Which pair of molecules has the strongest dipole – dipole interactions ?

Options

A. `NH_(3) and CH_(4)`
B. `NH_(3) and NH_(3)`
C. `CH_(4) and CH_(4)`
D. `CO_(2) and CO_(2)`

Explanation

To determine which pair of molecules has the strongest dipole-dipole interactions, we need to analyze the polarity of the given molecules. Here’s a step-by-step solution:

### Step 1: Understand Dipole-Dipole Interactions
Dipole-dipole interactions occur between polar molecules. A polar molecule has a significant difference in electronegativity between its atoms, resulting in a permanent dipole moment.

**Hint:** Look for molecules with significant electronegativity differences to identify polar molecules.

### Step 2: Analyze the Given Options
We will evaluate the polarity of the molecules in each option:

1. **Ammonia (NH3) and Methane (CH4)**:
- Ammonia is polar due to the presence of a lone pair on nitrogen, which creates a dipole.
- Methane is non-polar because it has a symmetrical tetrahedral structure.
- **Conclusion**: This pair cannot form dipole-dipole interactions.

2. **Ammonia (NH3) and Ammonia (NH3)**:
- Both molecules are polar.
- They can interact with each other through dipole-dipole interactions.
- **Conclusion**: This pair can form strong dipole-dipole interactions.

3. **Methane (CH4) and Methane (CH4)**:
- Both molecules are non-polar.
- They cannot form dipole-dipole interactions.
- **Conclusion**: This pair cannot form dipole-dipole interactions.

4. **Carbon Dioxide (CO2) and Carbon Dioxide (CO2)**:
- CO2 is a linear molecule, and the dipoles cancel out, making it non-polar.
- **Conclusion**: This pair cannot form dipole-dipole interactions.

### Step 3: Compare the Results
From the analysis:
- The only pair that can form dipole-dipole interactions is **Ammonia and Ammonia**.
- Other pairs either consist of non-polar molecules or a mix of polar and non-polar molecules, which do not lead to dipole-dipole interactions.

### Final Answer
The pair of molecules with the strongest dipole-dipole interactions is **Ammonia (NH3) and Ammonia (NH3)**.

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