Question 9:
Which of the following has the highest heat of vaporization (J/g)?
Water has the highest heat of vaporization among the options at approximately 2260 J/g, due to extensive hydrogen bonding requiring more energy to break during vaporization.
Correct Answer: (C) Water
Water’s heat of vaporization is 2257-2260 J/g at boiling point (100°C), highest due to three strong hydrogen bonds per molecule. This exceeds alcohols, aiding evaporation cooling in biology and industry.
Options Comparison
| Option | Compound | Heat of Vaporization (J/g at ~boiling point) | Explanation |
|---|---|---|---|
| (A) | Methanol | ~1100-1170 | Moderate H-bonding (one per molecule); lower molar mass but weaker network than water. |
| (B) | Ethanol | ~841 | Longer chain reduces H-bond density; value drops with carbon atoms. |
| (C) | Water | ~2260 | Strongest intermolecular forces from max H-bonds; anomaly for its size. |
| (D) | Butanol | ~580-590 | Longest chain, weakest relative H-bonding; lowest value. |
Key Factors
Heat of vaporization decreases down alcohols (methanol > ethanol > butanol) as nonpolar chains dominate over H-bonding. Water’s value per gram is uniquely high, explaining high boiling point despite low mass. For exams, recall water ~40% higher than methanol.
