Question

Calculate free energy (in $kJ$ ) when $1$ mole of a an ionic salt $MX$ (s) is dissolved in water at $25^{\circ} C$. Given
Lattice energy of $MX =780 \,kJ\, mol ^{-1}$
Hydration energy of $MX =-775.0 \,kJ\, mol ^{-1}$
Entropy change of dissolution at $25^{\circ} C =45\, J\, mol ^{-1} K ^{-1}$

Answer 1

$\Delta H _{\text {dissolution }}=\Delta H _{\text {(ionisation) }}+\Delta H _{\text {(hydration) }}$
$=780-775=5 \,kJ\, mol ^{-1}$
$\Delta G _{\text {dissolution }}=\Delta H _{\text {dissolution }}- T \Delta S _{\text {dissolution }}$
$=5000-298 \times 45=-8410 \,J =8.41\, kJ \,mol ^{-1}$