Question

Coordination complex \[\left[ {Al{{\left( {OH} \right)}_3}{{\left( {H_2O} \right)}_3}} \right]\] (s) more simply written as \[Al{\left( {0H} \right)_3}\] (s) is:
A. Acidic
B. Basic
C. Neutral
D. Amphoteric

Answer 1

Hint:In order to solve this question we will see whether the given compound has acidic properties, i.e., can it donate a proton in aqueous medium. We will also see whether it shows basic nature, i.e., accepting a proton or donating a hydroxide ion in the medium. Then we have the third category which is amphoteric nature, it refers to those entities that can behave as both acids as well as bases. Let us see under which category the compound given in the question falls.

Complete step by step answer:
In the question we have aluminium hydroxide.
Now aluminium can act as both acids and bases and thus has both acidic and basic properties and is therefore amphoteric in nature. Let us see how.
In acids like hydrogen chloride, it behaves like a Bronsted-Lowry base by donating its hydroxide ion and forming a conjugate acid (aluminium trichloride)
\[3{{ }}HCl{{ }} + {{ }}Al{\left( {OH} \right)_3}\; \to {{ }}AlC{l_3}\; + {{ }}3{{ }}{H_2}O\]
In basic medium, it acts as a Lewis acid by binding hydroxide ions
\[Al{\left( {OH} \right)_3}\; + {{ }}O{H^ - }\; \to {{ }}Al{\left( {OH} \right)_4}^ - \]
The reactions can also be demonstrated as:
 $
Al{(OH)_3} + 3{H^ + } \to A{l^{3 + }} + 3{H_2}O \\
Al{(OH)_3} + O{H^ - } \to {[Al{(OH)_4}]^ - } \\
 $
Hence the correct answer is Option D amphoteric.

Additional Information:
The Bronsted–Lowry theory is basically an acid–base reaction theory and the fundamental concept of this particular theory is that when an acid and a base react with each other, the acid undergoes changes and gets converted to forms its conjugate base, and the base gets converted into its conjugate acid by exchange of a proton (the hydrogen cation).
Also, a Lewis acid is that chemical entity which contains an orbital that is empty and hence is capable of accepting an electron pair from a Lewis base to form a Lewis adduct.

Note:
It is important to understand the basic concepts behind the acid base theory and how substances can act as both acids and bases leading to them being called amphoteric substances. Sometimes oxides formed by metals, for example, oxide of aluminium $A{l_2}{O_3}$ , are amphoteric in nature too. Amphoterism depends on the oxidation state of the oxide.