The data below list the lattice enthalpy (energy required to separate the crystal…

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

Problem

The data below list the lattice enthalpy (energy required to separate the crystal into gaseous ions) and the molar hydration enthalpy of the ions for three lithium halides at

25\,\text{°C}

Salt	$\Delta H_{\text{lattice}}$ ( $\text{kJ}\,\text{mol}^{-1}$ , + values endothermic)	$\Delta H_{\text{hydration}}$ ( $\text{kJ}\,\text{mol}^{-1}$ , - values exothermic)
LiF	+1030	-920
LiCl	+845	-835
LiI	+705	-760

Assume that the molar entropy of dissolution,

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

, is approximately

+37\,\text{J}\,\text{mol}^{-1}\,\text{K}^{-1}

for each salt and is temperature-independent over the range considered. Use these data to analyze the thermodynamic favorability and relative solubilities of the three salts at

298\,\text{K}

. (a) Determine the molar enthalpy of solution,

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

, for each salt. (b) Using the given

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

value, calculate the standard free-energy change,

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

, for dissolution of each salt at

298\,\text{K}

. (c) Rank the three salts from most soluble to least soluble in water at

298\,\text{K}

, based on your calculated

\Delta G^\circ

values. (d) Justify the trend you obtained in part (c) by referring explicitly to the competing energetic factors that contribute to

\Delta G^\circ

Related theory

Browse the problem library