Lecture Notes: Vapor Pressure

Key Concepts

• Vapor Pressure: Measure of the pressure exerted by a vapor in equilibrium with its non-vapor phases.

• Factors Affecting Vapor Pressure:

  • Temperature: As temperature increases, vapor pressure increases.
  • Nature of the Liquid: Different liquids have varying vapor pressures at a given temperature due to differing molecular structures and intermolecular forces.

Important Details

• Dynamic Equilibrium: Vapor pressure is achieved when the rate of evaporation equals the rate of condensation.

  • At this point, the vapor pressure remains constant.

• Boiling Point: The temperature at which the vapor pressure of a liquid equals the external pressure.

  • Normal boiling point occurs when vapor pressure equals the standard atmospheric pressure (1 atm).

• Clausius-Clapeyron Equation: Describes the relationship between vapor pressure and temperature.

  • Equation: ( \ln \left( \frac{P_1}{P_2} \right) = \frac{\Delta H_{vap}}{R} \left( \frac{1}{T_2} - \frac{1}{T_1} \right) )
  • Variables:
    • ( P_1, P_2 ): Vapor pressures at temperatures ( T_1, T_2 )
    • ( \Delta H_{vap} ): Enthalpy of vaporization
    • ( R ): Universal gas constant

Practical Applications

• Understanding vapor pressure is crucial in fields such as meteorology, cooking, and chemical engineering.
• It explains phenomena like:
  • Why water boils at lower temperatures at high altitudes.
  • How pressure cookers work by increasing external pressure to increase boiling point.

Summary

• Vapor pressure is an essential concept in understanding the phase changes and boiling of liquids.
• Dynamic equilibrium and the Clausius-Clapeyron equation are important in predicting and explaining variations in vapor pressure with temperature changes.