Physical chemistry

1. States of Matter: Liquids and Solids
1. Comparison of Gases, Liquids, and Solids
2. Phase Transitions
3. Phase Diagrams

2. States of Matter
 Comparison of gases, liquids, and
solids.
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Pres
enta
tion
of
Lect
ure
Outl
ines
,
11–
2
– Gases are compressible fluids. Their molecules are widely
separated.
– Liquids are relatively incompressible fluids. Their molecules are
more tightly packed.
Solids are nearly
incompressible and
rigid. Their molecules
or ions are in close
contact and do not
move.

3. Changes of State
 A change of state or phase transition is a change
of a substance from one state to another.
solid
liquid
gas
melting freezing
condensation
boiling
sublimation condensation or
deposition

4. Vapor Pressure
 Liquids are continuously vaporizing.
– If a liquid is in a closed vessel with space above it, a partial pressure of the
vapor state builds up in this space.
– The vapor pressure of a liquid is the partial pressure of the vapor over the
liquid, measured at equilibrium at a given temperature.
The vapor pressure of a liquid depends on
its temperature.
–As the temperature increases,
the kinetic energy of the
molecular motion becomes
greater, and vapor pressure
increases.
–Liquids and solids with
relatively high vapor pressures
at normal temperatures are said
to be volatile.

5. Ilustration of Vapor Pressure
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Budda
Water
Budda (air pressure)
will keep the liquid
water down while the
heat vaporizes the
surface water.
Who’s this?

6. Ilustration of Vapor Pressure
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Water
When the vapor
pressure exceeds
the air pressure,
Budda cannot keep
the liquid water
down.
Then Budda rises
and vapor bubbles
form throughout the
liquid. And it boils!

7. Boiling Point
 The temperature at which the vapor
pressure of a liquid equals the pressure
exerted on the liquid is called the boiling
point.
– As the temperature of a liquid increases,
the vapor pressure increases until it
reaches atmospheric pressure.
– At this point, stable bubbles of vapor form
within the liquid. This is called boiling.
– The normal boiling point is the boiling
point at 1 atm.

8. Freezing Point
 The temperature at which a pure liquid changes to a crystalline solid,
or freezes, is called the freezing point.
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– The melting point is identical to the freezing point and is defined
as the temperature at which a solid becomes a liquid.
– Unlike boiling points, melting points are affected significantly by
only large pressure changes.

9. Heat of Phase Transition
 To melt a pure substance at its melting point requires an extra boost of energy to
overcome lattice energies.
– The heat needed to melt 1 mol of a pure substance is called the
heat of fusion and denoted DHfus.
For ice, the heat of fusion is 6.01 kJ/mol
kJ
01
.
6
H
);
l
(
O
H
)
s
(
O
H fus
2
2 
D

To boil a pure substance at its boiling point requires an extra boost of energy to
overcome intermolecular forces.
–The heat needed to boil 1 mol of a pure substance is called the
heat of vaporization and denoted DHvap.
–For water, the heat of vaporization is 40.66 kJ/mol.
kJ
66
.
40
H
);
g
(
O
H
)
l
(
O
H vap
2
2 
D


10. A Problem to Consider
 The heat of vaporization of ammonia is 23.4 kJ/mol. How much heat is required
to vaporize 1.00 kg of ammonia?
– First, we must determine the number of moles of ammonia in 1.00 kg (1000 g).
3
3
3
3
3
NH
mol
8
.
58
NH
g
0
.
17
NH
mol
1
NH
g
10
.00
1 



11. A Problem to Consider
 The heat of vaporization of ammonia is 23.4 kJ/mol. How much heat
is required to vaporize 1.00 kg of ammonia?
– Then we can determine the heat required for vaporization.
kJ
10
1.38
kJ/mol
23.4
NH
mol
8
.
58 3
3 



12. Clausius-Clapeyron Equation
 We noted earlier that vapor pressure was a function of temperature.
– It has been demonstrated that the logarithm of the vapor
pressure of a liquid varies linearly with absolute temperature.
– Consequently, the vapor pressure of a liquid at two different
temperatures is described by:
)
( 2
1
vap
1
2
T
1
T
1
R
H
P
P
ln 
D

A Problem to Consider
Carbon disulfide, CS2, has a normal boiling point of 46°C (vapor pressure = 760 mmHg)
and a heat of vaporization of 26.8 kJ/mol. What is the vapor pressure of carbon disulfide at
35°C?
–Substituting into the Clausius-Clapeyron equation, we obtain:
Hg)
mm
(760
P
ln 2
 )
( K
308
1
K
319
1
K)
J/(mol
8.31
J/mol
10
26.8 3



361
.
0
)
K
10
(-1.12
K)
(3225 1
-
4
-






13. A Problem to Consider
 Carbon disulfide, CS2, has a normal boiling point of 46°C (vapor pressure = 760
mmHg) and a heat of vaporization of 26.8 kJ/mol. What is the vapor pressure of
carbon disulfide at 35°C?
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– Taking the antiln we obtain:
361)
antiln(-0.
Hg)
mm
(760
P2

Hg
mm
760
361)
antiln(-0.
P2 

Hg
mm
530
P2 

14. Phase Diagrams
 A phase diagram is a graphical way to summarize the
conditions under which the different states of a substance
are stable.
– The diagram is divided into three areas representing each
state of the substance.
– The curves separating each area represent the boundaries
of phase changes.

