Question

Assertion: Both reactions are Lewis acid-base reactions?
(i)- $N{{H}_{3}}+B{{F}_{3}}\to {{H}_{3}}N-B{{F}_{3}}$
(ii)- $Mg+S\to M{{g}^{2+}}+{{S}^{2-}}$

Reason: Lewis acid-base reaction involves the donation of lone pair electrons from base to acid. This donation results in a coordinate bond.
(a)- Both assertion and reason are correct and the reason is the correct explanation for the assertion.
(b)- Both assertion and reason are correct and the reason is not the correct explanation for the assertion.
(c)- Assertion is correct but the reason is incorrect.
(d)- Assertion is incorrect but the reason is correct.
(e)- Both assertion and reason are incorrect.

Answer 1

Hint: According to Lewis acid-base concept: If a substance is able to accept a pair of the electron because it has vacant orbitals then it will act as an acid and if the substance is able to donate a pair of the electron due to the presence of lone pair then it is a base.

Complete step by step answer:
Many theories came one by one for defining the nature of an acid and a base. Bronsted-Lowery's concept of acids and bases was broader than the Arrhenius concept in the sense that it could not explain not only all the Arrhenius acids and bases but included many more substances as bases. However, it did not have a much larger scope for acids.
This theory explained that:
If a substance is able to accept a pair of the electron because it has vacant orbitals then it will act as an acid and if the substance is able to donate a pair of the electron due to the presence of lone pair then it is a base.
Some examples of acids are $B{{F}_{3}},\text{ }AlC{{l}_{3}},\text{ }FeC{{l}_{3}}$ etc and some examples of bases are $N{{H}_{3}},\text{ }RN{{H}_{2}},\text{ }ROH$ etc.
It is obvious that when the reaction takes place between an acid and a base according to this concept, then the bond will be a coordinate bond or a dative bond between them.
So the reaction (i)- $N{{H}_{3}}+B{{F}_{3}}\to {{H}_{3}}N-B{{F}_{3}}$ is an Lewis acid-base reaction. But the reaction (ii)- $Mg+S\to M{{g}^{2+}}+{{S}^{2-}}$ is not a Lewis acid-base reaction because there is no formation of a coordinate bond.
So, the assertion is wrong but the reason is right.
So, the correct answer is “Option D”.

Note: We can also say that the molecules that have vacant d-orbital available can act as a Lewis acid. G.N. Lewis in 1923, proposed a broader concept of acids and bases.