Which of the following is a Bronsted-Lowry acid but not an Arrhenius acid?

Options

A. HCI
B. `NH_(4)^(+)`
C. `BF_(3)`
D. `CH_(3)COOH`

Answer

To determine which of the following is a Bronsted-Lowry acid but not an Arrhenius acid, we need to understand the definitions of both types of acids.

### Step-by-Step Solution:

1. **Understand Bronsted-Lowry Acid**:
- A Bronsted-Lowry acid is defined as a substance that donates a proton (H⁺) to another substance.

2. **Understand Arrhenius Acid**:
- An Arrhenius acid is defined as a substance that increases the concentration of H⁺ ions in aqueous solution when it ionizes.

3. **Identify the Characteristics**:
- We need to find a substance that can donate a proton (making it a Bronsted-Lowry acid) but does not increase the H⁺ concentration in an aqueous solution (making it not an Arrhenius acid).

4. **Evaluate Possible Candidates**:
- Consider common acids such as HCl, H₂SO₄, and NH₄⁺ (ammonium ion).
- HCl and H₂SO₄ are Arrhenius acids because they ionize in water to produce H⁺ ions.
- NH₄⁺ (ammonium ion) can donate a proton to a base (like water), thus acting as a Bronsted-Lowry acid.

5. **Conclusion**:
- NH₄⁺ does not ionize in water to increase H⁺ concentration in the same way that traditional Arrhenius acids do. Therefore, NH₄⁺ is a Bronsted-Lowry acid but not an Arrhenius acid.

### Final Answer:
The correct answer is **NH₄⁺ (ammonium ion)**.

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