Why do many fluorides tend to be less soluble than the corresponding chlorides?

The solubility of a salt in water depends upon two factors

(a) Lattice enthalpy

(b) Hydration enthalpy

If the hydration enthalpy is greater than lattice enthalpy (hydration enthalpy > lattice enthalpy), then the salt is soluble in water. However if hydration enthalpy < lattice enthalpy, the salt is insoluble in water.

Now, as the fluoride ion is smaller than the chloride ion, therefore the charge per unit volume (or charge density) is more in case of fluorides than chlorides. Hence, the lattice energy of fluorides is generally more than that of chlorides and if the lattice energy of a salt is high, its dissolution is less. This is why the solubility of fluorides is generally less than the corresponding chlorides.

 

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