The lattice energy of solid NaCl is 180 kcal/mol. The dissolution of the solid in water, in the form — Thermodynamics and Thermochemistry Chemistry Question
Question
The lattice energy of solid NaCl is 180 kcal/mol. The dissolution of the solid in water, in the form of ions is endothermic to the extent of 1kcal/mol. If the solvation energies of $\text{Na}^+$ and $\text{Cl}^-$ ions are in the ratio 6:5, what is the enthalpy of hydration of sodium ion?
Answer: B
💡 Solution & Explanation
Total hydration enthalpy $\Delta H_{\text{hyd}} = \Delta H_{\text{sol}} - \text{LE} = 1 - 180 = -179\text{ kcal/mol}$. With a 6:5 ratio, let $\text{Na}^+ = 6x$ and $\text{Cl}^- = 5x$. $11x = -179 \implies x = -16.27$. Enthalpy of hydration for $\text{Na}^+$ is $6x = 6(-16.27) = -97.62\text{ kcal/mol}$.
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