Question

Which of the following is a Lewis acid?
A. ${\rm{C}}{{\rm{l}}^ - }$
B. ${{\rm{H}}_{\rm{3}}}{{\rm{O}}^{\rm{ + }}}$
C. ${\rm{P}}{{\rm{F}}_{\rm{3}}}$
D. ${{\rm{C}}_{\rm{2}}}{{\rm{H}}_{\rm{5}}}{\rm{OH}}$

Answer 1

We know that Lewis acid can accept a pair of non-bonding electrons. An electron pair acceptor is called Lewis acid.

Complete answer:

A chemical species which has a vacant orbital and accepts an electron from a Lewis base is known as Lewis acid. Lewis bases have filled an orbital and contain an electron which do not take part in bonding but a Lewis adduct is formed by bonding with a Lewis acid. Lewis bases are nucleophiles as they donate electrons. Lewis acids are electrophiles because they accept electron pairs and have empty orbitals. All cations are Lewis acid as they take electrons. An atom or ion having incomplete octet can accept electrons, so they are Lewis acids.
From the above options, we can conclude that both ${{\rm{H}}_{\rm{3}}}{{\rm{O}}^{\rm{ + }}}$and ${\rm{P}}{{\rm{F}}_{\rm{3}}}$ are Lewis acid.
The + sign in the structure of ${{\rm{H}}_{\rm{3}}}{{\rm{O}}^{\rm{ + }}}$indicates that it can accept an electron pair from a Lewis base. In ${\rm{P}}{{\rm{F}}_{\rm{3}}}$, three fluorine atoms connected through one Phosphorus atom and it has incomplete octet. So, it can accept electron pairs from a Lewis base.

Hence, the correct answers are (B) ${{\rm{H}}_{\rm{3}}}{{\rm{O}}^{\rm{ + }}}$and (C) ${\rm{P}}{{\rm{F}}_{\rm{3}}}$.

Note:
Students might confuse between Lewis acid and Lewis base. A species which accepts an electron is known as Lewis acid. And a species which donates an electron is termed as Lewis base.