Distillation - Simple vs. Fractional

Introduction:

Earlier in your chemistry career you learned that Raoult's Law allows us to calculate the vapor pressure above a solution given the concentration of the solute (Xsolute) and the vapor pressure of the pure solvent:

Psolvent  =  Xsolute Posolvent

If both the solvent and solute are volatile, then Dalton's law of partial pressures, shows us that the total pressure above the solution is the sum of the partial pressures of each solvent and solute:

Ptotal  =  Psolvent  +  Psolute

Using this principles, we can construct a Raoult's law plot that shows the composition of both the liquid and the vapor above the solution:

If for instance, you start with a 50:50 mixture of two volatile components, A and B, you can see that the composition of the liquid is represented by point 'P' on the blue line.  However, the composition of the vapor above this solution is considerably enhanced in the lower boiling component 'A' (point 'Q' on the red line).  If this vapor is collected and cooled, the liquid will have the composition at point 'R'.  The process of going from liquid to vapor to liquid again (points 'PQR') is called a single plate distillation.  If you then take the liquid at point 'R' and collect the vapor above it (point 'S') and cool it to point 'T', you have completed a second single plate distillation and have further purified component 'A' (lower boiling component).  This distillation process can be continued until the desired degree of purity is achieved.


Purpose:

The purpose of this lab is to familiarize you with one of the most valuable techniques in Organic Chemistry, namely distillation.  You will be given a mixture of heptane and hexane (1:1) and asked to separate it into its pure components.


Procedure:

Today you will be comparing the efficiency, timing, and sample consumption of a simple distillation versus a fractional distillation.  Half of the class will perform the simple distillation while the other half performs the fractional distillation.  The procedure for each is identical, with the exception of a fractionating column for the fractional distillation (see figures below).

  1. Pour 25 mL of the (1:1)  normal heptane:hexane mixture into a clean, dry 50 mL round bottom flask.  Remember to add a boiling chip or two.
  2. Assemble the appropriate apparatus (see below).  You may use a 25mL graduated cylinder in place of a round bottom flask as the  receiver to measure the volume of distillate collected. Remember to lubricate all ground glass joints with a minimal amount of silicon grease.  Make sure that all joints are secured (blue clips) and that the whole apparatus is properly clamped.
  3. Make sure that the thermometer is placed ~0.5" – 1"  below the side arm of the condensing column. 
  4. Attach your water hoses (in at the bottom, out at the top).
  5. Use your round iron support ring and wire gauze to support the 50 mL heating mantel snugly under the round bottom flask.  Remember, the heating mantle must NOT be plugged directly into a 120 V outlet, it must be plugged into a variac.
  6. Have your apparatus checked by your instructor. 
  7. Gently turn on your water supply and turn up the variac to 65%.  You may need to reduce or increase this value depending on your combination of heating mantle and variac.  Some produce more power than others.
  8. Note the time when the heat is turned on.  Observe the liquid in the distillation flask and record the time when boiling starts.
  9. Record the head temperature at least once every 5 minutes until the vapor reaches the thermometer.
  10. When the vapor reaches the thermometer, start recording the temperature and volume of distillate collected every two minutes.  You will need to plot temperature vs. volume, so be as accurate as possible.
  11. Stop collecting distillate after the temperature has exceeded 75°C.
  12. Remove the heating mantle and allow the apparatus to fully cool to room temperature.
  13. Measure the volume of liquid remaining in the distillation pot.
  14. Your instructor will assist you in obtaining GC traces of your distillate and the residual liquid in the distillation pot.

Setup for Simple Distillation

Setup for Fractional Distillation

 

 

Conclusions:

 

  • Use your GC data to calculate the percent composition of the distillate vs. the 'pot' for both the simple vs. fractional distillation.
  • Plot the distillation curves (temperature (Y axis) vs volume (X axis)) for the simple, and fractional distillations. 
  • How did the purity of the fractions from the simple distillation compare with those from the fractional distillation?  How do you do you explain this?
  • How did the material recovery compare between the simple and fractional distillation?
  • Did the volume collected as distillate and still remaining in the 'pot' account for the total amount of mixture you started with?  Why?

 (Created by C.R Snelling)
 (Updated 9/30/05 by J. Neilan)