In the Laboratory
A Simple, Safe Way To Prepare Halogens and Study Their Visual Properties at a Technical Secondary School* Domingo A. Liprandi, Orlando R. Reinheimer, José F. Paredes, Pablo C. L’Argentière** Química Inorgánica, Departamento de Química General e Inorgánica (FIQ, UNL), Santiago del Estero 2829, 3000 Santa Fe, Argentina
The halogens are a group of the periodic table that is well suited to verifying the progressive variation of physicalchemical properties with increasing atomic number (1–4). However, the toxicity and hazardousness of the elements of this group make them difficult to obtain and manipulate during a laboratory class. Because of this, much experimental visual information and the possibilities for learning about the halogens’ behavior are lost. The procedure proposed here contributes to improving observation skills. This is in general a difficult task, but it is important because it allows what is seen to be connected with previous theoretical knowledge, favoring significant learning (5). The method is a relatively short, simple, safe, and visual way to prepare chlorine, bromine, and iodine with reactants and glassware that are easy to obtain and to handle. The method and equipment have been tested successfully for more than 10 years to teach part of the halogens’ chemical behavior in laboratory classes of Inorganic Chemistry at FIQ-UNL. To evaluate how the students perceive this experiment, concept maps and V-diagrams were used (5). After using this kind of tests, we concluded that the students understand the halogens’ chemistry much better when they carry out the procedure described in this paper. Teaching-Laboratory Policies The present experience and all activities that can result from it must be carried out under the guidance of a skillful
Figure 1. Glass equipment setup used to prepare halogens in aqueous solutions. Reactants are distributed as follows. A: Filtration flask, 20 mL of HCl (36%). B: Separatory funnel, 30 mL of NaClO (commercial grade). C: Dreschel flask, 20 mL of H2O (demineralized). D: Test tube, 6 mL of KBr (1 M) and 12 mL of petroleum ether 60–80. E: Test tube, 6 mL of KI (0.1 M) and 12 mL of petroleum ether 60–80. F: Filtration flask, NaOH (3 M). G: Compressor tubing clamps. *A preliminary report on this subject was presented at the Associação Brasileira de Química meeting, September 1995, XXXV Congresso Brasileiro de Química, Salvador, Bahia Brazil. **Corresponding author. Email:
[email protected].
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and trained teaching assistant, obeying the following safety rules carefully. 1. Safety goggles and gloves Complete approved safety goggles (not safety glasses) and impervious gloves must be worn in the laboratory at all times. 2. Appropriate dress Students and teaching assistants must be appropriately dressed in the laboratory at all times. This constitutes wearing a laboratory coat that protects the skin from the neck to below the knees, and closed-toe shoes. 3. Contact lenses Contact lens wearers are encouraged to wear glasses in the laboratory. 4. Hair Students with hair longer than shoulder length must wear it appropriately contained.
Experimental Details The halogens are obtained by electron transfer reactions (redox) in aqueous solution, at room temperature and acid pH, as reported elsewhere (3, 6–8). These procedures are the most universally adopted because they work satisfactorily with quite inexpensive and easy-to-get chemicals.
Procedure CAUTION: All the involved chemicals are highly toxic and harmful! They must be handled with care. Information about their characteristics can be found in http://www.siri.org/msds/. Lighted Bunsen burners are not allowed! A glass equipment setup (shown in Fig. 1) is built under a fume hood. Liquid reactants are measured with a pipet and the help of a valved pipet filler and added to the receptacles as indicated in the legend to Figure 1. NaClO is added only after verifying that the system is hermetic (this is critical!). Teflon–glass connections can be sealed with paraffin wax, and a vacuum achieved with a water pump. To start working, one or two bubbles per second must be obtained in C (the letters A–G in the material that follows refer to the part of Figure 1 that is so labeled). This assures mild mixing in D and E, favoring the halogen solubility in petroleum ether 60–80 and avoiding excessive solvent volatility. No more NaClO is necessary after 30 mL is consumed. In F, a 3 M NaOH solution is needed in such a quantity that ca. 1 cm of the lower end of the glass tube is immersed in it. The end point of the experiment is reached when the aqueous solution in E becomes a pale amber color (be careful!). The vacuum is reduced while the separatory funnel is open completely; these two actions must be carried out slowly and simultaneously. The separatory funnel is closed immediately after the pressure equilibrium is recovered throughout the system. At this point compressor tubing clamps (G) are tightly fastened.
