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Effect of Temperature upon Spontaneity of a Reaction

Case ΔH ΔS° ΔG° = ΔH – TΔ Remarks
I - +
Always negative. For example,
C(s) + O2(g) → 2NO2(g)
Spontaneous at all temperatures; reverse reaction always non-spontaneous
II + -
Always positive. For example,
N2(g) + 2O2(g) → 2NO2(g)
Non-spontaneous at all temperatures; reverse
reaction takes place
III + +
Positive at low temperatures, negative at high temperature. For examples,
2NO2(g) → 2NO(g) + O2(g)
Non-spontaneous at low temperatures but becomes spontaneous at higher temperatures
IV - -
Negative at low temperatures, positive at high temperature. For example,
PCl3(g) + Cl2(g) → PCl5(g)
Spontaneous at low temperatures but at high temperatures reverse reaction becomes spontaneous

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