For two reactions carried out at both 298 K and 500 K

CHEMISTRY · Multi-part · A Level · Chemistry · Thermodynamics · Thermodynamics

Problem

For two reactions carried out at both 298 K and 500 K, the standard enthalpy and entropy changes are listed below. Reaction C:

\Delta H^{\circ}_C = +95\,\text{kJ\,mol}^{-1}

\Delta S^{\circ}_C = +250\,\text{J\,mol}^{-1}\,\text{K}^{-1}

Reaction D:

\Delta H^{\circ}_D = -40\,\text{kJ\,mol}^{-1}

\Delta S^{\circ}_D = -120\,\text{J\,mol}^{-1}\,\text{K}^{-1}

C and D share a common intermediate and can therefore be coupled. (a) Without performing a numerical calculation, predict the sign of

\Delta G^{\circ}

for Reaction C at 298 K and at 500 K. Justify your prediction by discussing the relative contributions of the enthalpy and entropy terms. (b) Perform an explicit calculation of

\Delta G^{\circ}

for Reaction D at 298 K and at 500 K. Show all work and report each result with appropriate units. (c) Using your answers from parts (a) and (b), determine at which of the two temperatures (if either) the coupling of C and D will yield an overall negative

\Delta G^{\circ}

. Support your conclusion with quantitative reasoning. (d) Discuss one practical consideration—other than temperature—that experimental chemists must control to ensure that the coupled pathway proceeds as intended.

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