Lab #11 - Fractional Distillation
Introduction: Our main source
of hydrocarbons is petroleum. The process of separating, purifying,
and increasing the yield of the desirable components of petroleum are collectively
referred to as petroleum refining.
Since petroleum is a complex mixture of hydrocarbons,
it has no fixed boiling point. The boiling points of the various components
may range from as low as 20oC to as high as 400oC.
The difference in the boiling points of the components of the mixture makes
possible the initial rough separation of petroleum by fractional distillation.
In general, the smaller the molecule, the lower its boiling point. As
a result, in the fractional distillation of petroleum, molecules are separated
according to size. In this experiment you will separate a mixture of
3 hydrocarbons by fractional distillation.
Problem: How can the difference in boiling
points of three hydrocarbons be used to separate them?
Apparatus: Use the diagram below to help set up your equipment.
|250 mL distillation flask or Florence flask
||one-hole rubber stopper for condenser
|bent glass tubing (approximately 60o)
||2 pieces of rubber tubing
|two-hole rubber stopper for flask
|large watch glass
Materials: A mixture of hydrocarbons and water (prepared
by the teacher)
||Place 50 mL of the unknown mixture in the round bottomed distillation
flask. Add several boiling chips. Assemble the apparatus as shown above.
Check that all your connections are tight. When satisfied have the
teacher check them as well.
||Heat the mixture slowly and carefully. Once the
temperature starts to rise start recording the temperature every 30 seconds
until heating is discontinued.
|Note: The leveling off of temperature indicates that one of
the components of the mixture is boiling. Liquid will begin to collect
in the collection flask. Remember that the thermometer is measuring
the temperature of the vapors not the temperature of the liquid in the flask.
||Once the temperature has begun to rise again, immediately change
the collection flask to collect the next component that distills. Carefully
stopper your first flask and set it aside away from the bunsen flame.
||The temperature will again level off. Collect your second distillate
and stopper it once the temperature starts to rise again.
||The temperature will rise again until the third and final distillate
starts to distill. What should remain is water after this. DO
NOT boil to dryness!
||While the apparatus is cooling. Use a 25 mL graduated cylinder
to record the volume of each fraction you distilled.
||Construct a graph of temperature versus time. Plot the time
along the horizontal axis. From the graph, determine the temperature
that most likely represents the boiling point of each fraction component of
the mixture. Record these temperatures.
||Calculate the percentage composition of the mixture by volume, using
the initial volume of the mixture and the volume of the fractions.
||Why does the temperature rise as the distillation proceeds?
||What do the plateaus on the graph of temperature versus time represent?
||Why is there a relatively rapid rise in temperature between the plateaus
on the graph of temperature versus time?
||Account for the difference in boiling points between the factional
components of the mixture.
||What was the percentage composition by volume of the mixture?
||How would a fractionating column help effect a better separation
of the components of a mixture?