Consider the evaporation of methanol at 25.0 °C : CH₃OH(l) → CH₃OH(g) c. Explain why methanol spontaneously evaporates in open air at 25.0 °C

Key Concepts

Evaporation

Evaporation is the process by which molecules at the surface of a liquid gain enough energy to enter the gas phase. This occurs when some molecules have sufficient kinetic energy to overcome intermolecular forces, allowing them to escape into the air. The rate of evaporation increases with temperature, as higher temperatures provide more molecules with the energy needed to evaporate.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. For methanol at 25.0 °C, the vapor pressure is significant enough that some molecules will escape into the air, leading to spontaneous evaporation. When the vapor pressure exceeds the atmospheric pressure, evaporation occurs more readily.

Thermodynamics and Spontaneity

Thermodynamics helps explain the spontaneity of processes based on changes in energy and entropy. The evaporation of methanol is spontaneous at 25.0 °C because it increases the entropy of the system; as liquid methanol transitions to gas, the disorder increases, favoring the process. This aligns with the second law of thermodynamics, which states that spontaneous processes tend to increase the overall entropy of the universe.