Question

Problem:Discuss the justification for this statement: “Although one does not normally regard ${\mathbf{NH}}_{\mathbf{4}}^{\mathbf{+}}$as an acid, it is actually only slightly weaker as an acid than hydrocyanic acid, HCN, in aqueous solution.”

Short answer

Ammonium ions are less acidic than Hydrocyanic acid because it has more pH value than Hydrocyanic acid.

Step-by-step solution

Acidity of Ammonium ion

The ammonium ion $\left({\text{NH}}_{\text{4}}^{\text{+}}\right)$is formed by pronation of ammonia on dissolving on water released the proton and form the ammonia shown as follows:

${\mathrm{NH}}_4^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{NH}}_4+{\mathrm{H}}^{+}$

Ammonium ions are considered a weak acid as they donate their proton to neutralize their positive charge and show a pH of around 5.5.

Acidity of Hydrocyanic acid

Hydrocyanic acid is the acid in the water as it releases the proton and forms the stable conjugate base cyanide ion shown as follows:

$\mathrm{HCN}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CN}}^{-}+{\mathrm{H}}^{+}$

Hydrocyanic acid shows a pH of around 5.2.

Comparing acidity

Acid showing less pH is more acidic.

Ammonium ion showed a pH of around 5.5, and Hydrocyanic acid showed a pH of around 5.2, making the Hydrocyanic acid stronger than the ammonium ion in the aqueous solution.