Question

For an ionic compound to be soluble in water, which of the following is correct

• A. Lattice energy < hydration energy
• B. Lattice energy > hydration energy
• C. Either a or b
• D. All ionic compounds are soluble in water

Answer 1

The correct option is A Lattice energy < hydration energy

The energy change on formation of the solution is usually reported as the enthaply of solution, $\Delta E_{solution}$. It is the sum of the lattice energy, $\Delta E_{lattice}$, the energy required to break the ionic solid apart into separated ions, and the hydration energy, $\Delta E_{hydration}$, the energy released when the separated ions become surrounded by water molecules.

$\Delta E_{solution}$ = $\Delta E_{hydration}+\Delta E_{lattice}$

If there is a net release of energy, that is, if more energy is released on hydration than is stored in the ionic solid lattice, then solubility is favored. If there is a net gain of energy, that is, if less energy is released on hydration than is stored in the ionic solid lattice, then solubility is unfavored.

If lattice energy (cost) of ionic compound is smaller than that of hydration energy (gain), the given compound will dissociate and it will be soluble in water.