Vapor Pressure Study Notes

Vapor Pressure

Learning Objectives

  • Interpret a vapor pressure diagram.

Understanding Changes of Physical State

  • Phase Transitions: Includes evaporation, condensation, freezing, and melting of substances, crucial for natural processes like Earth's water cycle.

  • Closed Container Dynamics: In a closed container, when a liquid vaporizes, gas molecules cannot escape, resulting in equilibrium when:

    • The rate of molecules vaporizing equals the rate of molecules condensing.

    • This equilibrium state is termed dynamic equilibrium where processes (vaporization and condensation) occur simultaneously at equal rates.

Equilibrium Vapor Pressure

  • Definition: The pressure exerted by vapor in equilibrium with a liquid in a closed container at a specific temperature, referred to as equilibrium vapor pressure or simply vapor pressure.

  • Measurement Technique: To measure vapor pressure, a liquid sample is placed in a closed container, and a manometer measures the pressure from the vapor in equilibrium with the liquid.

Intermolecular Forces and Vapor Pressure

  • Impact of Intermolecular Attractions:

    • The chemical identity of molecules in a liquid affects the types and strengths of intermolecular attractions, impacting vapor pressure.

    • Strong intermolecular forces hinder vaporization, leading to lower vapor pressure.

    • Conversely, weak intermolecular attractions facilitate vaporization, resulting in higher vapor pressure.

Example: Relative Vapor Pressures of Four Compounds

  1. Diethyl Ether:

    • Small dipole with predominantly London dispersion forces.

    • Largest among the four but with the weakest intermolecular forces, resulting in the highest vapor pressure.

  2. Ethanol:

    • Smaller size than diethyl ether, weaker dispersion forces.

    • Exhibits hydrogen bonding, leading to stronger intermolecular forces than diethyl ether, and consequently, lower vapor pressure.

  3. Water:

    • Smaller size, with weak dispersion forces but extensive hydrogen bonding.

    • Strong intermolecular attractions result in fewer molecules escaping the liquid, yielding lower vapor pressure than diethyl ether and ethanol.

  4. Ethylene Glycol:

    • Contains two -OH groups, leading to extensive hydrogen bonding.

    • Larger molecule resulting in stronger London dispersion forces, leading to the slowest vaporization rate and the lowest vapor pressure.

Temperature and Vapor Pressure

  • Temperature Dependency: As temperature rises, vapor pressure also increases due to enhanced average kinetic energy among molecules.

  • Kinetic Energy Distribution: At higher temperatures, more molecules possess sufficient kinetic energy to overcome intermolecular forces, facilitating vaporization.

  • Vapor Pressure Increase:

    • Higher escape rate of molecules contributes to elevated vapor pressure.

Boiling Point Dynamics

  • Definition: The boiling point of a liquid occurs when its vapor pressure equals the external atmospheric pressure.

  • Normal Boiling Point: Defined as the boiling point when the surrounding pressure is 1 atm (101.3 kPa).

  • Graphical Representation: The variation in vapor pressure with temperature for different substances indicates the relationship between boiling point and surrounding pressure.

Application Example: Boiling Points Under Different Conditions

  • Leadville, Colorado (Elevation: 10,200 feet, Pressure: 68 kPa):

    • Utilizing the vapor pressure graph for water, the boiling point was determined to be approximately 90 °C at 68 kPa.

  • Base Camp on Mount Everest (Ethyl Ether Boiling Point: 10 °C):

    • Graph analysis calculated the approximate atmospheric pressure to be around 40 kPa.