Question

Be sure to answer all parts. From the values of ΔH and ΔS, predict which of the following reactions would be spontaneous at 21°C: reaction A: ΔH = 10.5 kJ/mol, ΔS = 30.0 J/K·mol; spontaneous nonspontaneous impossible to tell reaction B: ΔH = 1.8 kJ/mol, ΔS = −113 J/K · mol, spontaneous nonspontaneous impossible to tell If either of the reactions is nonspontaneous, can it(they) become spontaneous? yes, reaction A can become spontaneous yes, reaction B can become spontaneous yes, both reactions can become spontaneous no, neither reaction can become spontaneous If either of the reactions is nonspontaneous but can become spontaneous, at what temperature might it become spontaneous? 0 °C

Answer 1

Answer:-

For a reaction to be spontaneous , the value of ∆G (free-energy change) must be negative.

Reaction A:-

T = 21°C = (273.15 + 21) K = 294.15 K

∆H = 10.5 kJ /mol = 10500 J/mol

∆S = 30 J /K-mol

∆G = ∆H - T∆S

∆G = 10500 - (294.15 × 30) = (10500 -8824.5) J/mol

∆G = +1675.5 J/mol

Hence, reaction A is non-spontaneous.

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Reaction B :-

​​​​​​T = 294.15 K

∆H = 1.8 kJ / mol = 1800 J /mol

∆S = -113 J /K-mol

∆G = 1800 - ( 294.15 × -113) = (1800 + 33238.95) J/mol = +35038.95 J/mol

Hence, Reaction B is non-spontaneous.

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Reaction A can become spontaneous at relatively higher temperature because on increasing the temperature, the (-T∆S) becomes more negative. At 334 K (approx.) or higher temperature than 334 K , reaction A becomes spontaneous.