Question

According to solubility rules, most F- containing compounds are insoluble but most Cl-, Br-, and I- containing compounds are soluble. Briefly explain this observation?

Answer 1

Two factors contribute to the reduced solubility....

Explanation:

`(i)` `"Enthalpy"`, `"a priori"`, we would expect that the bond strength of a binary salt, `MX`, would INCREASE as the size of the halide counterion DECREASED. And, certainly, fluoride is the SMALLEST halide ion. Recall that Coulomb's law is an inverse square law....`F_"electrostatic"=k_e(q_1q_2)/r^2`, where `k_e="Coulomb's constant"`, `(k_e = 9.0×10^9* N* m^2* C^(−2))`.

And also please note the dissolution reaction of ionic salt in water is CLEARLY a bond breaking chemical reaction,,,

`MX(s) stackrel(H_2O)rarrM^+ + X^-`

`(ii)` `"Entropy"`, it is a fact that the dissolution of the fluoride anion is ENTROPICALLY disfavoured, in that the smaller fluoride is more effectively solvated given its smaller charge density.

PS Could we use the same arguments to account for the reduced acidity of `HF` in aqueous solution, versus that of `HCl`, `HBr`, and `HI`?

PPS I would like to see some data on the solubilities of the common alkali halides...a scientist interrogates data, typically they don't make pronunciamentoes (except for me here!).