Simple and Fractional Distillation Experiment

One of the most important tasks in chemistry is the separation of organic compounds which are not usually found in pure form naturally or as products of chemical synthesis. Distillation is a common method for purifying liquids based upon their boiling points and their differences in vapour pressure. Distillation is a process in which one liquid is separated from another liquid, or a liquid from a non-volatile solid. During the distillation process, the component with the lower boiling point will vapourize first and thereby will travel through the condenser to liquefy in the collection flask before the component of higher boiling point. This process works best when the boiling points of the components are significantly different (simple distillation). Components can be effectively separated by a one-step vaporization. Fractional distillation is used when the boiling temperatures are very similar and the vapour produced is a combination of the mixture. The addition of a fractionating column allows numerous small distillations to occur within the column as the vapour rises towards the condenser.

This experiment will demonstrate the separate a 50:50 mixture of toluene and cyclohexane by two methods and will require two separate equipment setups – one with the fractionating column (fractional distillation) and condenser and one with only the condenser (simple distillation). The results will show the contrast between the two methods and their effeciencies.

Procedure:
See FOL accessed February 3, 2013 and Lab Manual pages 23-32.1,2 Modifications – Use the FOL procedure.
Make sure the thermometer bulb is low enough to achieve good contact with the vapour to be sure of accurate temperature readings. The heating mantle temperature must be watched to maintain continuous boiling within the flask

Chart 1. Graphical Representation of Simple Distillation over Time

Chart 2. Graphical Representation of Fractional Distillation over Time

Compound Descriptions
A) Hexane – Clear, colourless liquid
B) Toluene – Clear colourless liquid
C) Distilled Cyclohexane – Clear, colourless liquid

Conclusion:
Upon comparison of the distillation curves of the simple and fractional distillation demonstrate that fractional distillation separates the two compounds more effectively. Simple distillation separates the majority of the two compounds near the beginning and the end of the distilling process, fractional distillation produced much more pure fractions. In simple distillation there is only the condenser column which allows for less surface area for the different compounds to fully separate. Hexane molecules gain higher kinetic energy through boiling faster than the toluene molecules due to their lower molecular weight and lesser intermolecular forces thus hexane has a lower boiling temperature than the toluene. The longer fractionating distillation column as well as the condenser column of the fractional distillation setup allows the higher kinetic energy component (hexane) to separate from the lower-energy toluene molecules (toluene). Possible sources of error include uneven heating which can be prevented by the addition of boiling beads and failure to maintain the boiling temperature, both of which could result in poor separation of the components and an impure distillate. Questions

1. See attached graphs.
2. From the distillation curves it can be estimated that approximately 15 mL of the liquid boiled below 85oC without the column and 13 mL with the column. 3. The method using the fractional column is more efficient at separating the mixture into its components. 4. One physical property of a pure liquid is that it has a constant boiling point at a constant pressure. This physical property helps in the determination of a liquids’ purity and its identification. Sometimes mixtures may boil off a vapor which has the same ratio of constituents as the liquid mixture which means the chemical composition stays the same in both the liquid and vapour form. These are called azeotropes. Impure mixtures of this type will have a constant boiling point as well but it will not have the same as the boiling point as the pure components. 5. A reduction in the atmospheric pressure will result in a decrease in the boiling point of a liquid. 6. A pure liquid within a distilling flask does not vapourize all at once when the boiling temperature is reached because vapourization is not an instantaneous process.

Vapourization involves the exciting of the atoms and as they reach the surface of the liquid they must overcome the pressure and attractive forces to become vapour. Vapourization is an equilibrium reaction and as some atoms vapourize an equal number of atoms are continually condensing into liquid. 7. It is dangerous to heat a liquid in a distilling apparatus that is closed tightly at every joint and has no vent to the atmosphere because as the liquid boils the vapour pressure within the flask increases with no vent it could have explosive results. As the liquid becomes a vapour or gas the volume expands and will need an outlet to ease the pressure. 8. The cooling water within the condenser must enter to bottom and exit the top because the water must be circulating and the theory of condensing requires a vapour to cool to become liquid. In order to condense the vapour must come into contact with a cool surface and as that surface is exposed to the hot vapour it would heat itself if the water was not constantly circulating away from the heat. Having the water enter the lower end of the condenser also prevents air from becoming trapped as bubbles within the condenser jacket.

9. When a distilling flask is filled more than two-thirds full, there is a chance that the liquid will boil over into the condenser or receiving flask without vaporizing creating an impure distillate. There would also be no room for expansion during the phase change from liquid to vapour leading to an explosive danger. If the flask were too full, the distillation process would also be slow as there would be little surface area for rapid evaporation of the boiling liquid.

References:

1. Brown, William, H., Poon, Thomas Introduction to Organic Chemistry, 4th ed. John Wiley and Sons, Inc. United States. @ 2011. 2. Hart, David, J., Craine, Leslie, E., Hart, Harold, Vinod, T.K. Laboratory Manual to Accompany – Organic Chemistry A Short Course, 13th ed. Brooks/Cole Cengage Learning. Belmont, CA. @2012.