Question

How do we assess whether an acid is strong in aqueous solution?

Answer 1

Acidity is an experimental phenomenon, and strong acids behave as strong acids do in water.

Explanation:

In general when hydrogen is bound to a strongly electronegative atom, then that hydrogen is acidic. However, this criterion fails when we consider the acidity of say `HF`, a weak acid, with a very electronegative conjugate base, `F^-`.

As with every other reaction, we look at the reaction itself:

`HA +H_2O rightleftharpoons H_3O^+ + A^-`

The extent of reaction depends on (i) the strength of `H-A` bond, and (ii) the solvation of the `A^-` conjugate base. (This is an enthalpy effect and an entropy effect,

i.e. `DeltaG^@=DeltaH^@-TDeltaS^@`

Anions for which the negative charge is delocalized, i.e. `NO_3^-`, or `SO_4^(2-)` tend to have strong conjugate acids. This is in contrast to `HF`, whose conjugate base, `F^-` is strongly polarizing and tends to induce solvent order. `HF` is thus a weaker acid than `HCl` or `HBr` on the basis of this entropy effect.

So, you just have to learn the common strong acids.