Chapter 10 –
Liquids and Solids
10.8 – 10.9 Notes
AP Chemistry
Changes in State
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The state of matter a substance exists in
depends on its environment.
Three states of matter:
• Solid
• Liquid
• Gas
Vapor
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Vaporization (evaporation) is the
endothermic process in which a liquid
becomes a gas.
Heat of Vaporization:
• Energy needed to vaporize 1 mole of liquid
• Water has a high ΔHvap = 40.7 kJ/mol,
meaning it can absorb a lot of heat
Vapor Pressure
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Vapor – gas phase of a substance that exists
as a liquid at 25oC and 1 atm.
There is a dynamic equilibrium between a
liquid and its vapor.
A liquid/vapor system has reached
equilibrium when the rate at which liquid
molecules become vapor and vapor
molecules become liquid is the same.
Vapor pressure – pressure exerted by vapor
at equilibrium
Measuring Vapor Pressure
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Vapor pressure can be measured using
a simple barometer.
Patmosphere = Pvapor + P Hg column
Pvapor = Patmosphere - P Hg column
Vapor pressure of a substance increases
with temperature.
Graphical Analysis of Vapor
Pressure vs. Temperature
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A plot of vapor pressure vs. temperature is nonlinear.
ln(Pvap) vs. 1/T(K) is linear and follows y = m x + b
y = lnPvap, x = 1/T, m = -ΔHvap/R, C = a constant
characteristic of the liquid R = 8.31 J/(Kmol)
Clausius-Clapeyron Equation:
lnPvap = -(ΔHvap/R)(1/T) + C
Or
ln(Pvap,T1/Pvap,T2) = (ΔHvap (1/T2-1/T1))/R
This relationship is useful for determining either
enthalpies of vaporization from vapor pressure
measurements or vapor pressures at different
temperatures.
Example
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The boiling point of acetone at 760. torr
is 56.5oC. At an elevation of 5,300 ft the
atmospheric pressure is 630. torr. What
would be the boiling point of acetone
(ΔH = 30.2 kJ/mol) at this elevation?
What would be the vapor pressure of
acetone at 25oC at this elevation?
Sublimation
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A solid to gas phase change is called
sublimation.
This occurs because solids have vapor
pressures.
Changes of State
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Changes in state can be represented using
a heating curve (a plot of temperature vs.
time).
Solid↔Liquid = Heat of Fusion (ΔHfus)
Point at which solid and liquid have identical
vapor pressures
Liquid↔Gas = Heat of Vaporization (ΔHvap)
Point at which liquid and vapor have the
same vapor pressure under conditions in
which the system is under a constant
pressure of 1 atm.
ΔHvap> ΔHfus
Normal Melting and Boiling
Points
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Normal Melting Point – temperature at
which the solid and liquid states have the
same vapor pressure under conditions
where the total pressure is 1 atmosphere
Normal Boiling Point – temperature at
which the vapor pressure of a liquid is
exactly one atmosphere
Heating Curves
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When heating or cooling a substance at
a constant rate q = mcΔT.
At phase boundaries, the temperature
remains constant until the entire
substance has changed state. Only then
can the substance be heated or cooled.
Superheating and cooling can occur
when a liquid is heated or cooled rapidly.
Example
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A 0.250 g chunk of sodium metal is dropped
into a mixture of of 50.0 g of water and 50.0 g
of ice, both at 0oC.
The reaction is
2Na(s) + H2O(l)  2NaOH(aq) + H2(g)
ΔH = -368 kJ/mol
Will the ice melt? Assuming the final mixture
has a specific heat capacity of 4.18J/goC,
calculate the final temperature. The enthalpy
of fusion for ice is 6.02 kJ/mol.
Phase Diagrams
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Phase diagrams represent the phases of
a substance as a function of temperature
and pressure.
Phase diagrams represent a substance
in a closed system.
Phase Diagrams Continued
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Critical Temperature – temperature above
which no applied pressure can liquefy a
vapor
Critical Pressure – pressure required to
produce liquefaction at the critical
temperature
Critical Point – point at which the critical
temperature and pressure are achieved
Triple Point – point at which the vapor
pressure of the solid, liquid, and gaseous
phases are in equal