Lecture13222
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Liquid and Solid States: Qualitative Aspects
of Phase Changes
Lecture 13
Three states of mattereverywhere.
We consume water in three states
There are states of matter and phases.
Do not confuse them.
Two types of electrostatic forces at work:Intramolecular (bonding) forces exist within each molecule and influence the chemical properties of the substance;Intermolecular (nonbonding) forces exist between the molecules and influence the physical properties of the substance.
A macroscopic comparison:Gas conforms to shape and volume of container, has high compressibility and ability to flow.Liquid conforms to shape of container, its volume is limited by surface. It has very low compressibility and moderate ability to flow.Solid maintains its own shape and volume, has almost no compressibility and ability to flow.
Phase changes
Phase change definitionsMelting (fusion) is a process when a solid liquifies.Freezing (crystallization) is a process when a liquid solidifies.Vaporization is a process when a liquid vaporizes.Condensation is a process when a gas changes into a liquid.
Phase changes
Phase change definitionsSublimation is a process when a a solid becomes a gas without first becoming a liquid.Deposition is a process when a a gas becomes a solid without first becoming a liquid.
How enthalpy changes
Melting and vaporizing are endothermic processes.Condensing and freezing are exothermic processes.
Each phase change has a specific H.H2O(l) H2O(g); Hvaporization
H2O(g) H2O(l); Hcondensation
H2O(s) H2O(l); Hfusion
H2O(l) H2O(s); Hcrystallization
I2(s) I2 (g); Hsublimation
I2(g) I2 (s); Hdeposition
All the reverse processes have H of the same magnitude but opposite sign:Hvaporization= —Hcondensation
Hfusion= —Hcrystallization
Hsublimation= —Hdeposition
Enthalpy of phase changes
Enthalpy of phase changes
Heat of sublimation equals sum of the heats of fusion and vaporization:Solidliquid; Ho
fus
Liquidgas; Hovap
-----------------------------Solidgas; Ho
subl
Heating water
Phase changes of many substances
are reversible and reach equilibrium.
Liquid-gas equilibrium
When a system at equilibrium is
disturbed,it counteracts the
disturbance and eventually re-establishes a state of
equilibrium.
Molecular speed vs temperature
The higher the temperature is, the higher the vapor
pressure.The weaker the intermolecular forces are,
the higher the vapor pressure.
Vapor pressure vs temperature
Vapor pressure vs temperature
Rudolf Julius Emanuel Clausius (1822–1888), German scientist
Benoît Paul Émile Clapeyron (1799-1864), French scientist
The Clausius-Clapeyron equation:
A sample problem
on using the Clausius-Clapeyron equation.
The boiling point is the temperature
at which the vapor pressure equals the external pressure.The boiling point depends on
the applied pressure.
Solid, liquid, gas
Phase diagram of CO2
Features of phase diagram:Regions correspond to different phases of the substance.Lines between regions represent the phase transition curves.
Features of phase diagram:The critical point shows critical temperature and pressure, at which the phase boundary disappears.The triple point shows temperature and pressure, at which all phases co-exist.
Phase diagram of H2O
THE END