Fractional Distillation

An example of a simple distillation is the separation of a solution of salt and water into two separate pure substances. When the saltwater solution is heated to boiling, water vapor from the mixture reaches the condenser (see Figure 1) and the cold water circulating around the inside tube causes condensation of water vapor into droplets of liquid water. The liquid water is then collected at the lower end of the condenser. The non-volatile salt remains in the flask.

In this experiment, the initial mixture you distill contains two volatile liquids: ethanol and water. In this distillation, both of the liquids will evaporate from the boiling solution. Ethanol and water have normal boiling temperatures of 79°C and 100°C, respectively. One objective of the experiment is to observe what happens when a liquid-liquid mixture is heated and allowed to boil over a period of time. Throughout the distillation, volumes of distillate, called fractions, will be collected. The percent composition of ethanol and water in each fraction will be determined from its density. Water has a density of 1.00 g/cm³ (at 20°C) and ethanol has a density of 0.79 g/cm³ (at 20°C). The fractions you collect will have densities in this range.

OBJECTIVES

In this experiment, you will

• Observe what happens when a liquid-liquid mixture is heated and allowed to boil over a period of time.

• Determine percent composition of ethanol and water in the fraction from its density.

Figure 1

MATERIALS

PROCEDURE

1. Obtain and wear goggles! CAUTION: The compounds used in this experiment are flammable and poisonous. Avoid inhaling their vapors. Avoid contacting them with your skin or clothing. Be sure there are no open flames in the lab during this experiment. Notify your teacher immediately if an accident occurs.

2. Connect the Temperature Probe to LabQuest and choose New from the File menu. If you have an older sensor that does not auto-ID, manually set up the sensor.

3. Assemble the distillation apparatus as shown in Figure 1. Do not begin heating until your teacher has checked your apparatus.

4. Use a 100 mL graduated cylinder to obtain 60 mL of ethanol. Pour the ethanol into the flask. Put 60 mL of tap water into the same flask. Add 3–4 boiling chips to ensure the formation of many small bubbles during the boiling.

5. Make sure the stopper fitting the probe into the flask and the stopper fitting the glass bend into the condenser are tightly in place. Turn on the cold tap water so that it slowly flows up through the condenser.

6. Use a third utility clamp, fitted at the top of the ring stand, to secure the Temperature Probe wire as far as possible from the hot plate.

7. Label the three 100 mL beakers 1–3. Put beaker #1 in place at the end of the condenser.

8. On the Meter screen, tap Rate. Change the data-collection rate to 0.5 samples/second (interval of 2 seconds/sample) and the data-collection length to 1200 seconds. Select OK.

9. Have your instructor check your set-up, and then turn the hot plate on to its maximum setting. Caution: Do not burn yourself or the probe wire with the hot plate.

10. When everything is ready, start data collection. When the temperature reaches 50°C, turn the hot plate down to a medium setting.

11. When 30 mL of liquid (distillate) have been collected in beaker #1, remove it and insert beaker #2 in its place. This 30 mL portion is called Fraction 1. Set it aside until Step 14.

12. After you have collected 30 mL of Fraction 2 in beaker #2, quickly replace it with beaker #3. Set Fraction 2 aside until Step 15.

13. After you have collected 30 mL of Fraction 3 in beaker #3, turn off the hot plate. Set Fraction 3 aside until Step 16. Let the program continue to run until data collection stops after 20 minutes (or stop before 20 minutes has elapsed).

14. Determine and record the mass of a clean, dry 50 mL graduated cylinder. Pour the contents of Fraction 1 into the graduated cylinder. Read and record its volume, to the nearest 0.1 mL. Then determine and record the mass of the distillate plus the graduated cylinder.

15. Discard Fraction 1 as directed by your teacher. Pour the contents of Fraction 2 into the 50 mL graduated cylinder. Read and record its volume, to the nearest 0.1 mL. Then determine and record the mass of the distillate plus the graduated cylinder.

16. Repeat the Step 15 procedure for Fraction 3.

17. Examine the temperature vs. time graph. Find the initial boiling temperature of the mixture (the point where the rapidly rising initial temperature ends, and the slow increase in temperature begins). Record this value in your data table.

18. (optional) Print a copy of the graph. On this graph, locate and label the initial boiling temperature you obtained in Step 17.

Density of ethanol and water mixtures (20°C)

Processing the data

1. After finding the mass of the distillate by subtracting the mass of the graduated cylinder from the mass of the graduated cylinder + distillate, calculate the density of each fraction using the formula: density = mass/volume.

2. Using the density table above, determine the % ethanol corresponding to the density of each fraction. Record these values.

3. Using the values of % ethanol obtained in the previous step, determine the % water for each fraction.

4. What is the primary component of the first fraction you collected? Explain why it is not pure.

5. Did the density of the fractions increase or decrease as the experiment progressed? Explain.

6. What happened to the % of ethanol in the collected fractions as the experiment progressed? What happened to the % of water?

7. What could be done to subsequently increase the purity of the ethanol (reduce the water) in the first fraction? Explain.

8. In Step 17 of the procedure, you found (and recorded on your graph) the initial boiling temperature of the mixture. Is this value lower or higher than the normal boiling temperature of pure ethanol (79°C)?

DATA and calculations