Vapor Pressure

Introduced to this in AP Chemistry

What is it? §

Vapor Pressure: Pressure exerted by the vapor of a liquid (or solid) when condensation and vaporization are in dynamic equilibrium

Vapor pressure is a physical property that’s a result of IMFs

for AP Chemistry we’re going to be focusing on liquids not solids

Rate of vaporization = rate of condensation

Vapor Pressure (Molecular Level) §

Vapor Pressure on the Molecular Level

• Some molecules on the surface of a liquid escape the attraction of the IMFs: liquid -> gas phase
• These gas molecules collide with the container creating a pressure
• After some time the pressure will be constant = vapor pressure
• Dynamic Equilibrium: The point when as many molecules escape the surface as strike the surface and return to the liquid layer
• P${}_{vap}$ = $△$ Height (mmHg)

Volatility and Vapor Pressure §

• Volatile substances evaporate rapidly
  • Liquid with high vapor pressure
• In general molecules with greater molar mass have lower vapor pressure - why?
  • Molecules with higher molar mass need more energy to be evaporated which results in less vapor pressure
  • Less vapor pressure means that the IMFs are strong
  • Moris’s comments:
    • State a specific IMF not a general IMF
    • Explain why more molar mass leads to more IMF
    • Tie it back to Vapor Pressure
  • Sample Answer:
    • More polarizable (don’t have to use this) -> more instantaneous dipole moments -> increased LDF -> molecules stay in the liquid phase, so lower vapor pressure