Distillation
Distillation
is a commonly used method for purifying liquids and separating mixtures of
liquids into their individual components. Familiar examples include the
distillation of crude fermentation broths into alcoholic spirits such as gin
and vodka, and the fractionation of crude oil into useful products such as
gasoline and heating oil. In the organic lab, distillation is used for
purifying solvents and liquid reaction products.
To understand distillation,
first consider what happens upon heating a liquid. At any temperature, some
molecules of a liquid possess enough kinetic energy to escape into the vapor
phase (evaporation) and some of the molecules in the vapor phase return to the
liquid (condensation). An equilibrium is set up, with molecules going back and
forth between liquid and vapor. At higher temperatures, more molecules possess
enough kinetic energy to escape, which results in a greater number of molecules
being present in the vapor phase. If the liquid is placed into a closed
container with a pressure gauge attached, one can obtain a quantitative measure
of the degree of vaporization. This pressure is defined as the vapor pressure
of the compound, and can be measured at different temperatures.
When a solution reaches the
boiling point of constituent, the substances will vaporize. When vapours of
substance reach the condenser, the cool and condense into pure liquide. The
pure liquid is collected in the receiving flask until the solution stops
boiling
Condensation is done by the
condenser.Cool water is run through the condenser to cool the vapour below its
boiling point. Then the vapour is converted into liquide.
Fractional Distillation of Crude Oil
BOILING POINTS AND STRUCTURES OF HYDROCARBONS
The boiling points of
organic compounds can give important clues to other physical properties. A
liquid boils when its vapor pressure is equal to the atmospheric pressure.
Vapor pressure is determined by the kinetic energy of molecules. Kinetic energy
is related to temperature and the mass and velocity of the molecules. When the
temperature reaches the boiling point, the average kinetic energy of the liquid
particles is sufficient to overcome the forces of attraction that hold
molecules in the liquid state. Then these molecules break away from the liquid
forming the gas state.
Vapor pressure is caused
by an equilibrium between molecules in the gaseous state and molecules in the
liquid state. When molecules in the liquid state have sufficient kinetic
energy, they may escape from the surface and turn into a gas. Molecules with
the most independence in individual motions achieve sufficient kinetic energy
(velocities) to escape at lower temperatures. The vapor pressure will be higher
and therefore the compound will boil at a lower temperature.
BOILING
POINT PRINCIPLE:
Molecules which strongly
interact or bond with each other through a variety of intermolecular forces can
not move easily or rapidly and therefore, do not achieve the kinetic energy
necessary to escape the liquid state. Therefore, molecules with strong
intermolecular forces will have higher boiling points. This is a consequence of
the increased kinetic energy needed to break the intermolecular bonds so that
individual molecules may escape the liquid as gases.
THE BOILING POINT CAN BE A ROUGH MEASURE OF THE AMOUNT OF ENERGY NECESSARY TO SEPARATE A LIQUID MOLECULE FROM ITS NEAREST NEIGHBORS.
THE BOILING POINT CAN BE A ROUGH MEASURE OF THE AMOUNT OF ENERGY NECESSARY TO SEPARATE A LIQUID MOLECULE FROM ITS NEAREST NEIGHBORS.
MOLECULAR
WEIGHT AND CHAIN LENGTH TRENDS IN BOILING POINTS
A series of alkanes
demonstrates the general principle that boiling points increase as molecular
weight or chain length increases (table 1.).
Table
1. BOILING POINTS OF ALKANES
Formula
|
Name
|
Boiling Point C
|
Normal State at Room Temp.
+20 C
|
CH4
|
Methane
|
-161
|
gas
|
CH3CH3
|
Ethane
|
- 89
|
|
CH3CH2CH3
|
Propane
|
- 42
|
|
CH3CH2CH2CH3
|
Butane
|
-0.5
|
|
CH3CH2CH2CH2CH3
|
Pentane
|
+ 36
|
liquid
|
CH3(CH2)6CH3
|
Octane
|
+125
|
|
The reason that longer
chain molecules have higher boiling points is that longer chain molecules
become wrapped around and enmeshed in each other much like the strands of
spaghetti. More energy is needed to separate them than short molecules which
have only weak forces of attraction for each other.
FOCUS
ON FOSSIL FUELS
Petroleum refining is
the process of separating the many compounds present in crude petroleum. The
principle which is used is that the longer the carbon chain, the higher the
temperature at which the compounds will boil. The crude petroleum is heated and
changed into a gas. The gases are passed through a distillation column which
becomes cooler as the height increases. When a compound in the gaseous state
cools below its boiling point, it condenses into a liquid. The liquids may be
drawn off the distilling column at various heights.
Although all fractions
of petroleum find uses, the greatest demand is for gasoline. One barrel of
crude petroleum contains only 30-40% gasoline. Transportation demands require
that over 50% of the crude oil be converted into gasoline. To meet this demand
some petroleum fractions must be converted to gasoline. This may be done by
"cracking" - breaking down large molecules of heavy heating oil;
"reforming" - changing molecular structures of low quality gasoline
molecules; or "polymerization" - forming longer molecules from
smaller ones.
For example if pentane
is heated to about 500 C the covalent carbon-carbon bonds begin to break during
the cracking process. Many kinds of compounds including alkenes are made during
the cracking process. Alkenes are formed because there are not enough hydrogens
to saturate all bonding positions after the carbon-carbon bonds are broken.
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