Question

In the reaction, $Alc{l_3} + C{l^ - } \to {[AlC{l_4}]^ - }$,$AlC{l_3}$ acts as a
A.Salt
B.Lewis base
C.Lewis acid
D.Bronsted acid

Answer 1

Hint: In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. This theory complements the model of oxidation and reduction reactions. This theory suggests that acids react with bases to share a pair of electrons, with no charge in the oxidation numbers of atoms.

Complete step by step answer:
Lewis acid is any substance such as the ${H^ + }$ ion that can accept a pair of non-bonding electrons. In other words, Lewis acid is an electron pair acceptor.
Further, a Lewis base is any substance such as ion that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor.
Now, in case of the above given reaction,
Electronic configuration of Al is
$1{s^2}2{s^2}2{p^6}3{s^2}3{p^1}$
Now, the three valence electrons are used to form bonds with 1 Cl each.
In the p orbit, 2 orbitals are vacant due to which it is electron deficient.
Now, according to the Lewis theory, a species is an acid if it is electron deficient and accepts lone pairs of electrons.
So, $AlC{l_3}$ is a Lewis acid.

Hence, option C is correct.

Note:
If two Lewis acids are in the same group of the periodic table, then acidity decreases as the size of the atom increases. Lewis acidic strength is proportional to the tendency to accept lone pairs of electrons. If the bond is highly polar, then the proton tends to leave the molecule more easily and thus making it a strong acid.