Question

Which of the following compounds is protonic acid?
A. $S{O_2}{(OH)_2}$
B. $B{(OH)_3}$
C. $PO{(OH)_3}$
D. $SO{(OH)_2}$

Answer 1

Hint: The term protonic is used to distinguish traditional acids from lewis acids from lowry-Bronsted acids in non-aqueous solvent. It produces ${H^ + }$ ion in the aqueous solutions.

Complete step by step answer: Let’s discuss each of the above options to find out the correct answer.
In aqueous solutions ${H_2}S{O_4}$ (sulphuric acid) ionizes completely to form hydronium ions $({H_3}{O^ + })$ and hydrogen sulphate ions $(HSO_4^ - )$. Since it is a strong mineral acid and in dilute solutions, hydrogen sulphate ions also dissociate to give hydronium ions and sulphate ions as :
$
{H_2}S{O_4}(aq) \to {H_3}{O^ + } + HSO_4^ - \\
  HSO_4^ - \to {H_3}{O^ + } + SO_4^{2 - } \\
$
Hence we can say it is protonic acid.
Now in the case of $B{(OH)_3}$ i.e. boric acid or orthoboric acid, it does not ionize in water to give $({H^ + })$ ions. Hence it is not a protonic acid. As it accepts a pair of electrons from hydroxide ions so it is a lewis acid. The reaction is given below:
$B{(OH)_3} + 2{H_2}O \to {[B(O{H_4})]^ - }(aq) + {H_3}{O^ + }$
In case of $PO{(OH)_3}$, i.e. ${H_3}P{O_4}$, it ionizes in an aqueous solution mostly to give a combination of its three anions as:
$
  {H_3}P{O_4} + {H_2}O\underset {} \leftrightarrows {H_3}{O^ + } + {H_2}PO_4^ - \\
  {H_2}PO_4^ - + {H_2}O\underset {} \leftrightarrows {H_3}{O^ + } + HPO_4^{2 - } \\
  HPO_4^{2 - } + {H_2}O\underset {} \leftrightarrows {H_3}{O^ + } + PO_4^{3 - } \\
$
Hence it is also a protonic acid.
For the case of $SO{(OH)_2}$i.e. ${H_2}S{O_3}$, in aqueous solution it dissociates as:
$
  {H_2}S{O_3} + {H_2}O \to {H_3}{O^ + } + HSO_3^ - \\
  HSO_3^ - + {H_2}O \to {H_3}{O^ + } + SO_3^{2 - } \\
$
Hence, it is also a protonic acid.
Hence the correct answers are A, C, D.

Note: The concept of protonic acid is given by both Arrhenius and Bronsted. Since in the case of the protic solvent, there are O-H or N-H bonds which form a hydrogen bond with themselves. These O-H or N-H bonds serve as a source of proton donors. Also, as we know ${H^ + }$ion is nothing but a proton, which is not capable of independent existence, so it exists as hydronium ion $({H_3}{O^ + })$.