4.66 - Thermodynamics study questions - 5

Q466-05 Consider the following reaction in the Contact Process for the production of sulfuric acid:
Temperature /K Equilibrium constant Kc /dm3 mol-1
298 9.77 x 1025
500 8.61 x 1011
700 1.75 x 106

The value of ΔG for the reaction is -140 kJ at 298 K. What can be deduced by the sign of ΔG

The reaction is spontaneous under standard conditions.

The values of ΔH and ΔS for the reaction at 298K are ΔH= -196 kJ and ΔS = -188 JK-1. State and explain what will happen to the spontaneity of the reaction is the temperature of the reaction is increased.

As the entropy term is negative, the second part of the Gibbs free energy relationship, (-TΔS), becomes positive. The higher the temperature the more positive the term. Hence, a temperature will be reached at which the magnitude of the second term is larger than that of the enthalpy term. At this temperature the reaction becomes non-spontaneous.

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