A chemist attempts to predict the standard free energy of dissolution of three…

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

Problem

A chemist attempts to predict the standard free energy of dissolution of three chloride salts in water at

298\,\text{K}

using only published values of lattice enthalpy (

\Delta H_{\text{lat}}

) and ion-hydration enthalpy (

\Delta H_{\text{hyd}}

). The table below summarizes the enthalpic data and experimentally measured entropy changes (

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

Salt	$\Delta H_{\text{lat}}$ ( $\text{kJ\,mol}^{-1}$ )	$\Delta H_{\text{hyd}}$ ( $\text{kJ\,mol}^{-1}$ )	$\Delta S^\circ_{\text{exp}}$ ( $\text{J\,mol}^{-1}\,\text{K}^{-1}$ )
NaCl	787	−783	+43
MgCl₂	2526	−2520	−70
NH₄Cl	705	−715	+112

Assume the chemist calculates

\Delta G^\circ_{\text{calc}} = \Delta H_{\text{lat}} + \Delta H_{\text{hyd}} - T(\Delta S^\circ_{\text{exp}})

. (a) For each salt, calculate

\Delta G^\circ_{\text{calc}}

298\,\text{K}

. Report values to three significant figures. (b) Rank the salts from most thermodynamically favorable to least favorable for dissolution according to your calculated

\Delta G^\circ_{\text{calc}}

values. (c) Experimental solubility measurements give the order NH₄Cl > NaCl ≫ MgCl₂. Identify and explain two limitations of the chemist’s calculation that could account for the disagreement with experiment. (d) Propose one additional experimental measurement (beyond

\Delta S^\circ

) that would improve the reliability of a theoretical prediction of

\Delta G^\circ

for these salts, and justify how the measurement addresses at least one limitation identified in part (c).

Related theory

Browse the problem library