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

Options

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

Answer

To determine which of the given options can act as a Bronsted acid but not as a Lewis acid, we need to understand the definitions of both types of acids:

1. **Bronsted Acid**: A substance that can donate a proton (H⁺).
2. **Lewis Acid**: A substance that can accept an electron pair.

Now, let's analyze each option step by step:

### Step 1: Analyze Option 1 - OH⁺
- **Bronsted Acid**: OH⁺ can donate a proton (H⁺) to become H₂O, so it acts as a Bronsted acid.
- **Lewis Acid**: OH⁺ has a positive charge and only one lone pair on oxygen, which means it has an electron deficiency. It can accept an electron pair, thus acting as a Lewis acid.
- **Conclusion**: This cannot be the answer since it acts as both a Bronsted and Lewis acid.

### Step 2: Analyze Option 2 - AlCl₃
- **Bronsted Acid**: AlCl₃ does not have any protons to donate, so it cannot act as a Bronsted acid.
- **Lewis Acid**: Aluminum in AlCl₃ has only 6 electrons in its valence shell, making it electron deficient. It can accept an electron pair, thus acting as a Lewis acid.
- **Conclusion**: This cannot be the answer since it does not act as a Bronsted acid.

### Step 3: Analyze Option 3 - FeCl₃
- **Bronsted Acid**: FeCl₃ does not have any protons to donate, so it cannot act as a Bronsted acid.
- **Lewis Acid**: Iron in FeCl₃ has empty d-orbitals, allowing it to accept electron pairs, thus acting as a Lewis acid.
- **Conclusion**: This cannot be the answer since it does not act as a Bronsted acid.

### Step 4: Analyze Option 4 - NH₃
- **Bronsted Acid**: NH₃ can donate a proton (H⁺) to become NH₂⁻, so it acts as a Bronsted acid.
- **Lewis Acid**: NH₃ has a lone pair of electrons on nitrogen but does not have any empty orbitals to accept electrons. Therefore, it cannot act as a Lewis acid.
- **Conclusion**: This is the correct answer since NH₃ acts as a Bronsted acid but not as a Lewis acid.

### Final Answer:
The species that can act as a Bronsted acid but not as a Lewis acid is **Option 4: NH₃**.

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