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

1 Answer
Sep 29, 2016

Answer:

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.