Liquids

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Liquids Vapor Pressure
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Vapor

• Gas phase of a substance that is normally a
  liquid at room temperature
• Some evaporation occurs at all temperatures
• The easier a substance evaporates, the weaker the
  intermolecular forces

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Fig a open system Evaporation continues
until all liquid is gone Fig b closed system
Rate of evaporation rate of condensation
System appears static
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How does evaporation occur?
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The molecules in the liquid phase have a range of
kinetic energies. Some of them have enough
kinetic energy to escape from the liquid phase
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Vapor Pressure

• Pressure exerted by a vapor over its liquid
• How is vapor pressure affected by temperature?
• The higher the temperature, the higher the vapor
  pressure

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For all liquids, vapor pressure increases as
temperature increases
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Vapor Pressure

• Vapor pressure does NOT depend on how much liquid
  is present
• some liquid is present can be a teaspoon or a
  gallon
• Vapor pressure depends only on temperature

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Intermolecular Forces in Liquids

• WEAK FORCES
• High vapor pressure
• High rate of evaporation
• Low boiling point
• Small Hv

• STRONG FORCES
• Low vapor pressure
• Low rate of evaporation
• High boiling point
• Large Hv

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Phase Changes

• Melting
• Boiling (occurs throughout, constant temp)
• Evaporation (occurs at surface temperature
  drops)
• Sublimation
• Freezing
• Deposition
• Condensation

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Phase Changes are accompanied by energy changes

• Which phase changes absorb energy?
• Which phase changes release energy?

Melting, boiling
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Its always from the systems perspective

• Endothermic Process Energy is absorbed
• Exothermic Process Energy is released

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Boiling Point

• Temperature at which
• vapor pressure of a liquid
• external or atmospheric pressure
• Normal Boiling Point is temperature at which
  vapor pressure of the liquid 1 atm
• Substances with high bps have strong molecular
  interactions

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Melting Point

• temperature at which the liquid phase and solid
  phase of a substance can coexist at equilibrium
• The higher the melting point, the stronger the
  molecular interactions

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Freezing Point

• Temperature at which a liquid is converted to a
  crystalline solid
• How does freezing point compare to melting point?

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Strong or Weak attractive forces?

• High vapor pressure
• Large Hv
• High boiling point
• Low boiling point
• Low vapor pressure
• Small Hv
• Evaporates rapidly
• Evaporates slowly

• Weak
• Strong
• Strong
• Weak
• Strong
• Weak
• Weak
• Strong

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Boiling and Pressure

• If you increase the external pressure (say you
  are camping in Death Valley), the boiling point
  is ________ than 100oC
• If you decrease the external pressure (say you
  are eating Raman noodles at the top of Mt.
  Whitney), the boiling point is ________ than 100oC

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Boiling vs. Evaporation

• BOILING
• Occurs throughout the liquid
• Occurs at a definite temperature
• Occurs at a constant temperature
• Only spontaneous when the vapor pressure equals
  the external pressure

• EVAPORATION
• Occurs at the surface of the liquid (or solid)
• Occurs to some extent at all temperatures
• The liquid cools off as some of it evaporates
• Spontaneous all the time