Question

Which of the following is a Lewis acid?

• A. HCl
• B. HO$^-$
• C. H$_2$O
• D. Co$^{3+}$
• E. NH$_3$

Hint:

All simple cations (like $H^+$, $Cu^{2+}$, $Fe^{3+}$) and molecules with incomplete octets (like $BF_3$, $AlCl_3$) are Lewis acids.

Answer 1

The Correct Option is D

To determine which of the given options is a Lewis acid, we need to understand the concept of Lewis acids and bases. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor.

1. Let's evaluate each option:
   • HCl: Hydrochloric acid is a classical example of a Bronsted-Lowry acid since it donates a proton (H⁺) in aqueous solutions. However, it is not typically considered a Lewis acid.
   • HO^-: Hydroxide ion is a Lewis base because it has a pair of unshared electrons that can be donated to form a bond with an electron pair acceptor.
   • H₂O: Water is typically a Lewis base because it has two lone pairs of electrons on the oxygen atom, which can be donated.
   • Co³⁺: Cobalt(III) ion is a metal cation that can accept a pair of electrons due to its vacant orbitals, making it a Lewis acid. Metal ions are generally Lewis acids as they can accept electron pairs from ligands.
   • NH₃: Ammonia is a Lewis base because the nitrogen atom has a lone pair of electrons that can be shared with a Lewis acid.
2. From the analysis above, Co³⁺ is identified as the Lewis acid among the options, as it can accept electron pairs.

Hence, the correct answer is Co³⁺.