Which pair of molecules has the strongest dipole-dipole interactions? a. NH3 and CH4 b. CO2 and CH4 c. NH3 and NH3 d. CO2 and CO2 e. CH4 and CH4
The Correct Answer and Explanation is:
Correct Answer: c. NH₃ and NH₃
Explanation:
Dipole-dipole interactions are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. These interactions only occur between molecules that have a permanent dipole moment, meaning the molecules must be polar.
Let’s examine the molecules listed in the options:
- NH₃ (ammonia): A polar molecule. It has a trigonal pyramidal geometry due to the lone pair on nitrogen, and a significant difference in electronegativity between nitrogen and hydrogen. This gives NH₃ a permanent dipole.
- CH₄ (methane): A nonpolar molecule. It has a tetrahedral shape with equal electronegativity pulling in all directions, cancelling out dipoles.
- CO₂ (carbon dioxide): A nonpolar molecule, even though the C=O bonds are polar. Its linear geometry causes the dipoles to cancel out, resulting in no net dipole moment.
Now, evaluating the options:
- a. NH₃ and CH₄: NH₃ is polar, CH₄ is nonpolar → no dipole-dipole interaction.
- b. CO₂ and CH₄: Both are nonpolar → no dipole-dipole interaction.
- c. NH₃ and NH₃: Both are polar → strong dipole-dipole interactions. Additionally, NH₃ can form hydrogen bonds, a special type of strong dipole-dipole interaction, due to the N-H bond.
- d. CO₂ and CO₂: Both are nonpolar → no dipole-dipole interaction.
- e. CH₄ and CH₄: Both are nonpolar → no dipole-dipole interaction.
Therefore, NH₃ and NH₃ not only exhibit dipole-dipole interactions but also hydrogen bonding, which is among the strongest intermolecular forces for neutral molecules. This makes them the pair with the strongest dipole-dipole interactions in the given list.
