Question

Identify Bronsted-Lowry acids in the reaction given below?
$\left[ {{\text{Al}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{6}}}^{{\text{3 + }}}} \right]{\text{ + HC}}{{\text{O}}_{\text{3}}}^{\text{ - }}\underset {} \leftrightarrows \left[ {{\text{Al}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{5}}}{{\left( {{\text{OH}}} \right)}^{{\text{2 + }}}}} \right]{\text{ + }}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$
A B C D
The correct answer is:
A.A, C
B.A, D
C.B, D
D.B, C

Answer 1

(1) There are many concepts regarding acids and bases. One such concept is the Bronsted-Lowry concept or the protonic concept. According to this concept, an acid is a species which can lose a proton and a base is a species which can accept a proton. Or in other words, an acid can be considered to be a proton donor and a base can be considered to be a proton acceptor.
(2) When a Bronsted-Lowry acid donates a proton, the remaining part of the acid is called a conjugate base. For example, the conjugate base of hydrogen chloride is chloride ion.
${\text{HCl}} \to {{\text{H}}^{\text{ + }}}{\text{ + C}}{{\text{l}}^{\text{ - }}}$
When a Bronsted-Lowry base accepts a proton, the remaining part of the base is called a conjugate acid. For example, the conjugate acid of water is hydronium ion.
${{\text{H}}_{\text{2}}}{\text{O + }}{{\text{H}}^{\text{ + }}} \to {{\text{H}}_{\text{3}}}{{\text{O}}^{\text{ + }}}$

Complete step by step answer:
The given reaction is:
$\left[ {{\text{Al}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{6}}}^{{\text{3 + }}}} \right]{\text{ + HC}}{{\text{O}}_{\text{3}}}^{\text{ - }}\underset {} \leftrightarrows \left[ {{\text{Al}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{5}}}{{\left( {{\text{OH}}} \right)}^{{\text{2 + }}}}} \right]{\text{ + }}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$
We need to find out the conjugate acids in this reaction between hexaaqua aluminium (III) ion (A) and bicarbonate ion (B).
An acid-base reaction involves two acids and two bases. The general acid-base reaction can be written as:
${\text{Aci}}{{\text{d}}_{\text{1}}}{\text{ + Bas}}{{\text{e}}_{\text{2}}}\underset {} \leftrightarrows {\text{Aci}}{{\text{d}}_{\text{2}}}{\text{ + Bas}}{{\text{e}}_{\text{1}}}$
The same subscript is used to represent the conjugate acid and base. Thus, acid and base with the subscript 1 forms one conjugate pair and the acid and base with the subscript 2 forms another conjugate pair.
In the given acid-base reaction between hexaaqua aluminium (III) ion (A) and bicarbonate ion (B), the hexaaqua aluminium (III) ion loses a proton from one of its water molecule to give pentaaqua hydroxo aluminium (III) ion (C). Therefore, A acts as a Bronsted-Lowry acid.
${\left[ {{\text{Al}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{6}}}} \right]^{{\text{3 + }}}} \to {\left[ {{\text{Al}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{5}}}{\text{OH}}} \right]^{{\text{2 + }}}}{\text{ + }}{{\text{H}}^{\text{ + }}}$
Similarly, the pentaaqua hydroxo aluminium (III) ion (C) can accept a proton to form the hexaaqua aluminium (III) ion (A). So, C is a Bronsted-Lowry base.
${\left[ {{\text{Al}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{5}}}{\text{OH}}} \right]^{{\text{2 + }}}}{\text{ + }}{{\text{H}}^{\text{ + }}} \to {\left[ {{\text{Al}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{6}}}} \right]^{{\text{3 + }}}}$
Thus, A and C form a conjugate acid-base pair.
On the other hand, the bicarbonate ion (B) can accept a proton to form carbonic acid (D). So, B is a Bronsted-Lowry base.
${\text{HC}}{{\text{O}}_{\text{3}}}^{\text{ - }}{\text{ + }}{{\text{H}}^{\text{ + }}} \to {{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$
And, the carbonic acid (D) can lose a proton to give bicarbonate ion (B). So, D is a Bronsted-Lowry acid.
${{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}} \to {\text{HC}}{{\text{O}}_{\text{3}}}^{\text{ - }}{\text{ + }}{{\text{H}}^{\text{ + }}}$
Thus, B and D form another conjugate pair.

Hence option B Is correct.

Note:
Some other concepts for acids and bases are Arrhenius concept and Lewis concept. According to the Arrhenius concept, an acid is a species which liberates hydrogen ions in water and a base is a species which liberates hydroxide ions in water. According to the Lewis concept, an acid is a species which can accept an electron pair and a base is a species which can donate an electron pair.