What is Gibbs Free Energy?
Gibbs free energy (G) predicts whether a reaction is spontaneous at constant temperature and pressure. The formula ΔG = ΔH − TΔS combines enthalpy and entropy to determine the spontaneity and feasibility of reactions.
ΔG = ΔH − TΔS. If ΔG < 0, the reaction is spontaneous (favourable); if ΔG > 0, it is non-spontaneous (requires energy).
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Step-by-step worked examples
ΔH = −50 kJ/mol, ΔS = 120 J/(mol·K), T = 298 K. Is the reaction spontaneous?
ΔG = ΔH − TΔS = −50 − 298×(120/1000) ΔG = −50 − 35.76 = −85.76 kJ/mol ΔG < 0 → spontaneous (very favourable)
ΔH = 80 kJ/mol, ΔS = 100 J/(mol·K), T = 400 K. Is it spontaneous?
ΔG = 80 − 400×(100/1000) = 80 − 40 = 40 kJ/mol ΔG > 0 → non-spontaneous (requires energy)
At what temperature does ΔG = 0 for ΔH = 60 kJ, ΔS = 150 J/K?
0 = ΔH − TΔS → T = ΔH/ΔS T = 60 000 / 150 = 400 K (phase transition point)
Flashcards
Quick quiz
Q1.ΔG = ΔH − TΔS. When is reaction spontaneous?
Q2.What happens at ΔG = 0?
Q3.Entropy-driven reaction: ΔH and ΔS signs?
Q4.How does T affect ΔG = ΔH − TΔS if ΔS > 0?
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Common mistakes
ΔG = ΔH always. — Correct: ΔG = ΔH − TΔS; entropy term matters at higher T.
ΔG > 0 = spontaneous. — Correct: ΔG < 0 = spontaneous (negative is favourable).
Equilibrium means ΔG is very negative. — Correct: At equilibrium, ΔG = 0 (no driving force).
Entropy has no effect on ΔG at low T. — Correct: TΔS is always subtracted; small T means small entropy contribution.
FAQ
What is the Gibbs free energy formula?
ΔG = ΔH − TΔS. It combines enthalpy, entropy, and temperature to predict spontaneity.
Why does temperature affect ΔG?
Higher T amplifies the entropy term (−TΔS), making entropy-driven reactions more favourable.
Can a reaction have ΔG < 0 but be slow?
Yes — ΔG predicts spontaneity (direction), not reaction rate (activation energy matters).
What is a coupled reaction?
A non-spontaneous reaction (ΔG > 0) driven by coupling to a spontaneous one (ΔG << 0).