How does electronegativity affect acidity?

1 Answer
Aug 28, 2017

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.