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Question

H and S for the equilibrium reaction, H2O(l)H2O(g) at 1 atmospheric pressure are 40.63 kJmol1 and 108.8 JK1mol1 respectively. Calculate the temperature at which the reaction occur.
Also, predict the sign of free energy for this transformation above this temperature.

A
373.4K, negative
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B
373.4K, positive
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C
463.4K, negative
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D
463.4K, positive
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Solution

The correct option is A 373.4K, negative
Given:
H=40.63 kJ mol1
S=108.8 JK1mol1=0.1088 kJK1mol1

G=0 (as the system is in equilibrium)
Applying Gibb's free energy equation:
G=HT S
0=40.63T×0.1088
T=40.630.1088=373.4K

The sign of G above this temperature will become negative as magnitude of (TΔS>ΔH) and the reaction will be spontaneous.

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