Question

Which of \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}{\text{, }}{{\text{H}}_2}{\text{S}}\] or \[{{\text{H}}_2}{\text{O}}\] is the strongest acid?
A) \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\]
B) \[{{\text{H}}_2}{\text{S}}\]
C) \[{{\text{H}}_2}{\text{O}}\]
D) All same

Answer 1

Hint: Usually in an acid base reaction, a strong acid reacts with a strong base to form a weak acid and a weak base. But for the reaction between two acids, the stronger acid donates a proton to the weaker acid, so that the weaker acid acts as a base in this reaction.

Complete answer:
When \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] reacts with water molecule, it can protonated water molecule.
\[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }} + {{\text{H}}_2}{\text{O }} \to {\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_5}{\text{OH}}} \right]^{2 + }} + {\text{ }}{{\text{H}}_3}{{\text{O}}^ + }\]
In the above example, \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] acts as an acid and water acts as a base. A proton is transferred from \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] to water molecule. Thus, compared to water molecule, \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] is the stronger acid. \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] is the conjugate base of \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_5}{\text{OH}}} \right]^{2 + }}\]. \[{{\text{H}}_3}{{\text{O}}^ + }\] is the conjugate acid of water. An acid and its conjugate base differ only by one proton. A base and its conjugate acid differ only by a proton.
When \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] reacts with sulphide ion, it can protonate sulphide ion.
 \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }} + {{\text{S}}^{2 - }}{\text{ }} \to {\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_5}{\text{OH}}} \right]^{2 + }} + {\text{ H}}{{\text{S}}^ - }\]
In the above example, \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\]acts as an acid and sulphide ion acts as a base. A proton is transferred from \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] to sulphide ion. Thus, compared to sulphide ion, \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] is the stronger acid. \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] is the conjugate base of \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_5}{\text{OH}}} \right]^{2 + }}\]. \[{\text{H}}{{\text{S}}^ - }\] is the conjugate acid of \[{{\text{S}}^{2 - }}\] .
Hence, among \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}{\text{, }}{{\text{H}}_2}{\text{S}}\] or \[{{\text{H}}_2}{\text{O}}\] the strongest acid is \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] .

So, the correct option is the option (A).

Note: In the complex cation \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\], the oxygen atom of water molecule, donates an electron pair to the central aluminium metal, and forms a coordinate covalent bond. Due to this, the electron density on oxygen atom decreases and the oxygen atom gains partial positive charge. To compensate for the decrease in the electron density, the oxygen atom attracts the electron pair of oxygen-hydrogen bond. This facilitates in the easy removal of proton and increases the acidity of the complex cation \[{\left[ {{\text{Al}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3 + }}\] .