Question

In the reaction, $LiH + Al{H_3} \to LiAl{H_4}$, $Al{H_3}$ and $LiH$ act as:
(A) Lewis acid and Lewis base
(B) Lewis Base and Lewis acid.
(C) Bronsted base and Bronsted acid.
(D) None of the above

Answer 1

In order to find which is the acid and which is the base in the formation of $LiAl{H_4}$, we should try to understand which type of acid-base reaction is taking place in the above given reaction. There are three types of acid-bases theory such as the Arrhenius theory, Bronsted-Lowry theory and Lewis theory.

Complete Solution :
On seeing the equation given in the question, we can say that
- By using Arrhenius definition, we cannot classify the acid and bases from the above equation because in the above equation there are no $O{H^ - }$ ions.
- By using Bronsted -Lowry definition also we cannot classify acids and bases from the above equations because the stuff which was given by $LiH$ to the $Al{H_3}$ is not the proton, but the protide (${H^ - }$) ion.
- We can classify the acid and bases in the above equation by Lewis theory.
Lewis acid is a substance which can accept or receive unshared pairs of electrons, while Lewis base is the substance which can give or donate its unshared pair of electrons.
From the above give reaction, we can split the reactions as:
$LiH \to L{i^ + } + {H^ - }$
$Al{H_3} + {H^ - } \to AlH_4^ -$
- $LiH$ is giving a pair of electrons to $Al{H_3}$ and $Al{H_3}$ is accepting a pair of electrons from $LiH$.
Therefore, $Al{H_3}$ is a Lewis acid and $LiH$ is a Lewis base.
So, the correct answer is “Option A”.

Note: Let us now see some of the uses of Lewis acid and Lewis base.
- Lewis acids are usually used in the Friedel crafts reactions.
- Lewis bases are used for modifying the selectivity and the activity of metallic catalysts.