Lesson 5 of 12 · 7 min
K, Q and Gibbs energy
NCERT § "Relationship between Equilibrium Constant K, Reaction Quotient Q and Gibbs Energy G"
Thermodynamics has its own verdict on direction, the sign of ΔG. When the technician added N₂, Q said 'forward'. Does ΔG agree?
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The lesson in notes
In short
ΔG = ΔG° + RT ln Q for a reaction mixture of any composition.
At equilibrium ΔG = 0 and Q = K, giving ΔG° = −RT ln K = −2.303 RT log K.
K = e^(−ΔG°/RT); a negative ΔG° means K > 1 and a positive ΔG° means K < 1.
When ΔG < 0 the forward reaction proceeds (Q < K); when ΔG > 0 the reverse is favoured (Q > K).
Use R = 8.314 J K⁻¹ mol⁻¹ with ΔG° in J mol⁻¹, not kJ mol⁻¹.