Forces and Bonding: Phase Distribution

• To explore the nature of intermolecular forces using a simple chemical system
• To explore the nature of dynamic equilibrium in this chemical system
• To learn something about gas chromatography

Gas Chromatography. The detection and identification of the components of mixtures is an important problem in chemistry, with wide practical importance in forensics, cosmetics, and the food, liquor, petroleum, and paint industries. Modern chemists often use chromatography for such separations. There are many types of chromatography, differing in instrumentation and sophistication. Among these are paper chromatography, which you may have used to separate the components of ink; thin layer chromatography (TLC), a simple but very elegant chromatographic method; liquid chomatography (LC); and gas chromatography (GC). We will use gas chromatography in this experiment to explore the nature of intermolecular forces, so some background on the method is in order.

Gas chromatography may be used to detect any reasonably volatile (i.e., vaporizable) substance. Thus gases, liquids, and solutions of volatile solutes are amenable to analysis by GC. Gas chromatography is carried out using an instrument called a gas chromatograph, which has a simple basic design consisting of 3 components. These are the injection port, the column, and the detector. These three components may be separately thermostatted to any desired temperature between room temperature and 250 ^oC. A small, precisely known volume of the sample to be analyzed is introduced to the instrument via syringe at the injection port. For liquid analytes, a volume of about 1 mL is normally sufficient. For analytes in the gas phase, a volume of about 1 mL is required. The injector is kept sufficiently hot to vaporize the sample rapidly once it is injected. The sample vapor is then picked up by a flow of carrier gas (usually He), and is swept into the column. The column is a long glass tube coated with a material that attracts molecules of sample according to their structures. This material is called the stationary phase because it is fixed in place as the sample moves through it. Molecules that are strongly attracted "stick" to (or are retained by) the column more tightly and take more time to pass through it than do weakly attracted molecules, which pass through relatively rapidly. The retention time for a substance is simply the total amount of time it takes after injection for the substance to pass through the column. As the sample passes through the column, then, it is separated into its components, which emerge at different times and pass into the detector. The detector is a device that detects molecules of sample and produces an electrical signal when they pass through it. The electrical signal is sent to a recording device, which shows a trace corresponding to the electrical signal produced by the detector. In modern instruments, a computer serves as the recording device.

The result of doing GC is a chromatogram, a plot of detector signal output versus time elapsed since sample injection. Substances may be identified by their retention times, which are characteristic and reproducible for a given set of instrument operating conditions (temperatures of injector, column, and detector; carrier gas flow rate, and column stationary phase). The amount of substance in moles is proportional to the intensity of the signal, given in relative terms by the area under the signal. We can have the computer determine these areas by integration.

Problem and Hypothesis. Today you will use GC to explore the behavior of a simple chemical system, and to answer some focus questions about the system. The system consists of two liquid solvents, water and 1,1,2,2-tetrachloroethane, and a third liquid, the solute, that dissolves to some extent in both solvents. You will put the two solvents in contact, add the solute, and explore the manner in which the solute distributes itself between the two solvents. Your goal will be to develop answers to the following focus questions below.

1. Can you think of any way other than GC to measure the amounts of solute in the two solvents?
2. Give a molecular-level description of the process by which a solute distributes between 2 solvents, and of the equilibrium situation.
3. Would you expect temperature to have an effect on the distribution of solute between two solvents? What effect?
4. Can you state with confidence which of the two solvents exerts stronger intermolecular forces on your solute? Explain.
5. Which solvent does the solute prefer?
6. What happens to the distribution if you change the amount of one of the solvents?
7. What happens to the distribution if you change the amount of solute?
8. As you vary the relative amounts of solvents, or the amount of solute, is there any property of the distribution that remains unchanged?
9. Does it matter which solvent the solute is initially dissolved in?
10. What is the relationship between the distribution and the molecular structures of solvents and solute?
11. Based on your results, can you predict how a different solute might distribute between the two solvents?

• 5 Pasteur pipets
• 1 Pasteur pipet bulb
• 2 2-mL graduated pipets
• 1 syringe pipet pump
• 2 1-dram vials with septum sealed caps
• 1 1-mL syringe
• 1,1,2,2-tetrachloroethane
• distilled water
• acetone (CH₃C(O)CH₃)
• ethanol (CH₃CH₂OH)
• isopropyl alcohol ((CH₃)₂CHOH)
• methylethyl ketone (CH₃CH₂C(O)CH₃)
• gas chromatograph

Safety glasses must be worn at all times in the laboratory. Most organic liquids are toxic and flammable. Avoid contact with skin, and keep away from open flames or sources of heat.

Record all data in your notebook. Obtain the necessary equipment and clean the glassware thoroughly using brushes and Alconox detergent. Rinse with distilled water and dry thoroughly.

Two solutes will be investigated initially by your section. Your instructor will assign you one of them to study. In the fume hood, use a 1-mL graduated pipet and syringe pipet pump to tranfer 1.00 mL of 1,1,2,2-tetrachloroethane (hereafter TCE) to a 1-dram screw-cap vial. Then transfer 1-mL of water to the vial, and cap it. What do you observe? Obtain about 0.50 mL of your assigned solute in a second 1-dram vial.

Devise and carry out a series of experiments to establish the gas chromatographic characteristics of each of the three liquids (water, TCE, and solute). Plan to use a 1-mL syringe to make injections of 0.2 mL in volume. The instructor will demonstrate the proper technique for making injections and for operating the computer.

Add 5 drops of solute to the solvents vial, cap it, and shake for a few seconds. What do you observe? Make at least 4 injections from the TCE layer, then (at least) 4 injections from the water layer. Based on your results, can you answer any of the focus questions?

Add 0.5 mL of water to the vial, cap, and shake. Again make at least 4 injections from each layer. What do you notice? Do your results help you with any of the focus questions?

Add another 5 drops of solute to the solvents vial, cap, shake, and sample the layers.

Convene your group with another group that has studied the second solute. Compare, discuss, and interpret results, particularly with respect to focus question 10.

Your instructor will assign you another solute to study, different from the initial two. Based on the results for the two initial solutes, predict the manner in which the new solute will distribute between water and TCE, and register your prediction with the instructor before carrying out the measurements. Lewis structures and molecular shape considerations are important in making the prediction.

Clean-up. When you have finished all of your work:

• Clean Labkit glassware by recommended procedures, shake off excess water, and return to the Labkit.
• Clean Pasteur pipets, graduated pipets, and vials by recommended procedures, shake off excess water, and place in the drying oven.
• Return all borrowed equipment to the instructor before leaving lab.
• Clean up your work area before leaving lab.

Organic liquids should be disposed of in the chlorinated solvent waste container in the fume hood.