Which of the following substances is predicted to have the highest heat of vaporization?

• A. H2O
• B. NH3
• C. CH4
• D. CO2

Answer: (A)

Verified step by step guidance

1. Understand that the heat of vaporization is the amount of energy required to convert a substance from liquid to gas at its boiling point, which depends largely on the strength of intermolecular forces present in the substance.
2. Identify the types of intermolecular forces in each substance: H2O has strong hydrogen bonding, NH3 has hydrogen bonding but weaker than H2O, CH4 has only weak London dispersion forces, and CO2 has London dispersion forces as well.
3. Recall that hydrogen bonding is a particularly strong type of dipole-dipole interaction, which significantly increases the heat of vaporization compared to substances with only dispersion forces.
4. Compare the molecular structures and polarity: H2O is highly polar with two hydrogen bonds per molecule, NH3 is polar with one hydrogen bond per molecule, while CH4 and CO2 are nonpolar with only dispersion forces.
5. Conclude that because H2O has the strongest intermolecular forces (hydrogen bonding) among the options, it will have the highest heat of vaporization.