Question

How does electronegativity affect acidity?

Answer 1

The strength of an acid is given by the extent of the following equilibrium.......

Explanation:

`HX + H_2O rightleftharpoonsH_3O^+ + X^-`

Now for the hydrogen halides, the more ELECTRONEGATIVE the halide, the WEAKER the acid, i.e. for `HF`, significant amounts of `HF` remain undissociated, and when a fluoride salt, say `NaF`, is dissolved in water, the solution is ALKALINE given the following equilibrium.....

`F^(-) + H_2O rightleftharpoons HF + HO^-`

For `HCl`, `HBr`, and `HI`, because the conjugate base is larger, and more diffuse, and LESS charge-dense, the equilibrium lies almost entirely to the right.......

`HX +H_2OrarrH_3O^+ + X^-`, `X=Cl, Br, I, !=F`

So in this (special!) instance, electronegativity militates against acidity. Of course, here, an entropy effect also is important, in that the `F^-` ion is entropically disfavoured.