Experiment 3-Boiling PointsDistillation
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State of matter
Vaporization
Solid
Liquid
Gaz
Fig.1: The states of matter
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Vaporization
Vaporization is the process by which a liquid is transformed into a gas
It can be either through:
Evaporation
In which only molecules on the
surface level of a liquid gain
enough energy to break bonds
with one another and escape
as a gas. Thus evaporation is a
surface phenomenon
Fig.2: Evaporation vs. boiling
L
G
Fig.1: The sates of matter
Boiling
In which liquid molecules
throughout the entire bulk of the
liquid gain enough energy to
break bonds with one another
and escape as a gas. Thus boiling
is a bulk phenomenon
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Atmospheric pressure, Vapor pressure, and The boiling point
 Atmospheric pressure (Patm): is a downward pressure exerted, on all things, by the gases
of the atmosphere
 Atmospheric pressure at sea level is 1 atm.
 The higher you go above sea level → the lower the atmospheric pressure becomes
Vapor pressure (Pv): is an upward pressure exerted by a vapor that is
equilibrium with its condensed phase at a given temperature in a closed system
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Fig.3: Boiling occurs when Pv = Patm
Atmospheric pressure, Vapor pressure, and The boiling point
Fig.3: Boiling occurs when Pv = Patm
Boiling point (b.p.): As the temperature of a liquid increases it vapor pressure
increases, The temperature at which the vapor pressure becomes equal to the
atmospheric pressure is called the boiling point
• The higher the vapor pressure of a liquid at a certain temperature, the easier it gets to set
the liquid to boil, thus the lower its boiling point will be.
• The higher the atmospheric pressure above a liquid, the harder it gets to set the liquid to
boil, thus the higher its boiling point will be.
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Factors that Affect the Boiling Point
Molecular weight (M)
In the absence of any intermolecular forces,
the higher the molecular weight of a compound the higher its boiling point
Methane CH4
16 g/mol
-161 oC
Dichloromethane CH2Cl2
84.9 g/mol
39.6 oC
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Factors that Affect the Boiling Point
Intermolecular forces (IMF)
In the presence of intermolecular forces,
the stronger these forces are, the higher the boiling point
Acetone C3H6O
M= 85 g/mol
Weak dipole interactions
56 oC
Water H2O
M= 18 g/mol
Strong hydrogen bonding
100 oC
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Factors that Affect the Boiling Point
Branching (Ramification)
Branching decreases the surface area thus decreasing the intermolecular
force between molecules. As a result, the boiling point decreases
Less number of branches leads to increase the boiling point
n-Pentane C5H12
72 g/mol
36 oC
2-Methylbutane C5H12
72 g/mol
28 oC
2,2-Dimethylpropane C5H12
72 g/mol
9.5 oC
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Effect of Impurities on the Boiling Point
Impurities
Insoluble
No effect
Soluble
Non volatile (Solid solute)
• A solvent is a liquid that dissolves
a solid, liquid or gaseous solute
• A solute is a substance dissolved
in another substance
• A solute and a solvent make up a
solution
The presence of a solute, decreases
the vapor pressure of the liquid, so the
boiling point of the solution will be
more than that of the pure solvent.
This is called boiling point elevation.
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Fig.4: The effect of different kinds of impurities on the boiling point
The Boiling point of a mixture of two liquids
vapor
A
50
B
150
liquid
Fig.4: The equilibrium between
the liquid and gaseous phases
 For a given solution of A and B:
 The boiling point of this solution will lie between the boiling points of pure A and pure B
 The vapor in equilibrium with the solution of A and B will be richer in the more volatile
component
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Distillation
Distillation is a technique used to purify and separate a mixture
of liquids based on the difference in their boiling points

•
•
•
Conditions:
Components have different boiling points
Miscible liquids (homogenous mixture)
Soluble impurities
Distillation can be used:
- to purify a mixture of miscible liquids
- to separate a liquid from non volatile dissolved impurity
- to determine the boiling point of a pure liquid
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Distillation
Simple distillation
Fractionnal distillation
• The difference between the
boiling points is at least 70°C
• The difference between
the boiling points is ˂ 70°C
• Less time
• More time
• Non fractionating column
• Have Fractionating column
• One theoretical plate
• Large number of theoretical
plates
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The Simple Distillation Setup
h
a: A flat bottom flask, fastened by a clamp to a stand. It serves as
the distilling flask, in which the mixture of liquids to be separated
would be placed. The flask should not be more that half filled with
the solution to be distilled to avoid splashing
d
b: Boiling chips (stones), used to prevent superheating of the liquid.
