Question

Three reactions involving $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$ are given below:
i) $\text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4 }}}\text{+ }{{\text{H}}_{\text{2}}}\text{O }\to \text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ + }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$
ii) $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ + }{{\text{H}}_{\text{2}}}\text{O }\to \text{ HPO}_{4}^{-}\text{ }+\text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ }$
iii) $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ + O}{{\text{H}}^{-}}\text{ }\to \text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\text{ }+\text{ }{{\text{O}}^{\text{2-}}}\text{ }$
In which of the above reaction, does $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$ act as an acid?
A) (ii) only
B) (i) and (ii)
C) (iii) only
D) (i) and (iii))

Answer 1

According to Bronsted –Lowry concept the acid is the chemical substance that tends to donate a proton $\text{ }{{\text{H}}^{\text{+}}}\text{ }$ and base is a substance that accepts the proton. The reaction of acid produces conjugate base and conjugate acid respectively. Phosphoric acid $\text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\text{ }$ is a triprotic acid. It loses its proton to the base and forms phosphates.

Complete step by step solution:
According to Bronsted –Lowry acid-base concept, When an acid reacts with a base, the residual part of the acid tends to accept a proton hence act a conjugate base and the residual part of the base can donate the proton hence act as a conjugate acid. The general reaction is as follows,
$\text{ }\begin{matrix} \text{HA} & \text{+} & \text{B} & \rightleftharpoons & {{\text{A}}^{-}} & \text{+} & \text{B}{{\text{H}}^{\text{+}}} \\\ \text{(Acid)} & {} & \text{(Base)} & {} & \text{(conjugate base)} & {} & \text{(Conjugate base)} \\\ \end{matrix}$
Here, we have given the following reaction. The reaction is:
i) $\text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4 }}}\text{+ }{{\text{H}}_{\text{2}}}\text{O }\to \text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ + }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$
In this reaction, phosphoric acid $\text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\text{ }$reacts with the water. It is an acid-base reaction where phosphoric acid acts as an acid and water as a base. The reaction produces $\text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ }$ by abstracting a proton from $\text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\text{ }$and tends to donate a proton hence hydronium ion is conjugate acid. However, $\text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\text{ }$loses a proton and from$\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$, which tends to accept a proton. Thus here $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$acts as a conjugate base.
ii) $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ + }{{\text{H}}_{\text{2}}}\text{O }\to \text{ HPO}_{4}^{-}\text{ }+\text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ }$
In this reaction, phosphate ion $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$reacts with the water. It is an acid-base reaction where $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$ acts as an acid and water as a base. The reaction produces $\text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ }$ by abstracting a proton from $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$and tends to donate a proton hence hydronium ion is conjugate acid. However, $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$loses a proton and from$\text{HPO}_{4}^{-}\text{ }$, which tend to accept a proton .thus here $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$acts as an acid.
$\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\rightleftharpoons \text{ HPO}_{4}^{-}\text{(Base) }+\text{ }{{\text{H}}^{\text{+}}}\text{ }$
iii) $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ + O}{{\text{H}}^{-}}\text{ }\to \text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\text{ }+\text{ }{{\text{O}}^{\text{2-}}}\text{ }$
In this reaction, the phosphate ion accepts the proton from the hydroxide ion. The hydroxide ion acts as the acid while the phosphate ion acts as the base and forms${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}$.
Thus, from the above reaction in ii) reaction $\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ + }{{\text{H}}_{\text{2}}}\text{O }\to \text{ HPO}_{4}^{-}\text{ }+\text{ }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}\text{ }$,$\text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }$acts as the acid.

Hence, (A) is the correct option.

Note: Note that the water may act as the acid and base depend on the chemical substances with which it is reacting. Thus, it is amphoteric. The dihydrogen phosphate is an intermediate step in the dissociation of phosphoric acid.
$\begin{aligned} & \text{ }{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}\text{ }\rightleftharpoons \text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ + }{{\text{H}}^{\text{+}}}\text{ } \\\ & \text{ }{{\text{H}}_{\text{2}}}\text{PO}_{4}^{-}\text{ }\rightleftharpoons \text{ HPO}_{4}^{2-}\text{ + }{{\text{H}}^{\text{+}}} \\\ & \text{ HPO}_{4}^{2-}\text{ }\rightleftharpoons \text{ PO}_{4}^{3-}\text{ + }{{\text{H}}^{\text{+}}} \\\ \end{aligned}$
Dihydrogen phosphate is a weak acid compared to phosphoric acid.