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

1 Answer
anor277
Sep 29, 2016

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 HFHF, a weak acid, with a very electronegative conjugate base, F^-F−.

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

HA +H_2O rightleftharpoons H_3O^+ + A^-HA+H2O⇌H3O++A−

The extent of reaction depends on (i) the strength of H-AH−A bond, and (ii) the solvation of the A^-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.

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