Chemistry

Problem

A chemistry student investigates two dissolution processes carried out at 298 K. Process A:

\mathrm{NH_4Cl(s)} \rightarrow \mathrm{NH_4^+(aq)} + \mathrm{Cl^-(aq)}

Process B:

\mathrm{CaCO_3(s)} \rightarrow \mathrm{Ca^{2+}(aq)} + \mathrm{CO_3^{2-}(aq)}

(a) For each process, predict the sign of

\Delta S^\circ

and support your prediction by discussing changes in the amount of disorder at both the particulate level (ions vs. crystal lattice) and the macroscopic level (phase considerations). (b) Without performing numerical calculations, decide which process is expected to have the larger magnitude of

\Delta S^\circ

. Justify your selection by comparing relevant structural or stoichiometric factors. (c) The student proposes grinding each solid into a very fine powder before dissolution. Discuss qualitatively whether this physical change is expected to alter the sign or the magnitude of

\Delta S^\circ

for either process, and explain your reasoning. (a) Predict and justify the sign of

\Delta S^\circ

for Process A. (a) Predict and justify the sign of

\Delta S^\circ

for Process B. (b) Identify which process should exhibit the larger

|\Delta S^\circ|

and justify your choice. (c) Explain qualitatively how powdering the solids before dissolution would affect the sign and relative magnitude of

\Delta S^\circ

for each process.

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