Which of the following can act as a Bronsted acid but not as a Lewis acid?

Options

A. `OH^(+)`
B. `AlCl_(3)`
C. `FeCl_(3)`
D. `NH_(3)`

Answer

To determine which of the following can act as a Bronsted acid but not as a Lewis acid, we need to understand the definitions of Bronsted acids and Lewis acids.

### Step 1: Define Bronsted Acid and Lewis Acid
- **Bronsted Acid**: A substance that can donate a proton (H⁺ ion).
- **Lewis Acid**: A substance that can accept an electron pair.

### Step 2: Analyze the Given Options
We need to evaluate the substances given in the question to see if they can act as Bronsted acids and whether they can act as Lewis acids.

1. **OH⁺ (Hydroxonium Ion)**:
- **Bronsted Acid**: Yes, it can donate H⁺ (acting as a Bronsted acid).
- **Lewis Acid**: No, it cannot accept an electron pair. Therefore, it acts as a Bronsted acid but not as a Lewis acid.

2. **AlCl₃ (Aluminum Chloride)**:
- **Bronsted Acid**: No, it does not have H⁺ to donate.
- **Lewis Acid**: Yes, it can accept electron pairs to complete its octet.

3. **FeCl₃ (Iron(III) Chloride)**:
- **Bronsted Acid**: No, it does not have H⁺ to donate.
- **Lewis Acid**: Yes, it can accept electron pairs to complete its octet.

4. **NH₃ (Ammonia)**:
- **Bronsted Acid**: No, it does not donate H⁺; it acts as a Bronsted base by accepting H⁺.
- **Lewis Acid**: No, it does not accept electron pairs; it donates them.

### Step 3: Conclusion
From the analysis:
- The only substance that acts as a Bronsted acid but not as a Lewis acid is **OH⁺**.

### Final Answer
**OH⁺** can act as a Bronsted acid but not as a Lewis acid.

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