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

a- CO2 and CO2
b- CO2 and CH4
c- NH3 and NH3
d- NH3 and CH4
e- CH4 and CH4

Would it be C?

yes.

No, the correct answer is D- NH3 and CH4. NH3 (ammonia) and CH4 (methane) have dipole-dipole interactions, but NH3 has a stronger dipole moment due to the presence of a polar N-H bond. In contrast, CH4 is nonpolar with no dipole moment. Therefore, NH3 and CH4 have the strongest dipole-dipole interactions among the given pairs of molecules.

To determine which pair of molecules has the strongest dipole-dipole interactions, we need to consider the polarity and the strength of the dipole moment of each molecule.

Dipole-dipole interactions occur between polar molecules. A polar molecule is one that has a positively charged end and a negatively charged end, which we refer to as a dipole moment.

Let's analyze each pair of molecules:

a) CO2 and CO2: Carbon dioxide (CO2) is a linear molecule where the oxygen atoms pull the electron density away from the carbon atom resulting in a dipole moment. However, since both CO2 molecules have the same geometry and the same dipole moment orientation, the dipole-dipole interactions cancel out, making this pair have weaker dipole-dipole interactions.

b) CO2 and CH4: Carbon dioxide (CO2) is a polar molecule, as mentioned before. On the other hand, methane (CH4) is a symmetric tetrahedral molecule, resulting in a nonpolar molecule. The dipole-dipole interactions between a polar molecule (CO2) and a nonpolar molecule (CH4) are weaker than those between two polar molecules.

c) NH3 and NH3: Ammonia (NH3) is a polar molecule due to the difference in electronegativity between nitrogen and hydrogen atoms. When two NH3 molecules interact, the positive end of one molecule (hydrogen atom) will be attracted to the negative end (lone pairs of electrons) of the other molecule. Therefore, the dipole-dipole interactions in this pair are stronger than in the previous options.

d) NH3 and CH4: As discussed earlier, ammonia (NH3) is a polar molecule, while methane (CH4) is nonpolar. Similar to option b, the dipole-dipole interactions between a polar molecule (NH3) and a nonpolar molecule (CH4) are weaker.

e) CH4 and CH4: Both methane (CH4) molecules are nonpolar and have no dipole moment. Therefore, they do not have dipole-dipole interactions.

Considering these explanations, the answer is indeed c) NH3 and NH3, as they have the strongest dipole-dipole interactions among the given options.

CO2 and CO2