Question

1.[ Plot temperature (oC) vs. volume of distillate (ml) collected for the mixture with and without the fractionating column. Use the graph to determine the boiling ranges of the two compounds in the mixture and identify the compounds. Plot both sets of data on the same graph]- I know how to do that part and by the way the unknown compounds are acetone and 1-propanol. --Also plot on this graph an ideal distillation curve. Why are the real distillation curves different from the ideal? 2. Use a vapor/liquid diagram to explain the differences in the simple and fractional distillation plots. Describe what is happening in the fractionation column and explain why this results in a better separation with purer components isolated. 3. Why is it important to maintain a distilling rate of ~0.5 drop/s? Why not faster or slower? 4. What is an azeotrope? Sketch a distillation curve (as in Question 1) for a simple distillation of a mixture of toluene (70 ml) and water (30 ml). Explain the behavior of the curve.

Answer 1

1.- Two curves representing the temperature of the distilling vapor as a function of the volume of distillate. The black curve represents an ideal distillation in which the lower boiling component distills completely and then the higher boiling component distills. The gray curve is a more empirical plot in which the distillate at the beginning of the distillation is enriched in the amount of the lower boiling component and the distillate at the later stages of the distillation is enriched in the higher boiling component.

2.- In fractional distillation the distillation process continues. The condensed material is vaporized again. Follow the blue horizontal line across from the liquid curve at C2 to the vapor curve. There is another improvement in the concentration of the lower boiling component A. The vapor is condensed again. Follow the white vertical line down to the liquid curve. This number of times that the process of vaporization and condensation occurs depends on the efficiency of the distillation column. The more efficient the distillation column, the more times this happens and the purer the final product will be.

3.- Heat is slowly applied to the distillation flask. The amount of heat to apply is determined by the rate of distillation. The liquid should gently bubble and vaporize. As vapor rises from the liquid, it moves up the apparatus raising the temperature of the apparatus. The vapor will fill the distillation flask and most of the distillation head. The thermometer bulb should be completely surrounded by the vapor. If vapor creeps past the thermometer bulb without contacting it, the measured boiling point will be low. The vapor condenses in the condenser and drips into the receiving flask. Typically, the liquid should drip into the receiving flask at a rate of about 0.5 drops per second. If the rate of distillation is too rapid, the heat applied to the distillation flask must be decreased. With too rapid a rate, the measured boiling point is likely to be inaccurate and the purity of the distilled liquid will be compromised.

4.- An azeotrope or a constant boiling mixture is a mixture of two or more liquids whose proportions cannot be altered by simple distillation. This happens because, when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture. toluene / water mixture has bp 85.0oC, but bp toluene is 110.6oC and bp water is 100.0oC. For the toluene-water system, below 85oC the two liquid phases coexist. At 85oC the sum of the vapour pressures of water and toluene = one atmosphere, and boiling starts. In a mixture of toluene and water, boiling occurs at 85oC and both water and toluene are in the vapour that condenses to form two layers. The bottom layer of water has 0.06% toluene dissolved in it. The top layer of toluene has 0.05% water dissolved in it. The relative volumes are 18% water and 82% toluene. The excess toluene flows back into the flask and distils back over with more water.