Which of the following are Lewis acids ?
`H_2O, BF_3 , H^(+) , CN^(-) , CO_2`

Answer

To determine which of the given compounds are Lewis acids, we first need to understand the definition of a Lewis acid. A Lewis acid is defined as a substance that can accept an electron pair. Now, let's analyze each of the given compounds one by one.

### Step 1: Analyze H₂O (Water)
- **Structure**: Water has a bent shape with two hydrogen atoms bonded to an oxygen atom. The oxygen atom has two lone pairs of electrons.
- **Behavior**: Water can donate its lone pairs to form bonds (acting as a Lewis base), but it does not have a tendency to accept electron pairs.
- **Conclusion**: H₂O is **not** a Lewis acid.

### Step 2: Analyze BF₃ (Boron Trifluoride)
- **Structure**: BF₃ has a trigonal planar shape with boron at the center and three fluorine atoms surrounding it. Boron has only six electrons in its valence shell, which is less than the octet.
- **Behavior**: Since boron has an incomplete octet, it can accept a pair of electrons to complete its octet.
- **Conclusion**: BF₃ **is** a Lewis acid.

### Step 3: Analyze H⁺ (Hydrogen Ion)
- **Structure**: H⁺ is simply a proton, which means it has no electrons.
- **Behavior**: H⁺ can accept an electron pair from other species.
- **Conclusion**: H⁺ **is** a Lewis acid.

### Step 4: Analyze CN⁻ (Cyanide Ion)
- **Structure**: CN⁻ has a carbon atom triple-bonded to a nitrogen atom, and it carries a negative charge.
- **Behavior**: The cyanide ion has a complete octet and can donate a pair of electrons but does not accept electron pairs.
- **Conclusion**: CN⁻ is **not** a Lewis acid.

### Step 5: Analyze CO₂ (Carbon Dioxide)
- **Structure**: CO₂ has a linear structure with carbon in the center double-bonded to two oxygen atoms. The carbon atom has a complete octet.
- **Behavior**: CO₂ does not have a tendency to accept electron pairs, as its octet is complete.
- **Conclusion**: CO₂ is **not** a Lewis acid.

### Final Conclusion
Based on the analysis:
- The Lewis acids among the given options are **BF₃** and **H⁺**.

### Summary of Lewis Acids Identified:
- **Lewis Acids**: BF₃, H⁺
- **Not Lewis Acids**: H₂O, CN⁻, CO₂