO (g) and H₂O (l) are – 16.8, + 86.7 and – 237.2 kJ mol^–1 respectively. Predict the feasibility of the above reaction at the standard state.

Answer 1

Here, we are given

Δ_f G° (NH₃ ) = – 16.8 kJ mol^–1 

Δ_f G° (NO) = + 86.7 kJ mol^–1 

Δ_f G° (H₂O) = – 237.2 kJ mol^–1 

∴ Δ_f G° = ∑_f G° (Products) ∑ Δ_f G° (Reactants) 

=[14 × Δ_f G° (NO) + 6 × Δ_f G°(H₂O)] – [4 × Δ_f G° (NH₃ ) + 5 × Δ_f G° (O₂ )]

= [4 × (86.7) + 6 × (–237.2)] – [4 × (– 16.8) + 5 × 0] = – 1009.2 kJ

Since Δ_f G° is negative, the process is feasible.