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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.
ΔG = ΔH − TΔS
where:
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.
The signs of ΔH and ΔS together determine whether a reaction is feasible and how temperature affects feasibility.
ΔG = (negative) − T(positive) = negative − positive = always negative
The reaction is feasible at all temperatures. Both the enthalpy and entropy changes favour the reaction.
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