Solid sodium acetate dissociates completely when dissolved in water according to the…

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

Problem

Solid sodium acetate dissociates completely when dissolved in water according to the equation NaC2H3O2(s)→Na+(aq)+C2H3O2−(aq)\mathrm{NaC_{2}H_{3}O_{2}(s)} \rightarrow \mathrm{Na^{+}(aq)} + \mathrm{C_{2}H_{3}O_{2}^{-}(aq)} At 298 K the standard entropy change for the process is ΔSrxn∘=+216.0 J mol−1 K−1\Delta S^\circ_{\text{rxn}} = +216.0\,\text{J\,mol}^{-1}\,\text{K}^{-1}. Tabulated values are provided below. Substance  S∘(298 K)S^\circ(298\,\text{K}) NaC2H3O2(s)\mathrm{NaC_{2}H_{3}O_{2}(s)}  125.2 J mol−1 K−1125.2\,\text{J\,mol}^{-1}\,\text{K}^{-1} Na+(aq)\mathrm{Na^{+}(aq)}    59.0 J mol−1 K−159.0\,\text{J\,mol}^{-1}\,\text{K}^{-1} (a) Determine the standard molar entropy of the acetate ion, C2H3O2−(aq)\mathrm{C_{2}H_{3}O_{2}^{-}(aq)}, at 298 K. (b) Using molecular-level reasoning that considers solvation and ion--dipole interactions, explain why the dissolution process exhibits a large positive ΔS∘\Delta S^\circ, even though the ion in part (a) is surrounded by an ordered hydration shell.

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