The dissolution of sodium chloride in water at can be described by the sequence of…

CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry

Problem

The dissolution of sodium chloride in water at

298\,\text{K}

can be described by the sequence of elementary processes below. All values refer to one mole of

\text{NaCl}

. Process I:

\text{NaCl(s)} \rightarrow \text{Na}^+(g) + \text{Cl}^-(g)

\Delta S^\circ_I = +240.0\,\text{J mol}^{-1}\,\text{K}^{-1}

Process II:

\text{Na}^+(g) \rightarrow \text{Na}^+(aq)

\Delta S^\circ_{II} = -155.0\,\text{J mol}^{-1}\,\text{K}^{-1}

Process III:

\text{Cl}^-(g) \rightarrow \text{Cl}^-(aq)

\Delta S^\circ_{III} = -180.0\,\text{J mol}^{-1}\,\text{K}^{-1}

Consider the overall dissolution reaction:

\text{NaCl(s)} \rightarrow \text{Na}^+(aq) + \text{Cl}^-(aq)

(a) Calculate the standard entropy change

\Delta S^\circ_{\text{diss}}

for the dissolution of

\text{NaCl(s)}

298\,\text{K}

. (b) The standard enthalpy change for this dissolution is

\Delta H^\circ_{\text{diss}} = +3.9\,\text{kJ mol}^{-1}

. Using the sign and relative magnitudes of

\Delta H^\circ_{\text{diss}}

and your calculated

\Delta S^\circ_{\text{diss}}

, determine whether the dissolution of

\text{NaCl(s)}

is spontaneous at

298\,\text{K}

. Justify your answer quantitatively. (c) Predict qualitatively how increasing the temperature would affect the spontaneity of

\text{NaCl}

dissolution, based on the thermodynamic quantities involved.