How does electronegativity affect acidity?

1 Answer
anor277
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.

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