Thermo Chemistry

Question

From the values of ΔH and ΔS predict which of the following reactions would be spontaneous at 25°C. Calculate the minimum temperature at which each reaction will become spontaneous.

ΔH = -152 ^kJ/_mol, ΔS = 260 ^J/_{K · mol}

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Answer 1

Find ∆G:

∆G = ∆H - T∆S

∆H = -152 kJ/mol

∆S = 260 J/Kmol = 0.260 kJ/Kmol

T = 25ºC = 298K

∆G = -152 kJ/mol - (298K)(0.260 kJ/Kmol)

∆G = -152 kJ/mol - 77.48 kJ/mol

∆G = - 229 kJ/mol

This is a spontaneous reaction since ∆G is negative

To find the minimum temperature for spontaneity, find the temperature at which ∆G first becomes negative:

∆H - T∆S ≤ 0

-152 - (T)(0.260) ≤ 0

-0.260 T ≤ 152

T ≤ -584.6 K