Vapor pressure

Definition

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid form at a given temperature. It reflects the tendency of molecules to escape from the liquid or solid phase into the gas phase, and is a critical concept in understanding phase transitions, such as evaporation and boiling.

5 Must Know Facts For Your Next Test

1. Vapor pressure increases with temperature, as higher temperatures provide more energy for molecules to escape from the liquid phase.
2. At a given temperature, different substances have different vapor pressures due to variations in intermolecular forces.
3. The equilibrium state of vapor pressure can be disturbed by changes in temperature or volume, leading to either an increase or decrease in the amount of vapor present.
4. Vapor pressure plays a crucial role in determining the boiling point of liquids; a liquid boils when its vapor pressure equals the external atmospheric pressure.
5. In mixtures, the total vapor pressure is affected by the presence of solutes, which can either increase or decrease the vapor pressure compared to that of pure solvents.

Review Questions

• How does temperature influence vapor pressure and what implications does this have for phase changes?
  • Temperature has a direct effect on vapor pressure; as temperature increases, vapor pressure also increases because more molecules gain sufficient energy to escape from the liquid into the vapor phase. This relationship is crucial for understanding phase changes like evaporation and boiling. For example, when a liquid's vapor pressure equals the external pressure, it begins to boil, transitioning rapidly from liquid to gas.
• Discuss Raoult's Law and how it relates to vapor pressure in solutions.
  • Raoult's Law states that the vapor pressure of a solvent in a solution is proportional to its mole fraction. This means that when solutes are added to a solvent, they disrupt the ability of solvent molecules to escape into the vapor phase, effectively lowering the overall vapor pressure compared to that of pure solvent. This principle helps explain phenomena like boiling point elevation and freezing point depression in solutions.
• Evaluate how understanding vapor pressure can impact real-world applications such as distillation or material selection for chemical processes.
  • Understanding vapor pressure is essential in processes like distillation, where separating components based on differences in their boiling points relies on their vapor pressures. By knowing the vapor pressures of various substances, chemists can optimize conditions for effective separation. Additionally, selecting materials for chemical processes requires consideration of their vapor pressures, especially in situations involving volatile compounds where high vapor pressures might pose safety risks or influence reaction kinetics.

Related terms

Boiling Point: The temperature at which the vapor pressure of a liquid equals the external pressure surrounding the liquid, leading to the formation of bubbles within the liquid.

Raoult's Law:

A principle that states the vapor pressure of a solvent in a solution is proportional to the mole fraction of the solvent present in that solution.

Sublimation: The process in which a solid transitions directly into a gas without passing through the liquid phase, often occurring at low pressures and higher temperatures.