Question

Which of the following is Lewis acid?
A) $N{H_3}$
B) $N{H_4}^ +$
C) $B{F_3}$
D) ${H_2}O$

Answer 1

We need to know that a Lewis acid is a substance that accepts a pair of electrons to form a covalent bond. A Lewis base is a substance that donates a pair of electrons to form a covalent bond. So, a Lewis acid-base reaction is represented by the transfer of a pair of electrons from a base to an acid.

Complete answer:
We will look at all the options one by one to answer this question:
Option A) this option is incorrect as ammonia is a Lewis base and not a Lewis acid, It will donate electrons to compounds that will accept them.
Option B) this option is incorrect as $N{H_4}^ +$ is neither a Lewis Base nor a Lewis acid because it does have any available lone pair of Electrons to donate so that it can said to be a Lewis base, but also because it's octet is complete it doesn't have enough vacancy to accept any electron pair from other Lewis base, which means that it cannot be called a Lewis acid.
Option C) this option is correct as $B{F_3}$ acts as a Lewis acid when it accepts the lone pair of electrons that $N{H_3}$ donates.
Option D) this option is incorrect as water is a Lewis base.

So, the correct answer is “Option C”.

Note:
We have to remember that a Lewis base is any substance, such as the OH- ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor. In the Lewis theory, an acid is any ion or molecule that can accept a pair of nonbonding valence electrons.