Although molecules in a liquid are held together by intermolecular forces, molecules continually escape from its surface. The pressure produced at liquid–gas equilibrium is called vapor pressure.
Basic Knowledge
When a liquid and a gas coexist in a closed container, the number of molecules reaching the gas phase becomes equal to the number returning to the liquid phase, and liquid–gas equilibrium is reached. The vapor pressure at this point is the saturated vapor pressure. An important point is that vapor pressure depends only on temperature, not on container volume or the amount of liquid.
The temperature at which vapor pressure equals external pressure is the boiling point. At standard atmospheric pressure (1.013×105 Pa), the boiling point of water is 100℃ and that of ethanol is 78.3℃. At places with low atmospheric pressure, such as the summit of Mount Fuji, the boiling point decreases.
Saturated vapor pressure (vapor pressure at liquid–gas equilibrium; depends only on temperature)
Boiling point (temperature at which vapor pressure equals external pressure)
Heat of vaporization and heat of condensation (heat accompanying the liquid↔gas phase change)
Clausius–Clapeyron equation (ln P = -ΔHvap/RT + C)
Surface tension (force that causes a liquid surface to contract; N/m)
Viscosity (resistance to flow; greater when intermolecular forces are stronger)
Further Study (Principles and Applications)
The vapor-pressure curve increases exponentially as temperature rises. Compared with water, ethanol has weaker hydrogen bonding and smaller intermolecular forces, so its vapor pressure is higher at the same temperature.
A pressure cooker raises the internal pressure by sealing the container, increasing water's boiling point to around 120℃ and shortening cooking time. In contrast, vacuum distillation is an industrial technique that evaporates organic substances that decompose easily at high temperatures at low temperatures. Vapor-pressure lowering is treated in Lesson 2 as a colligative property of solutions.
- Saturated vapor pressure is determined only by temperature
- Vapor pressure = external pressure ⇒ boiling
- Higher boiling point ⇔ stronger intermolecular forces
- Substances with hydrogen bonding (H2O, NH3, HF) have unusually high boiling points
- Pressure cooker = high pressure ⇒ higher boiling point
- Vacuum distillation = low pressure ⇒ distillation at low temperature
- The heat of vaporization of H2O is especially large (40.7 kJ/mol)
Important Notes (Common Confusions and Frequent Mistakes)
① Vapor pressure is independent of container volume. Even when compressed, it remains constant if saturated. ② Evaporation occurs at the surface, while boiling involves vaporization throughout the liquid. ③ If the amount of gas in the container does not exceed the amount corresponding to the vapor-pressure value, no liquid remains and everything becomes gas. ④ The boiling point is conventionally defined as the temperature at 1 atm.
Practice
- When water is placed in a sealed 1.0 L container at 27℃, what is the partial pressure of water vapor in the container? (The saturated vapor pressure at 27℃ is 3.6×103 Pa.)
- At the summit of Mount Fuji (atmospheric pressure 6.4×104 Pa), at approximately what temperature does water boil?
- Between water and ethanol at the same temperature, which has the higher vapor pressure? Answer with the reason.