Question

Ionic salts NaCl and AgBr crystallize in the same crystal system (face centered cubic).While NaCl is well soluble in water (6.14 M at 25 ºC), AgBr has poor solubility in the same solvent (7.3 × 10-7 M). The radii of the ions are: Ag+ 115 ppm, Na+ 102 ppm, Clˉ 181, Brˉ 196 ppm. Provide an explanation for the difference in solubility

Answer 1

Solubility of a substance depends upon two factors:

Lattice enthalpy and hydration enthalpy

Lattice enthalpy is the energy required to break 1mol of compound into ions.

Hydration enthalpy is the energy released when a substance undergo hydration.

For a substance to be soluble in water , lattice enthalpy should be less and hydration enthalpy should be high.

Lattice enthalpy is directly proportional to product of charge and inversely proportional to size of ions. Hydration enthalpy depends on charge to radius ratio of the ions

Due to high charge to radius ratio of NaCl its hydration enthalpy is greater than AgBr. Hence NaCl is well soluble in water.

Solubility also depends upon type of interaction present.

NaCl is ionic compound , hence soluble in water.

But in AgBr there is covalent interations (Ag+ due to its pseudo noble gas configuration polarizes large Br- ) hence less soluble in water.