Hexane, C6H14 (MW=86 g/mol) has a boiling point of 68°C. Ethanol, CH3CH2OH (MW= 46 g/mol) has a boiling point of 78°C. Mark each of the following statement as True or False.
a. Ethanol must have stronger IMF, based on its higher boiling point
b. Ethanol has a higher boiling point because of greater London dispersion forces
c. Both hexane and ethanol have hydrogen bonding
d. Ethanol has a higher boiling point due to hydrogen bonding
e. Hydrogen bonding and London dispersion forces are at cross purposes here (one favors ethanol, the other favors hexane); in this case the hydrogen bonding evidently “wins”.
The Correct Answer and Explanation is:
Correct Answers:
a. True
b. False
c. False
d. True
e. True
Explanation
Boiling point is directly influenced by the strength of intermolecular forces (IMFs)—stronger IMFs require more energy (heat) to overcome, resulting in a higher boiling point.
(a) True: Ethanol has a higher boiling point (78°C) than hexane (68°C), despite its lower molar mass. This suggests that ethanol has stronger intermolecular forces than hexane. Stronger IMFs correlate with higher boiling points.
(b) False: London dispersion forces (LDFs) are temporary, weak attractions that increase with molecular size and surface area. Hexane, being larger and more hydrophobic, actually experiences stronger London dispersion forces than ethanol. Therefore, LDFs do not explain ethanol’s higher boiling point.
(c) False: Hydrogen bonding occurs when a hydrogen atom is directly bonded to a highly electronegative atom (like O, N, or F) and interacts with a lone pair on another electronegative atom. Ethanol (CH₃CH₂OH) has an –OH group, which allows hydrogen bonding. Hexane (C₆H₁₄), however, is a nonpolar hydrocarbon with only C–H bonds, which cannot hydrogen bond. Therefore, only ethanol exhibits hydrogen bonding.
(d) True: Ethanol’s higher boiling point is primarily due to hydrogen bonding, a much stronger IMF than London dispersion forces. The polar –OH group in ethanol allows molecules to associate through these strong interactions, requiring more energy to break apart during boiling.
(e) True: This statement correctly acknowledges the interplay between two types of IMFs: London dispersion forces dominate in hexane (a larger, nonpolar molecule), while hydrogen bonding dominates in ethanol. Despite hexane’s stronger LDFs due to its size, the hydrogen bonding in ethanol is more significant, leading to its higher boiling point. Thus, “hydrogen bonding wins” in determining the boiling point comparison.
In summary, the key factor driving ethanol’s higher boiling point is its ability to hydrogen bond, which outweighs the stronger London dispersion forces present in the larger, nonpolar hexane molecules.
