Which of the following is not used as a Lewis acid

Options

A. `SnCl_(4)`
B. `FeCl_(3)`
C. KCl
D. `BF_(3)`

Explanation

To determine which of the following is not used as a Lewis acid, we need to understand the definition of a Lewis acid. A Lewis acid is a substance that can accept a pair of electrons (i.e., it acts as an electron pair acceptor).

Let's analyze each option provided:

1. **SnCl4 (Tin(IV) chloride)**:
- Tin (Sn) has vacant orbitals available for accepting electron pairs. Therefore, SnCl4 can act as a Lewis acid.

2. **FeCl3 (Iron(III) chloride)**:
- Iron (Fe) in FeCl3 also has vacant orbitals due to its oxidation state (+3). It can accept electron pairs, making FeCl3 a Lewis acid.

3. **KCl (Potassium chloride)**:
- KCl is an ionic compound composed of K+ and Cl-. The chloride ion (Cl-) is stable and does not have a tendency to accept electron pairs. Hence, KCl does not act as a Lewis acid.

4. **BF3 (Boron trifluoride)**:
- Boron (B) in BF3 has an empty p-orbital, which allows it to accept electron pairs. Therefore, BF3 is also a Lewis acid.

Based on this analysis, the compound that is **not used as a Lewis acid** is **KCl**.

### Summary of the Solution:
- **Lewis Acid Definition**: Accepts electron pairs.
- **SnCl4**: Lewis acid (has vacant orbitals).
- **FeCl3**: Lewis acid (has vacant orbitals).
- **KCl**: Not a Lewis acid (Cl- is stable and does not accept electron pairs).
- **BF3**: Lewis acid (has vacant orbitals).

### Answer:
**KCl is not used as a Lewis acid.**

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