Transcript Liquids & Vapor Pressure - Wappingers Central School District

Liquids & Vapor Pressure
Vapor
• Gas phase of substance that is normally liquid
at room temperature
• Some evaporation occurs at all temperatures
• The easier a substance evaporates, the weaker
the IMF
Fig a: open system
Evaporation continues until all liquid is gone
Fig b: closed system
Rate of evaporation = rate of condensation
System appears static due to equilibrium
How does evaporation occur?
molecules in liquid phase have range of KE;
some have enough KE to “escape” from liquid phase
Vapor Pressure
– Pressure exerted by vapor over its liquid
– VP is affected by temperature:
• higher the T, higher the VP
all liquids:
VP ↑as T ↑
Vapor Pressure
• VP does NOT depend on how much liquid
is present
• VP depends ONLY on temperature
IMF in Liquids
WEAK FORCES
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High VP
High rate evaporation
Low bp
Small Hv
STRONG FORCES
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Low VP
Low rate evaporation
High bp
Large Hv
Phase Changes
• Melting
• Boiling
–Vaporization
• occurs throughout liquid; T constant)
–Evaporation (occurs only at surface; T ↓)
• Sublimation
• Freezing
• Deposition
• Condensation
Phase Changes are accompanied
by energy changes
• Endothermic Process: Energy is absorbed
• Exothermic Process: Energy is released
• Which phase changes absorb energy
(endothermic)?
melting, boiling (vaporization), sublimation
• Which phase changes release energy
(exothermic)?
freezing , condensation, deposition
Boiling Point
• Temperature at which:
VP liquid = external atmospheric P
• Normal Boiling Point:
temp at which VP liquid = 1 atm
• Substances with high bp’s have strong IMF’s
Melting Point
• = temp at which liquid & solid phases of
substance coexist at equilibrium
• higher the mp, the stronger the IMF
Freezing Point
• Temp at which liquid is converted to
crystalline solid
• How does fp compare to mp?
They’re the same: for H2O = 0˚C, 273K
Strong or Weak attractive forces?
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High vapor pressure
Large Hv
High boiling point
Low boiling point
Low vapor pressure
Small Hv
Evaporates rapidly
Evaporates slowly
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Weak
Strong
Strong
Weak
Strong
Weak
Weak
Strong
Boiling and Pressure
• If increase external pressure (ex: camping in
> than 100oC
Death Valley), boiling point is ____
• If decrease external pressure (ex: eating
Raman noodles at top of Mt. Whitney), the
boiling point is ____
< than 100oC
Boiling vs. Evaporation
BOILING
• occurs throughout
liquid
• occurs at definite T
• occurs at constant T
• only spontaneous
when VP = external
pressure
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EVAPORATION
occurs at surface of
liquid (or solid)
occurs at all T
liquid cools off as
evaporates
spontaneous all the
time