Chemistry / Unit 11 Chem 2112 short lesson, about 20 seconds
Temperature-Dependent Spontaneity and the Crossover Temperature
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The idea
For a reaction whose ΔH and ΔS share a sign, ΔG's sign depends on temperature, switching where ΔH equals TΔS exactly.
In practice
A reaction with ΔH and ΔS both positive is nonspontaneous at low temperature but spontaneous past its crossover point.
Where people slip
Calling a reaction "spontaneous" without stating the temperature ignores its own crossover.
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