Fractional Distillation - Teacher's Guide

Chemistry Activity
Fractional Distillation

Introduction

Distillation can be used to separate volatile liquids by a process known as fractional distillation. In this procedure, the liquid mixture is placed in the flask of a distillation apparatus. As the flask is heated, the lower boiling liquid vaporizes faster than the higher boiling liquid. Thus, although both components are in the vapor phase, the vapor is richer in the lower boiling liquid. The resulting vapor could be condensed and re-evaporated making the resulting vapor still richer in low boiling liquid. If the process is repeated a sufficient number of times, a separation of the two volatile components can be achieved.

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General Safety Guidelines
Introduction
Advance Preparation
Materials
Pre-Lab Discussion
Interaction in the Lab
Expected Answers to Data Analysis
Answers to Implications and Applications
Answers to Concluding Questions
Relevant National Science Education
Standards
Relevant New Jersey State Science
Education Standards

Advance Preparation

Prepare a 2:3 (by volume) mixture of 2-propanol and distilled water. Ordinary rubbing alcohol may be used; however, the rubbing alcohol is a 70%-by-volume mixture of 2-propanol and water. Thus, approximately equal volumes of the rubbing alcohol and water will be about right. If condensers and distilling flasks are unavailable, a large test tube fitted with a two-hole rubber stopper having a thermometer and a bent glass tube can be used. Whichever setup you use, it is often helpful to students to see the setup in advance. The thermometer bulb should be close to the bottom of the stopper and the bent glass tube is fitted with clear plastic tubing that leads into a large test tube immersed in a beaker of cold water. The water-alcohol mixture is heated in the test tube in this alternative setup. Of course, a smaller amount of the mixture to distill will be needed, i.e., only about 40 ml per student or group.
Materials

Nonconsumables
12 distilling flasks, 150-ml
12 boiling chips
12 condensers fitted with
rubber tubing
24 Ring stands with support
clamps
12 iron rings
48 test tubes, small
12 stirring rods
12 wire gauze, ceramic
centered
12 burners
12 thermometers, –10°C to
110°C x 1°C
12 adapters to fit condenser
(not absolutely essential)
60 test tubes, 20- x 150-mm
12 gunnels, long stem
12 graduated cylinders, 10 ml,
25 ml, and 50 ml
12 test tube racks

Consumables
Sugar, 10 g
Boiling chips
Filter paper or other porous paper
1200 ml 2-propanol/distilled water
mixture, (two parts 2-propanol
to three parts water)
Matches
Labels
Marking pencils
Graph paper
Rulers
Pencils

Pre-Lab Discussion

Select one or two procedures that are within student capabilities. Thoroughly discuss the procedure and demonstrate any new techniques prior to students' laboratory work. Caution students about the flammability of solvents used and the danger of having open flames while solvents are being used. Another possibility is to have different student groups do different procedures and make a group report about the procedure and results. However, do not allow procedures using acetone to be performed next to procedures requiring a burner. Hot plates can be substituted as a source of heat.

Help students with data tables and/or methods of recording observations. None of the procedures requires elaborate data tables, so designing them should be within the capability of the student. Point out that measurements must be recorded. It might be sufficient to simply identify the data to be recorded and leave the rest to the student. Equipment and materials for the laboratory should be easily accessible to students. Demonstrate the distillation assembly consisting of distilling flask, condenser, and adapter. Include the direction of water flow through the condenser. Most chemistry texts have solubility graphs showing the solubility of NaCl and KNO₃. You may want to include this in the pre- or post-lab discussion.
Interaction in the Lab

Circulate to answer student questions and help with equipment setup and technique. Acting as a safety officer is appropriate. Do not hesitate to correct students for unsafe practices. Encourage students to complete assigned procedures and report results regardless of their view about correctness of those results. If students decide better results can be achieved by repeating the procedure, encourage them to do so if there is time. Providing students with specific uses for separation techniques should be motivating.
Expected Answers to Data Analysis

Students will probably agree that it is impossible to tell that the original solution is a mixture of liquids. If heating is done slowly and carefully, students should obtain three separate fractions of liquid. Fraction 1 appears to be a pure substance having a boiling point of about 81°C; Fraction 2 is a mixture of substances similar to the original solution; and Fraction 3 seems to be a pure substance with a boiling point the same as water. (Actually, if properly collected, Fraction 1 is a constant boiling azeotrope that is 90% alcohol and 10% water with a constant boiling point of 80.5°C. It cannot be further separated by distillation, and thus it has the appearance of a single substance. Do not dwell on the purity of this fraction but emphasize that a separation has occurred.)

The initial heating and subsequent graph of temperature versus time are keys to determining the temperature ranges over which to collect the separate fractions. There are two distinct plateaus or flat segments indicating that there are two different substances in the mixture, leading to the suggestion that three fractions should be collected to separate the two substances. Fraction 1 should be collected about 78-82°C, Fraction 2 from 82°C to about the boiling temperature of water, and Fraction 3 beginning at about the boiling point of water. (The boiling point of both liquids depends upon atmospheric pressure at time of the activity. Boiling points are lowered by a decrease in pressure.) A typical set of properties determined for the solution and three fractions is shown in the following table. To obtain density data comparable to that listed above, students will need to make the mass and volume measurements carefully.

Typical student data for fractional distillation activity.

Property Solution Fraction 1 Fraction 2 Fraction 3
Flammability yes yes yes no
Odor medium strong medium none
Density ~0.91 g/ml ~0.084 g/ml ~0.93 g/ml ~1.00 g/ml
Boiling point variable ~81°C variable 100°C
Dissolves sugar? yes no slight yes

Relevant National Science Education Standards

Unifying Concepts and Processes — The activity involves understanding the different components in mixtures and their interactions that are involved in the process of separation.
Science as Inquiry — The activity involves conducting systematic observations, interpreting and analyzing data, drawing conclusions, and communicating results.
Physical Science — The activity examines matter as mixtures or as pure substances and the interaction of energy with a liquid mixture that ultimately lead to the separation of the mixtures components.

Relevant New Jersey State Science Education Standards

5.1 The activity involves conducting systematic observations, interpreting and analyzing data, drawing conclusions, and communicating results.

5.6 The activity examines matter as mixtures or as pure substances.

5.7 The activity examines the effect of an energy transformation (the addition of heat to a liquid until it boils) on the composition of the liquid, that is, separation by distillation.

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Property	Solution	Fraction 1	Fraction 2	Fraction 3
Flammability	yes	yes	yes	no
Odor	medium	strong	medium	none
Density	~0.91 g/ml	~0.084 g/ml	~0.93 g/ml	~1.00 g/ml
Boiling point	variable	~81°C	variable	100°C
Dissolves sugar?	yes	no	slight	yes

5.1	The activity involves conducting systematic observations, interpreting and analyzing data, drawing conclusions, and communicating results.
5.6	The activity examines matter as mixtures or as pure substances.
5.7	The activity examines the effect of an energy transformation (the addition of heat to a liquid until it boils) on the composition of the liquid, that is, separation by distillation.