CHEM1201 Lecture Notes - Lecture 21: Vapor Pressure, Phase Rule, Dynamic Equilibrium

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Liquid and solid states:
- difference in volume and density between gases and condensed phases
- stronger intermolecular forces exist in liquids and solids than in gases
Liquids: evaporation
- Vaporisation:
o A few molecules at the surface have enough energy to escape the attractions
of their neighbours
o They enter the gaseous state
- Condensation:
o At the same time, some molecules in the gaseous state can re-enter the
liquid
Evaporation in a closed system:
- In a closed system, a dynamic equilibrium is established
- Dynamic equilibrium:
o Rate of evaporation = rate of condensation
o ***no NET changes
-
Equilibrium vapour pressure:
- in a closed system, the vapour pressure of a liquid comes to a constant value, called
the equilibrium vapour pressure
- the equilibrium vapour pressure is a measure of the tendency of molecules to escape
liquid phase vapour phase
- equilibrium vapour pressure is temperature dependent
-
- Temperature dependence of vapour pressure
o With temperature, more molecules have the required KE escape the liquid
o The equilibrium vapour pressure of a liquid will increase
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Document Summary

Difference in volume and density between gases and condensed phases stronger intermolecular forces exist in liquids and solids than in gases. Vaporisation: a few molecules at the surface have enough energy to escape the attractions of their neighbours, they enter the gaseous state. Condensation: at the same time, some molecules in the gaseous state can re-enter the liquid. In a closed system, a dynamic equilibrium is established. Dynamic equilibrium: rate of evaporation = rate of condensation, ***no net changes. Temperature dependence of vapour pressure: with temperature, more molecules have the required ke escape the liquid, the equilibrium vapour pressure of a liquid will increase. Vapour pressure curves: temperature dependence of equilibrium vapour pressure the vapour pressure increases strongly with temperature volatile liquids (weak intermolecular interactions) have high equilibrium vapour pressures. When the vapour pressure = atmospheric pressure, a liquid starts to boil (open system).

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