15. Phase Diagrams
 Below is a typical phase diagram. It consists of three
curves that divide the diagram into regions labeled
“solid, liquid, and gas”.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.

16. Phase Diagrams
 Curve AB, dividing the solid region from the
liquid region, represents the conditions
under which the solid and liquid are in
equilibrium.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.

17. Phase Diagrams
 Usually, the melting point is only slightly
affected by pressure. For this reason, the
melting point curve, AB, is nearly vertical.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.

18. Phase Diagrams
 If a liquid is more dense than its solid, the
curve leans slightly to the left, causing the
melting point to decrease with pressure.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.

19. Phase Diagrams
 If a liquid is less dense than its solid, the
curve leans slightly to the right, causing the
melting point to increase with pressure.
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B
temperature
pressure
A
C
D
solid liquid
gas
.
.

20. Phase Diagrams
 Curve AC, which divides the liquid region
from the gaseous region, represents the
boiling points of the liquid for various
pressures.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.

21. Phase Diagrams
 Curve AD, which divides the solid region from the
gaseous region, represents the vapor pressures of
the solid at various temperatures.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.

22. Phase Diagrams
 The curves intersect at A, the triple point, which is
the temperature and pressure where three phases
of a substance exist in equilibrium.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.

23. Phase Diagrams
 The temperature above which the liquid state of a
substance no longer exists regardless of pressure
is called the critical temperature.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.
Tcrit

24. Phase Diagrams
 The vapor pressure at the critical temperature is
called the critical pressure. Note that curve AC
ends at the critical point, C.
B
temperature
pressure
A
C
D
solid liquid
gas
.
.
Tcrit
Pcrit
caffeine

25. BASIC CONCEPTS
 in phase equilibrium, when a substance passes from one phase to another, the
chemical composition does not change (ΔG=0)
allotropic transformations
of substances
25

26. BASIC CONCEPTS
 Phase - a homogeneous part of the system, which throughout its entire
length has the same thermodynamic properties and is separated from other
parts of the system by the interface
 A phase can be formed by one or more constituent substances
 A constituent substance or phase component is a substance that can be
isolated from the system and exist outside it.
 For example:In the air, nitrogen, oxygen, argon and other gases are
constituent substances
 In an aqueous solution of sodium chloride, NaCl and water H2O are
constituent substances
 The smallest number of constituent substances, through which the
composition of any phase is expressed, is called the number of
independent components of this system
26

27. PHASE EQUILIBRIUM WITHOUT CHEMICAL REACTION
 The number of independent components may or may not
coincide with the number of individual substances.
 In the case of phase equilibrium established without a
chemical reaction, the number of independent components is
equal to the total number of components
 For example: in a mixture consisting of gaseous nitrogen N2,
oxygen O2 and argon Ar, between which there is no
interaction, the number of constituent substances is equal to
the number of independent components, i.e. three
27

28. EQUILIBRIUM SYSTEM WITH A CHEMICAL REACTION
 The amounts of constituent substances depend on each other, and the composition of the
phases can be determined from the concentrations of not all, but only a part of the
substances.
 The number of independent components is equal to the number of constituents of
individual substances minus the number of equations relating these substances (their
concentrations).

 For example:in a mixture of three gases HI, I2 and H2, the following reaction is
possible:
Between the concentrations of three substances, a ratio is established, determined by the
equilibrium constant:Knowing the concentrations of the two constituent substances (for
example, HI and H2), it is possible to determine the concentration of the third component
(I2)
The number of independent components is equal to two: 3 - 1 = 2, where 3 is the number of
constituent substances, 1 is the number of equations relating their concentrations
If the concentrations of I2 and H2 in the equilibrium mixture are equal, then one more
condition is added that relates the concentrations of the two constituent substances in the
gas phase, and the number of independent components is equal to one: 3 – 2 = 1
28

29. Equilibrium system with a chemical reaction
29

30. DEGREES OF FREEDOM IN MECHANICS
 Degrees of freedom
30

31. THERMODYNAMIC DEGREES OF FREEDOM
 The thermodynamic degree of freedom (the number of degrees of freedom
or the variance of the system) is the number of parameters that can be
independently changed without changing the number and type of phases
of this system (i.e. so that new phases do not appear and old phases do not
disappear)
 Thermodynamic parameters that can be freely changed:
 temperature T
 pressure P
 volume V
 concentrations of substances Ci
31

32. Crystal Structure of Metals
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