Gibbs free energy: when is a reaction spontaneous?
A reaction has ΔH > 0 and ΔS > 0. When is it spontaneous?
Answer C: explanation for every choice
A
Spontaneity at all temperatures requires ΔH < 0 and ΔS > 0.
B
Spontaneity only at low temperatures applies when ΔH < 0 and ΔS < 0.
C
Correct. ΔG = ΔH − TΔS becomes negative once TΔS exceeds ΔH.
D
Never spontaneous applies when ΔH > 0 and ΔS < 0.
TakeawayΔG = ΔH − TΔS. The signs of ΔH and ΔS determine which temperatures make ΔG negative.
Equation usedGibbs free energy
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