Pure ethanol (C2H6O, molar mass 46 g/mol) and pure propane (C3H8, Molly mass 44g/mol) have similar dispersión force interactions in their pure liquids. But ethanol boils at +78 degrees C, while propane boils at -42 degrees C. Provide an explanation and use structure drawling to illustrate.
The Correct Answer and Explanation is :
Ethanol (C₂H₆O) and propane (C₃H₈) exhibit a significant difference in boiling points—78°C for ethanol and -42°C for propane—despite their similar molar masses (46 g/mol and 44 g/mol, respectively). This disparity is primarily due to the differences in their intermolecular forces.
Intermolecular Forces in Ethanol and Propane
Ethanol contains a hydroxyl group (-OH), which enables hydrogen bonding—a strong type of dipole-dipole interaction. Hydrogen bonds occur when a hydrogen atom covalently bonded to a highly electronegative atom (like oxygen) experiences attraction to a lone pair of electrons on another electronegative atom in a neighboring molecule. These bonds significantly increase the boiling point of ethanol by requiring more energy to break the intermolecular attractions during the phase transition from liquid to gas.
In contrast, propane is a nonpolar molecule consisting solely of carbon and hydrogen atoms arranged in a linear chain. The primary intermolecular forces in propane are London dispersion forces, which are weaker than hydrogen bonds. These forces arise from temporary fluctuations in electron distribution, leading to momentary dipoles that induce attraction between molecules. Due to the weaker nature of dispersion forces, propane molecules require less energy to separate, resulting in a much lower boiling point.
Structural Illustrations
Below are the structural formulas of ethanol and propane, highlighting the presence of the hydroxyl group in ethanol responsible for hydrogen bonding and the absence of such a group in propane.
Ethanol (C₂H₆O):
H H
| |
H - C - C - O - H
| |
H HPropane (C₃H₈):
H H H
| | |
H - C - C - C - H
| | |
H H HConclusion
The higher boiling point of ethanol compared to propane is attributed to the presence of hydrogen bonding in ethanol, which introduces stronger intermolecular attractions than the dispersion forces present in propane. This difference necessitates more energy (in the form of heat) to convert ethanol from a liquid to a gaseous state, thereby elevating its boiling point relative to propane.