Question

Explain the concept of relative strength of acids.

Answer 1

We can rank the strengths of acids by the extent to which they ionize in aqueous solution. The reaction of an acid with water is given by

$H{A}_{\left(aq\right)}+H_2{O}_{\left(l\right)}⟶H_3{O}_{\left(aq\right)}^{+}+A_{\left(aq\right)}^{-}$

A strong acid yields $100$% (or very nearly so) of $H_3{O}^{+}$ & $A^{-}$ when the acid ionizes water. A weak acid gives small amounts of $H_3{O}^{+}$ & $A^{-}$. The relative strengths of acids may be determined by measuring their equilibrium constants. Let it be $K_a$. It is called acid-ionization constant & is given by,

$K_a=\frac{\left[H_3{O}^{+}\right]\left[A^{-}\right]}{\left[HA\right]}$

The stronger acid has a larger ionisation constant than does a weaker acid. the $K_a$ increases as the strengths of the acid increases.

For $C{H}_{3}C{O}_{2}H<HN{O}_2<{HS{O}_4}^{-}$

$K_a$ $1.8\times {10}^{-5}$ $4.6\times {10}^{-4}$ $1.2\times {10}^{-2}$

Another measure is its percent ionisation i.e. % ionisation= $\frac{\left[H_3{O}^{+}\right]}{[HA{]}_o}\left(aq\right)\times 100$