Gibbs Free Energy and Temperature

Whether a process is spontaneous is decided by the Gibbs free energy change, ΔG = ΔH − TΔS. The balance weighs the enthalpy term, ΔH (heat exchanged with the surroundings), against the entropy term, TΔS (energy dispersal in the system), which grows as the temperature rises. Work through the three stages in order.

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ΔG = ΔH − TΔS

Temperature

Choose a process:

The four cases
	ΔS positive (favours)	ΔS negative (opposes)
ΔH negative (favours)	Spontaneous at all temperatures	Spontaneous only below the crossover
ΔH positive (opposes)	Spontaneous only above the crossover	Never spontaneous

Remember

ΔG = ΔH − TΔS. A process is spontaneous when ΔG < 0.
A negative ΔH (heat released to the surroundings) favours a process; a positive ΔS (energy more widely dispersed) favours it. Temperature multiplies only the entropy term.
When ΔH and ΔS have the same sign, the balance levels at the crossover temperature T = ΔH ÷ ΔS. Convert ΔS from J to kJ first.
The values for the antibody, DNA and drug are representative. For proteins and DNA, ΔH and ΔS themselves change with temperature, and the DNA crossover also depends on concentration, so the model gives a simplified picture.