Question

Which of the following are Lewis acid base reactions:
A.$CuC{l_2} + 4{H_2}O \to {\left[ {Cu{{\left( {{H_2}O} \right)}_4}} \right]^{2 + }} + 2C{l^ - }$
B.$KI + 4S{O_2}(s) \to {K^ + }\left[ {{{(S{O_4})}_4}{I^ - }} \right]$
C.$B{F_3} + N{H_3} \to \left[ {N{H_3}.B{F_3}} \right]$
D.$SO_3^{2 - } + O \to SO_4^{2 - }$

Answer 1

To answer this question you must recall the Lewis theory of acids and bases. The species that can accept an electron pair are termed as Lewis acids while those species which can donate a pair of electrons is known as a Lewis base. If a reaction involves the donation and acceptance of electrons then it must be a Lewis acid base reaction.

Complete answer:

In option A, we have the reaction, $CuC{l_2} + 4{H_2}O \to {\left[ {Cu{{\left( {{H_2}O} \right)}_4}} \right]^{2 + }} + 2C{l^ - }$
$C{u^{2 + }}$ ion has vacant orbitals and can act as a Lewis acid while oxygen in a water atom has two lone pairs and can act as a Lewis base. The two substances undergo a Lewis acid base reaction.
In option B, we have the reaction, $KI + 4S{O_2}(s) \to {K^ + }\left[ {{{(S{O_4})}_4}{I^ - }} \right]$
The central Sulphur atom in $S{O_2}$has vacant $d -$orbitals and can thus act as a Lewis acid while iodide ion due to the presence of lone pairs can act as a Lewis base. The two substances undergo a Lewis acid base reaction.
In option C, we have, $B{F_3} + N{H_3} \to \left[ {N{H_3}.B{F_3}} \right]$
$B{F_3}$ is an electron deficient species and nitrogen atom in $N{H_3}$atom carries a lone pair. Thus, the complex formed between the two is as a result of an acid base reaction.
In option D, we have, $SO_3^{2 - } + O \to SO_4^{2 - }$
The central Sulphur atom in $SO_3^{2 - }$has vacant $d -$orbitals and can thus act as a Lewis acid while oxygen atom is a Lewis base. The two substances undergo a Lewis acid base reaction to form the adduct $SO_4^{2 - }$

Thus, the correct answers are A, B, C, D.

Note: The Lewis theory of acids and bases describes Lewis acids and Lewis bases as electron pair acceptors and donors respectively. Lewis acids are generally electron deficient substances or substances with the presence of empty orbitals and can thus accept electron pairs from a Lewis base. A Lewis base has the ability to donate a pair of electrons to a Lewis acid forming a coordinate covalent bond. A Lewis base is generally a substance carrying an extra lone pair of atoms. The product formed when a Lewis base donates its electron pair to an acid is known as Lewis adduct.