Liquids & Vapor Pressure. Vapor Gas phase of a substance that is normally a liquid at room...
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![Page 1: Liquids & Vapor Pressure. Vapor Gas phase of a substance that is normally a liquid at room temperature. Some evaporation occurs at all temperatures. Generally,](https://reader036.fdocuments.us/reader036/viewer/2022062517/56649efb5503460f94c0e9c2/html5/thumbnails/1.jpg)
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.
• Generally, the easier a substance evaporates, the weaker the intermolecular forces.
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Fig a: open systemopen system. Evaporation continues until all liquid is gone. Fig b: closed systemclosed system. Eventually, rate of evaporation = rate of condensation. System appears static.
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At equilibrium, the rate of evaporation is equal to the rage of condensation.
At equilibrium, the vapor pressure is constant.
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The molecules in the liquid phase have a range of kinetic energies. Some of them have enough kinetic energy to “escapeescape” 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.
Measuring 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.
• As long as some liquid is present, can be a teaspoon or a gallon.
• Vapor pressure depends only on temperature.
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Which has the highest v.p.?
90
50C
20C70C
90C
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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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Boiling Point
• Temperature at which the vapor pressure of a liquid is equal to the external pressure.
Temperature where: VPliq = Pext
• Usually the external pressure is atmospheric pressure.
Temperature where: VPliq = Patm
• Atmospheric pressure varies from day to day.
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Boiling Point
• Temperature at which the vapor pressure of a liquid = external or atmospheric pressure.
• Normal Boiling PointNormal Boiling Point is temperature at which vapor pressure of the liquid = 1 atm.
• Substances with high boiling points have strong molecular interactions & low vapor pressures.
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Normal Boiling Point
• Temperature at which the vapor pressure of a liquid is equal to exactly 1 atmosphere or 760 torr or 101.3 kPa.
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Boiling So, what are 2 ways to get a liquid to boil?
Heat it up or reduce the air pressure above it’s surface
Pext = Patm
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Raise the vapor pressure of the liquid by heating it up.
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Reduce the external pressure using a vacuum pump.
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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
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Can you explain this …
• Drinking Bird
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Phase Changes
• Melting• Boiling (occurs throughout, constant
temperature)– 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, boilingMelting, boiling
Condensation, FreezingCondensation, Freezing
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It’s always from the system’s perspective.
• Endothermic Process: Energy is absorbed
• Exothermic Process: Energy is released
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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?
They’re the same.