Question

An example of Lewis acid - Lewis base reaction is:
(A) $\text{BC}{{\text{l}}_{3}}\text{ + C}{{\text{l}}^{-}}\to \text{ BCl}_{4}^{-}$
(B) $\text{HCN + }{{\text{H}}_{2}}\text{O}\to \text{ }{{\text{H}}_{3}}{{\text{O}}^{+}}\text{ + C}{{\text{N}}^{-}}$
(C) ${{\text{H}}_{2}}{{\text{O}}_{2}}\text{ + 2B}{{\text{r}}^{-}}\text{ + 2}{{\text{H}}^{+}}\to \text{ 2}{{\text{H}}_{2}}\text{O + B}{{\text{r}}_{2}}$
(D) $\text{2N}{{\text{H}}_{3}}\text{ }\rightleftharpoons \text{ }{{\text{N}}_{2}}\text{ +}\text{ 3}{{\text{H}}_{2}}$

Answer 1

Hint: In the given problem, firstly we will study about the Lewis acid and base. And then by studying all the reactions we will identify the correct Lewis acid - Lewis base reaction among the given options.

Complete step by step solution:
- In the given question, we have to choose the correct example of Lewis acid - Lewis base reaction among the given reactions.
- Firstly, we should know what Lewis acid and Lewis base are.
- Lewis acids are those species of the molecules which can accept a lone pair of an electron to complete its octet or vacant orbitals.
- For example, $\text{B}{{\text{X}}_{3}}$ where X is a halide such as chlorine, fluorine. Etc.
- Whereas Lewis bases are those species of the molecules which can donate a lone pair of an electron to the Lewis acid.
- For example, $\text{N}{{\text{H}}_{3}}$.
- So, among the given options only reaction A shows the Lewis acid - Lewis base reaction i.e.
 $\text{BC}{{\text{l}}_{3}}\text{ + C}{{\text{l}}^{-}}\to \text{ BCl}_{4}^{-}$
- Because in this reaction, chlorine ions act as a donor of lone pair electrons whereas boron trichloride acts as an acceptor.
- So, they combine and form a compound tetra chloro borate anion.
- Whereas in reaction B, the HCN is considered as Bronsted - Lowry acid because it donates a proton to the water.
- In reaction C, the loss of the oxygen from hydrogen peroxide takes place and bromine ion converts into bromine molecule.
- So, there is no transfer or acceptance - a donation of the lone pair electrons.
- Whereas the reaction D, is an example of a decomposition reaction.

Therefore, option A is the correct answer.

Note: Decomposition reaction is the reaction in which the substance breaks into two or more molecules. Bronsted - Lowry base is the species which can accept the proton whereas Bronsted - Lowry acid is the species which can donate the proton.