Vapor pressure

Definition

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid form at a given temperature. This concept is crucial in understanding how real gases behave, as it helps explain the conditions under which a substance can exist in multiple phases, such as liquid and gas. Additionally, vapor pressure plays a significant role in thermodynamics of solutions and mixtures, influencing properties like boiling point and solubility.

5 Must Know Facts For Your Next Test

1. Vapor pressure increases with temperature because more molecules have sufficient energy to escape from the liquid or solid phase into the vapor phase.
2. At any given temperature, different substances have unique vapor pressures, which can influence their evaporation rates and boiling points.
3. Vapor pressure is an essential factor in processes like distillation and evaporation, where separation of components based on their volatility is required.
4. In a closed container, vapor pressure reaches a maximum value where the rate of evaporation equals the rate of condensation, establishing dynamic equilibrium.
5. The presence of solutes in a solution lowers the vapor pressure compared to that of the pure solvent, a phenomenon explained by Raoult's Law.

Review Questions

• How does temperature affect vapor pressure and what implications does this have for real gases?
  • As temperature increases, the kinetic energy of molecules also increases, leading to a higher number of molecules escaping from the liquid phase into the vapor phase. This results in an increase in vapor pressure. For real gases, this relationship indicates that at higher temperatures, gases will behave more ideally as their vapor pressures rise, making it easier to predict their behavior under varying conditions.
• Discuss Raoult's Law and its significance in understanding 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 law is significant because it allows us to predict how adding solutes affects the overall vapor pressure. By understanding this relationship, we can determine how solutions behave differently from pure solvents, particularly regarding boiling points and evaporation rates.
• Evaluate how vapor pressure influences both boiling point and phase transitions within mixtures.
  • Vapor pressure directly influences boiling point; when a liquid's vapor pressure equals atmospheric pressure, it boils. In mixtures, varying vapor pressures of components affect their phase transitions. For instance, if a mixture has components with significantly different vapor pressures, those with higher pressures will evaporate first, altering the composition of both the liquid and vapor phases. This concept is crucial for processes like fractional distillation, where separation relies on differences in vapor pressures.

Related terms

Phase Equilibrium:

The state in which the rates of evaporation and condensation are equal, resulting in no net change in the amount of liquid or vapor present.

Raoult's Law:

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

Boiling Point: The temperature at which the vapor pressure of a liquid equals the external atmospheric pressure, causing the liquid to turn into vapor.