Question

Identify the pair containing only Lewis acids

• A. BF\(_3\), NH\(_3\)
• B. H\(^+\), BF\(_3\)
• C. F\(^-\), H\(_2\)O
• D. NH\(_4^+\), NH\(_3\)

Hint:

To quickly identify Lewis acids, look for three main categories: 1. Positive ions (like \(H^+\), \(Mg^{2+}\)). 2. Molecules with an incomplete octet on the central atom (like \(BF_3\), \(AlCl_3\)). 3. Molecules with a central atom that can accommodate more than an octet and is bonded to electronegative atoms (like \(SiF_4\), \(PCl_5\)).

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We need to identify the pair of chemical species where both members are Lewis acids.

Step 2: Key Formula or Approach:
- A

Lewis acid is a chemical species that can accept a pair of electrons. Common examples include cations, molecules with an incomplete octet, and molecules where the central atom can expand its octet.
- A

Lewis base is a chemical species that can donate a pair of electrons. Common examples include anions and molecules with lone pairs of electrons on the central atom.

Step 3: Detailed Explanation:
Let's analyze each species in the options:
-

BF\(_3\): Boron trifluoride. The boron atom has only 6 valence electrons (an incomplete octet), so it can accept an electron pair. It is a

Lewis acid.
-

NH\(_3\): Ammonia. The nitrogen atom has a lone pair of electrons that it can donate. It is a

Lewis base.
-

H\(^+\): A proton. It has an empty 1s orbital and readily accepts an electron pair. It is a

Lewis acid.
-

F\(^-\): A fluoride ion. It is an anion with four lone pairs of electrons, making it an electron pair donor. It is a

Lewis base.
-

H\(_2\)O: Water. The oxygen atom has two lone pairs of electrons that it can donate. It is a

Lewis base.
-

NH\(_4^+\): Ammonium ion. Nitrogen has a full octet and no lone pairs to donate. It is generally not considered a Lewis base. While it is a Brønsted-Lowry acid (can donate a proton), it is not a typical Lewis acid as it cannot accept an electron pair directly without first losing a proton. However, compared to Lewis bases, it is acidic.
Now let's evaluate the pairs:
- (A) BF\(_3\) (acid), NH\(_3\) (base). Incorrect.
- (B) H\(^+\) (acid), BF\(_3\) (acid). Both are Lewis acids. Correct.
- (C) F\(^-\) (base), H\(_2\)O (base). Both are Lewis bases. Incorrect.
- (D) NH\(_4^+\) (Brønsted-Lowry acid), NH\(_3\) (base). Incorrect.

Step 4: Final Answer:
The pair containing only Lewis acids is H\(^+\), BF\(_3\).