We’re being asked to identify the compound with the highest boiling point among the given choices.
Recall that there are several types of intermolecular forces (IMF):
1. Ion-dipole interaction – occurs between an ion and a polar covalent compound; strongest IMF
2. Hydrogen bonding – occurs in compounds where hydrogen is directly connected to an electronegative element such as N, O, or F; 2nd strongest IMF
3. Dipole-dipole interaction – occurs between two polar covalent compounds; 3rd strongest IMF
4. Dispersion forces – occurs in all compounds and is the primary IMF exhibited by nonpolar compounds; weakest IMF
Compounds with strong intermolecular forces have high boiling points. This is because they require more energy to be able to break the bonds during the phase transition.
So, we need to identify which among the given compounds has the strongest intermolecular forces.
For CH4: Since hydrogen cannot be the central atom, carbon goes in the center. The total number of valence electrons present in CH4 is:
Group Valence Electrons
C 4A 1 × 4 e– = 4 e–
H 1A 4 × 1 e– = 4 e–
Total: 8 valence e–
The Lewis structure of CH4 is:
The electronegativity difference between C (EN = 2.5) and H (EN = 2.2) is 0.3 so the C–H bond is nonpolar, which means CH4 is a nonpolar molecule. Since CH4 is nonpolar, it exhibits dispersion forces.
Which of the following has the highest boiling point?
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