Transcript Physical Properties - Winthrop University

Chapter 8: Physical Equilibria
• In this chapter, we are going to discuss the
equilibrium between phases of matter and the
thermodynamics behind the process
• What can we use this for?
– Environmental Sciences: Toxin cleanup,
wastewater purification
– Biochemistry/Biology: Gas exchange, blood flow
– Materials Science: Colloids, Biomaterials
Equilibrium
•
i.
ii.
iii.
Remember:
At equilibrium, the forward rate of
change is equal to the reverse raate
G=0
A phase change is the change of
matter from one state to another
Solid -> liquid -> gas -> liquid -> solid
Vapor Pressure
If we place some water vapor in
the headspace above the Hg in
a barometer, it will exert
pressure on the Hg
This pressure is proportional to
the amount of water we put into
the space UNTIL we add so
much that liquid water starts
appear on the surface of the
Hg.
No more H2O vapor can form
There is an equilibrium between
the 2 states
Vapor Pressure
•The pressure exerted on the surface of the
Hg increased until we started to get liquid
appearing on the Hg
•At this point, the rate of vaporization
equals the rate of condensation and the
pressure measure is called the Vapor
Pressure
Vapor Pressure
At a fixed temperature, as long as some
liquid is present, the vapor exerts a
characteristic pressure regardless of the
amount of liquid present
The vapor pressure of a given phase of a
substance is the pressure exerted by its
vapor when the vapor is in dynamic
equilibrium with the condensed phase
Vapor Pressure
The vapor pressure of a liquid @ a given temperature is
expected to be low and its enthalpy of vaporization high if
the intermolecular forces are strong
8.3: Variation of Vapor
Pressure with Temperature
• What determines the vapor pressure of
a liquid?
The Intermolecular Forces
or
How easily the molecules can escape
and enter the gas phase
Variation of Vapor Pressure with
Temperature
Variation of Vapor Pressure with
Temperature
• We can model this behaviour with the
Clausius-Clapeyron Equation:
P2  Hvap 1 1 
ln 
ln  
R
P1 
T1 T2 
The higher the Hvap, the more
energy required to vaporize the
molecule
The Vapor Pressure Increases
with increasing temperature
Or
The more molecules move, the
more they go into the gas phase

8.4: Boiling
• What happens when we boil a liquid (let’s say
H2O) at atmospheric pressure?
• At 1 atm and 100°C, the water boils
throughout the volume
– You see bubbles coming from all over/throughout
the pot, not just vapor coming off the surface
• This is the normal boiling point of water
– The Temperature at which the vapor pressure
equals 1 atm
• When the atmospheric/outside pressure is
greater than 1 atm, the boiling point is ____?
– Conversely, at lower pressures, the boiling point is
____
Boiling
Boiling Occurs when the vapor
pressure of a liquid is equal to the
atmospheric pressure
Strong Intermolecular forces usually
lead to higher normal boiling points
8.5: Freezing and Melting
• Remember: In a liquid, the molecules can slip past
each other and in a solid, they can’t
• At the freezing temperature, the liquid and solid
phases are in equilibrium
• The melting temperature is the same as the freezing
temperature, it just depends on which way you look
at it (are you trying to melt a solid or freeze a liquid?)
• The higher the pressure, the higher the melting
(freezing) point. Why is this?
– Less pressure, less inhibition of the molecules to
eject into the gas phase
• Water is unique
– Higher pressures equals a lower melting point
8.6: Phase Diagrams
• A phase diagram plots the
pressure versus
temperature for a given
sample
• We use them to determine
the state of a sample at
given conditions
• Phase boundaries
separate the different
phases and represent
where the phases are in
equilibrium with each
other
Triple point: Where 3 phase lines meet.
All 3 phases are in equilibrium
Representative Phase Diagrams
Sulfur has two solid forms,
rhombic and monoclinic
They differ in the way they are
packed.
There are 3 triple points
Representative Phase Diagrams
There are at least 10 kinds of
ice.
8.7: Critical Properties
What happens when we
move to point C on the
phase curve.
As the temperature and
pressure increase, the
liquid and vapor phases
remain in equilibrium, but
the density of the gas
phase increases
At 218 atm and 374°C, the
interface between liquid
and vapor phases
disappears
8.7: Critical Properties
Critical Temp (Tc)
or Critical
Pressure (Pc)
reached
8.7: Critical Properties
A gas can be liquified at
any pressure or
temperature BELOW Tc or
Pc
Above the critical point, the
material becomes a
Supercritical Fluid
•Many commercial uses
of these