They allow for smooth boiling
f
b
c: The hot plate (heat mantle), used to heat up the mixture of
liquids in the distilling flask
a
d: The Claisen adapter (the three way adaptor), Which connects the
distilling flask with the condenser (e) and the thermometer (h)
c
e
g
Fig.4: A simple distillation setup
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The Simple Distillation Setup
h
e: The condenser, also fastened by a clamp to a stand. It has an
inner tube covered by an outer jacket. Hot vapors coming out of
the distilling flask pass through the inner tube of the condenser
and are cooled down by the cold water passing through the
outer jacket. (blue arrow indicates the flow of water entering
the outer jacket from the tap and red arrow indicates the flow of
water getting out of it in to the sink. The flow of cold water
should always be against the flow of vapors for the condenser
jacket to be completely filled with cooling water
f: The bent adaptor which connects the condenser with the
receiving flask (g) making sure that the condensed liquid enters
it
d
f
b
a
e
g
c
Fig.4: A simple distillation setup
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The Simple Distillation Setup
g: The receiving flask in which the distillate (the liquid
vaporized then condensed) is collected
h: The thermometer, which is used to monitor the
temperature of the hot gases going into the condenser. It
should be placed such that its bulb is just below the side
arm of the Claisen adapter
Note
 When the glassware are connected together, a lubricant
(vacuum grease) is applied to the connections to avoid
breakage of the glass, and leakage of the hot gases
 Also clips could be used on the joints to make sure the
glassware don’t separate
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The Distillation Process
Given a homogeneous mixture of A and B such that A is more volatile than B:
Case 1: The difference in the boiling point of A and B is more than 70 oC
• In order to separate the two from each other, the mixture is placed in
a simple distillation setup
• As the temperature in the distilling flask increases, A which has a lower boiling point, will
start to vaporize out of it and into the condenser where it is cooled down and condenses
back to liquid
• Liquid A moves through the condenser and into the receiving flask via the receiving adapter
(first distillate)
• During this process, as long as only component A is being vaporized, the temperature
registered by the thermometer remains constant and is equal to the boiling point of liquid A
• With time all of liquid A will be vaporized, and collected away from liquid B, which remains in
the distilling flask
• Thus resulting in two separate fraction: Fraction 1 containing only A, and fraction 2
containing only B
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The Distillation Process
Given a homogeneous mixture of A and B such that A is more volatile than B:
Case 2: The difference in the boiling point of A and B is less than 70 oC.
In such a case, as liquid A is being vaporized and collected, molecules of liquid B
will gain enough energy to start co-vaporizing along with liquid A. The temperature
registered by the thermometer will start to move up, away from the boiling point of A,
indicating that both A and B are being co-vaporized
In this case there will be three fraction:
-
Fraction 1 containing only A (collected when the temperature registered by the thermometer is
constant and equal to the boiling point of A)
-
Fraction 2 (co-fraction) containing both A and B (collected when the temperature registered by the
thermometer is moving away from the boiling point of A until it is just about the boiling point of B)
-
Fraction 3 containing only B which is left in the distilling flask
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The Distillation Process
Note that if the difference in the boiling points of A and B is too little fractions 1 and 2 will come
out together, resulting in one Co-fraction that is enriched in liquid A
In order to completely separate liquid A from liquid B, the co-fraction should be taken and distilled
another time, to get another three fraction with a smaller volume co-fraction, which in turn
distilled again, and so on
To avoid repeating the distillation process an infinite number of times a
fractional distillation should be used to efficiently separate homogeneous
mixtures of liquid whose difference in boiling point is less than 70 oC
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Fractional Distillation
• The fractional distillation setup is the same as the simple one, but
with a fractionating column (i) in between the distilling flask and
the Claisen adaptor
• The fractionating column is a vertical column packed with glass
beads that has a large surface area providing an infinite number of
theoretical plates
• As the co-fraction vapor rises through the fractionating column, it
condenses upon touching the cold glass beads. As the condensate
goes down it will reach a part of the column sufficiently hot to boil
again. each time this cycle occurs, the rising vapor will become
richer in the more volatile component. Eventually the two
components of the mixture will be separated even if the difference
in their boiling points is less than 70 oC
i
Fig.4: A fractional distillation setup
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Thank You
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