Question

Which of the following is a Lewis acid?

• A. HCl
• B. OH
• C. $H_{2}O$
• D. $Co^{3+}$
• E. $NH_{3}$

Hint:

Lewis Acid = Acceptor, Lewis Base = Donor.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
This question asks us to identify a Lewis acid from a given list of species. We need to know the definitions of Lewis acids and Lewis bases.
Step 2: Key Formula or Approach:
- Lewis Acid: A species that can accept a pair of electrons to form a covalent bond. Electron-pair acceptors are typically cations or molecules with an incomplete octet. - Lewis Base: A species that can donate a pair of electrons to form a covalent bond. Electron-pair donors typically have lone pairs of electrons. Step 3: Detailed Explanation:
Let's analyze each option: - (A) HCl: In aqueous solution, HCl is a Brønsted-Lowry acid (proton donor). As a molecule, it's generally not considered a Lewis acid in the typical sense, though the H atom is electron-deficient. - (B) HO\(^-\) (Hydroxide ion): The oxygen atom has lone pairs of electrons that it can donate. Therefore, HO\(^-\) is a Lewis base. - (C) H\(_2\)O (Water): The oxygen atom in water has two lone pairs of electrons. It can act as a Lewis base by donating one of these pairs (e.g., to H\(^+\) to form H\(_3\)O\(^+\)). - (D) Co\(^{3+}\) (Cobalt(III) ion): This is a cation. Metal cations are electron-deficient and have empty orbitals, making them excellent electron-pair acceptors. They readily form coordinate bonds with Lewis bases (ligands) to form complex ions. Therefore, Co\(^{3+}\) is a Lewis acid. - (E) NH\(_3\) (Ammonia): The nitrogen atom has a lone pair of electrons that it can donate. Therefore, NH\(_3\) is a classic example of a Lewis base. Step 4: Final Answer:
The species that is a Lewis acid is Co\(^{3+}\).