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

• A. CH4 and CH4
• B. N2 and N2
• C. O2 and O2
• D. HCl and HCl

Answer: (D)

Verified step by step guidance

1. Step 1: Understand what dipole-dipole interactions are. These are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule.
2. Step 2: Identify which molecules are polar and which are nonpolar. Polar molecules have a permanent dipole moment due to differences in electronegativity and molecular geometry, while nonpolar molecules do not.
3. Step 3: Analyze each pair: CH4 (methane) is nonpolar because of its symmetrical tetrahedral shape; N2 (nitrogen gas) and O2 (oxygen gas) are nonpolar diatomic molecules with identical atoms; HCl (hydrogen chloride) is polar because of the difference in electronegativity between H and Cl.
4. Step 4: Since dipole-dipole interactions occur only between polar molecules, pairs of nonpolar molecules (CH4, N2, O2) will have very weak or no dipole-dipole interactions, relying instead on weaker London dispersion forces.
5. Step 5: Therefore, the pair with the strongest dipole-dipole interactions is HCl and HCl, because both molecules have permanent dipoles that attract each other.