Gibbs Free Energy

Gibbs free energy brings together everything we have covered in this course — enthalpy and entropy — into a single quantity that tells us whether a reaction is thermodynamically feasible. This is one of the most important concepts in chemistry, and a frequent topic in A-Level exam questions.

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The Gibbs Free Energy Equation

ΔG = ΔH − TΔS

where:

• ΔG = Gibbs free energy change (kJ mol⁻¹)
• ΔH = enthalpy change (kJ mol⁻¹)
• T = temperature in kelvin (K)
• ΔS = entropy change (convert to kJ K⁻¹ mol⁻¹ to match ΔH units)

A reaction is thermodynamically feasible (spontaneous) when ΔG < 0 (negative).

A reaction is not feasible when ΔG > 0 (positive).

When ΔG = 0, the system is at equilibrium.

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The Four Combinations of ΔH and ΔS

The signs of ΔH and ΔS together determine whether a reaction is feasible and how temperature affects feasibility.

Case 1: ΔH negative, ΔS positive

ΔG = (negative) − T(positive) = negative − positive = always negative

The reaction is feasible at all temperatures. Both the enthalpy and entropy changes favour the reaction.