The observation that ethanol has a greater vapor pressure than water at any given temperature can be attributed to the differences in intermolecular forces present in each liquid.
Water, with its strong hydrogen bonding due to the presence of two hydrogen atoms bonded to oxygen, exhibits significant cohesive forces that keep its molecules tightly packed. This leads to a lower vapor pressure because more energy is required to break these interactions and allow water molecules to escape into the vapor phase.
Ethanol, on the other hand, while it also forms hydrogen bonds, has a much lesser extent of these interactions compared to water. Ethanol’s molecular structure includes a hydrocarbon chain that does not participate in hydrogen bonding. This results in weaker overall intermolecular forces in ethanol compared to water, allowing it to have a higher vapor pressure. The weaker attractions between ethanol molecules mean that they can more easily enter the vapor phase, contributing to its higher vapor pressure.
In summary, the strength of intermolecular forces in water is greater than in ethanol due to the highly polar nature of water and its ability to form extensive hydrogen bonds, resulting in lower vapor pressure for water compared to ethanol.