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 and their dipole moments. Here’s a step-by-step solution:

### Step 1: Identify the Molecules
The question provides us with pairs of molecules. We need to analyze each pair to determine their polarity:
1. Ammonia (NH₃) and Methane (CH₄)
2. Ammonia (NH₃) and Ammonia (NH₃)
3. Carbon Dioxide (CO₂) and Carbon Dioxide (CO₂)
4. Methane (CH₄) and Methane (CH₄)

### Step 2: Determine Polarity of Each Molecule
- **Ammonia (NH₃)**: This molecule is polar due to the presence of a lone pair on nitrogen, which creates a dipole moment. The nitrogen atom is more electronegative than hydrogen, resulting in a separation of charges.

- **Methane (CH₄)**: This molecule is non-polar. It has a symmetrical tetrahedral shape, and the dipole moments of the C-H bonds cancel each other out, resulting in no net dipole moment.

- **Carbon Dioxide (CO₂)**: This molecule is also non-polar. Even though it has polar C=O bonds, the linear shape causes the dipole moments to cancel each other out, resulting in no net dipole moment.

### Step 3: Analyze the Interactions
- **Ammonia and Methane (NH₃ and CH₄)**: Since ammonia is polar and methane is non-polar, there will be no dipole-dipole interactions between them.

- **Ammonia and Ammonia (NH₃ and NH₃)**: Both molecules are polar and will exhibit strong dipole-dipole interactions due to the presence of a dipole moment and potential hydrogen bonding.

- **Carbon Dioxide and Carbon Dioxide (CO₂ and CO₂)**: Both molecules are non-polar, so there will be no dipole-dipole interactions.

- **Methane and Methane (CH₄ and CH₄)**: Both molecules are non-polar, resulting in no dipole-dipole interactions.

### Step 4: Conclusion
The pair of molecules that exhibits the strongest dipole-dipole interactions is **Ammonia (NH₃) and Ammonia (NH₃)** due to their polar nature and the presence of hydrogen bonding.

### Final Answer
**Ammonia (NH₃) and Ammonia (NH₃)** have the strongest dipole-dipole interactions.

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