SCH4C            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 Rings stands
clamps water condenser
condenser clamp one-hole rubber stopper for condenser
bent glass tubing (approximately 60o) 2 pieces of rubber tubing
two-hole  rubber stopper for flask wire gauze
evaporating dishes iron ring
large watch glass Bunsen burner

Materials:   A mixture of hydrocarbons and water (prepared by the teacher)

Procedure:
 
1.   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.
2. 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.
3. 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.
4. The temperature will again level off. Collect your second distillate and stopper it once the temperature starts to rise again.
5. 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!
6. While the apparatus is cooling.  Use a 25 mL graduated cylinder to record the volume of each fraction you distilled.

Calculations:
1.   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.
2. Calculate the percentage composition of the mixture by volume, using the initial volume of the mixture and the volume of the fractions.

Questions:
1. Why does the temperature rise as the distillation proceeds?
2. What do the plateaus on the graph of temperature versus time represent?
3. Why is there a relatively rapid rise in temperature between the plateaus on the graph of temperature versus time?
4. Account for the difference in boiling points between the factional components of the mixture.
5. What was the percentage composition by volume of the mixture?
6.   How would a fractionating column help effect a better separation of the components of a mixture?