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

Step 1: Understanding the Concept:
A Lewis acid is defined as a chemical species that can accept an electron pair.
Step 2: Key Formula or Approach:
Look for species that are electron deficient or have vacant orbitals (like metal cations or molecules with incomplete octets).
Step 3: Detailed Explanation:
(A) HCl is a Bronsted acid (proton donor).
(B) HO$^-$ and (E) NH$_3$ have lone pairs and are Lewis bases (electron donors).
(C) H$_2$O has lone pairs and usually acts as a Lewis base.
(D) Co$^{3+}$ is a transition metal ion with a high positive charge and vacant d-orbitals. It can readily accept electron pairs from ligands.
Therefore, Co$^{3+}$ acts as a Lewis acid.
Step 4: Final Answer:
The Lewis acid is Co$^{3+}$.