Journal of Chemical Education • Vol. 76 No. 4 April 1999 • JChemEd.chem.wisc.edu
In the Laboratory
Results and Discussion The presence of the halogens can be followed easily by some of their visual physical properties—in this case, color, aggregation state, and solubility. In A, chlorine is detected as a green-yellow gas. In D, bromine presents an orange-red color and a high solubility in petroleum ether. In E, iodine appears violet, mostly dissolved in petroleum ether; here it must be emphasized that the color is the same as that corresponding to the element in gaseous state, where the forces between molecules are minimum. In between there is a time when an intense amber solution (KI3) and a solid (I2) can be seen in the aqueous phase. Connecting this last observation with the gaseous aggregation state for chlorine, a trend in physical properties can be emphasized. That is: Within the halogen group, a higher atomic number is associated with an X2 molecule that has greater Van der Waals’ forces, which favors the solid state and also implies higher melting and boiling points. Despite the fact that bromine and iodine are mainly in the nonaqueous phase, all the redox reactions proceed in water. The experimental results also let us infer that the oxidation power decreases from Cl2 to I2, obeying the principle that “stronger oxidants can be used to prepare weaker ones”. In other words, in this case, the halogen with the lowest atomic number is useful for obtaining the other members of the group. This clearly shows a variation in chemical properties along the group. The last paragraph can be presented and justified with the corresponding global reactions, Keq and normal potentials (E°), as an approximation to reality (6, 9, 10). Electron transfer reactions are justified using potentials, whereas reactions that do not present changes in oxidation states are justified using equilibrium constants. In A: Cl{ 1/2Cl2 + e H+ + e + HClO H+ + Cl{ + HClO
1/ Cl 2 2
+ H2O Cl2 + H 2O
E° = {1.35 V E° = 1.63 V E°T = 0.28 V
As the ClO{ ion is unstable in an acid solution, HClO appears as one of the reactants. The following Keq demonstrates this: H+ + ClO {
HClO
Keq = 2.13 × 104
In D: Cl{ 2Cl2 Br{ 1/2Br2 1/ Cl + Br{ Cl{ + 1/2Br2 2 2 1/
E° = 1.35 V E° = {1.07 V E°T = 0.28 V
Then, most of the bromine is in the petroleum ether (p.e.) according to the experimental result: Br2(aq)
Br2(p.e.)
Keq > 1
In E: 1/
2Cl2 1/
I{
Cl{
2I2 1/ Cl + I { Cl{ + 1/2I2 2 2 Then, as in the bromine case:
I2(aq)
I2(p.e.)
E° = 1.35 V E° = {0.54 V E°T = 0.81 V Keq > 1
The Dreschel flask (C) is used to wash chlorine and to get the halogen in water solution. Such a solution is called chlorine water. Cl2(g) Cl2(aq) Keq = 6 × 10 {2 NaOH in F acts as a trap for the excess of chlorine owing to its instability in a basic solution (this holds not only for chlorine but for the other halogens as well) (6, 11). In this case, at room temperature the redox reaction is: e + 1/2Cl2 Cl{ E° = 1.35 V 2HO{ + 1/2Cl2 ClO{ + e + H2O E° = {0.40 V 2HO{ + Cl2 ClO{ + Cl{ + H2O E°T = 0.95 V Last but not least is the fact that at the end of the experiment, chlorine is in water, bromine and iodine are in petroleum ether 60–80, and all of them are ready to be used with other chemicals to check their reactivity in a safe way. This new activity has to be done according to the teaching assistant’s criteria. Beyond the End Point If chlorine keeps bubbling, once the pale amber solution is present in E, iodine production diminishes drastically. This is explained as due to the halogen mutual reactivity that generates an interhalogen compound. The pale amber solution contains ICl2{ ions, which appear according to the following reaction: e + 1/2Cl2 Cl{ E° = 1.35 V 4Cl{ + I2 2ICl2{ + 2e E° = {0.97 V 2Cl{ + Cl2 + I2 2ICl2{ E°T = 0.38 V Waste Materials W ARNING: All the waste chemicals that remain after the experiment must be neutralized with NaOH solution at room temperature under the fume hood before being disposed of properly. This is based on the strong link between the halogens and the pH of the system (6, 11) (see the last chemical equation of the “Results and Discussion” section). Acknowledgment We are indebted to UNL (CAI+D Program) for financial support. Literature Cited 1. de Barry Barnett, E.; Wilson, C. L. Inorganic Chemistry; Longmans, Green and Co.: London, 1957. 2. Gould, E. S. Inorganic Reactions and Structure; Henry Holt: New York, 1958. 3. Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry; Oxford University Press: Oxford, 1994. 4. Sanderson, R. T. J. Chem. Educ. 1994, 41, 361–366. 5. Novak, J.; Gowin, D. B. Learning How To Learn; Cambridge University Press: New York, 1984. 6. Cotton, F. A.; Wilkinson. G. Advanced Inorganic Chemistry; Interscience: New York, 1962. 7. Huheey, J. E.; Keiter, E. A; Keiter, R. L. Inorganic Chemistry: Principles of Structure and Reactivity; HarperCollins: New York, 1993. 8. Candom, F. E.; Murray, S. J. Chem. Educ. 1959, 36, 534. 9. Lattimer, W. M. The Oxidation States of the Elements and Their Potential in Aqueous Solutions; Prentice-Hall: Englewood Cliffs, NJ, 1952. 10. CRC Handbook of Chemistry and Physics, 69th ed.; Weast, R. C., Ed.; CRC: Boca Raton, FL, 1988. 11. Michalowski, T. J. Chem. Educ. 1994, 71, 560–562.
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