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Chem. Educator 2010, 15, 108–109
A New Procedure for the Lightning Experiment: Mn2O7 and Ethanol Francesco Caruso*,†, Francesco Giannici‡, Fulvio Caruso§, and Michele A. Floriano† †
Department of Physical Chemistry “Filippo Accascina”, Università di Palermo, Viale delle Scienze, Parco d’Orleans, Ed. 17, I 90128 Palermo, Italy,
[email protected], ‡Department of Inorganic and Analytical Chemistry “Stanislao Cannizzaro”, Università di Palermo, Viale delle Scienze, Parco d’Orleans, Ed. 17, I 90128 Palermo, Italy §Department of Electrical, Electronic and Telecommunication Engineering (DIEET), Università di Palermo, Viale delle Scienze, Parco d’Orleans, Ed. 9, I 90128 Palermo, Italy Received July 30, 2008. Accepted September 2, 2009.
Abstract: Formation of Mn2O7 and its reaction with ethanol is commonly referred to as “underwater lightnings”. Classic procedures are reviewed, along with their weak points, and physical and chemical properties of dimanganese heptaoxide are shown. A new improved approach is finally presented, safer and easier, and suited for lecturehall activities.
Introduction
2 HMnO4 = Mn2O7 + H2O
In order to demonstrate oxidation of organic compounds, it could be very nice making one of them react with a powerful oxidizing agent [1] and turn the reaction into a show. It would be possible, then, to talk about oxidation without invariably mentioning the burning of natural gas in mom’s cooking-stove. Furthermore, dimerization of inorganic chemical species is very seldom treated at the high school level and this demonstration could be a new way of introducing the concept in General Chemistry classes. As a matter of fact, only the formation of dimer dinitrogen tetroxide (N2O4) from radical ·NO2 has been previously treated, because of its traditional educational value in explaining the role of temperature in chemical equilibria [2–4]. An important dimerization equilibrium is proposed as an example of a pH-dependent reaction: 2 CrO42- + 2 H+ = Cr2O72- + H2O
(1)
More correctly, the reaction (1) is the balanced sum of the following ones [5]: (2)
2 HCrO4- = Cr2O72- + H2O
(3)
The experiment is performed as a demonstration in a Magic of Chemistry Show organized mainly for students of high schools. It involves the dimerization of permanganate ion (by protonation and dehydration – as occurs with the chromate ion) into the extremely more reactive dimanganese heptaoxide (Mn2O7) neutral specie: +
MnO4 + H = HMnO4 *
(4)
Address correspondence to this author. Department of Physical Chemistry. ‡ Department of Inorganic and Analytical Chemistry. § Department of Electrical, Electronic and Telecommunication Engineering †
Concentrated sulphuric acid subtracts water and moves equilibrium (5) to the right. The formation of the dimer is indicated by a colour change, from violet to dark green. At room temperature, dimanganese heptaoxide is an unstable hygroscopic oil (m.p. 5 °C; heat of formation at 20 °C -177.4 kcal) with a green metallic lustre in reflected light and looks dark red in transmitted light. It acts as a powerful oxidizing agent which produces oxygen. It claims to be stable below -10 °C if anhydrous conditions are maintained. It dissociates at 55 °C and ignites if heated above 95 °C [6–9]: 2 Mn2O7 4 MnO2 + 3 O2
(6)
during slow dissociation 2 Mn2O7 2 Mn2O3 + 4 O2
(7)
during explosive dissociation By addition of ethanol, the latter is oxidized to carbon dioxide:
CrO42- + H+ = HCrO4-
-
(5)
C2H5OH + 2 Mn2O7 2 CO2 + 4 MnO2 + 3 H2O
(8)
Manganese (VII) oxide can react explosively with most organic compounds and attacks acetic acid and anhydride and carbon tetrachloride even below room temperature. Reagents and Materials Concentrated sulphuric acid (H2SO4 at 95-98 %, CAS number: 7664-93-9) Very pure crystalline potassium permanganate (KMnO4, ACS reagent ≥ 99.0 %, CAS number: 7722-64-7) Ethanol (C2H5OH, CAS number: 64-17-5) Cold water A tall Pyrex® beaker
© 2010 The Chemical Educator, S1430-4171(10)12220-1, Published 01/27/2010, 10.1333/s00897102220a, 15100108.pdf
A New Procedure for the Lightning Experiment Mortar and pestle Spatula Pasteur pipettes A polystyrene container Clamp (if needed) Procedure This experiment was previously reported in a German chemistry demonstration book [8], in several web sites [10–13] and in a journal [14], featuring slightly incorrect chemical and practical remarks. Among them, the role of sulphuric acid instead of any other acid; use of macro scale quantities of reagents (200 mL of ethanol and 200 mL of sulphuric acid); use of common glassware instead of Pyrex® one; etc. The procedure commonly found in the cited references indicates to pour H2SO4 in a test tube, let tube edge dry and pour gently and slowly ethanol (n-propanol can be otherwise used [8]) to get the two liquids in two phases. It should be noted that, although very mixable, the two liquids can actually form two phases because of their quite different density: 0.8 g cm-3 for ethanol and 1.8 g · cm-3 for sulphuric acid (at room temperature) [15–16]. It should be also noted that the last step is really difficult to perform even for expert operators. It is hard to get a perfectly biphasic system as required. Then, the addition of small crystals of KMnO4 should cause tiny sparkles and cracklings at the interphase just like “underwater lightnings”. While testing this demonstration, the test tube caught fire and broke. On the contrary, according to the present method, risks are almost eliminated. The modified procedure is illustrated as follows. Using very clean and dry mortar and pestle, grind some permanganate crystals to a finely divided state. Now, using a Pasteur pipette, prepare a film of sulphuric acid on the bottom of a previously very well cleaned and dried Pyrex® vessel by dropwise addition of concentrated sulphuric acid. Containers must be Pyrex® made because of the local very large heat of reaction which could break common laboratory glassware and they also should be dry because water moves equilibrium (5) to the left. Fill the polystyrene container with little cold water to keep the reaction vessel at relatively low temperature (10 – 15 °C). With the spatula, sprinkle some permanganate powder (i.e. between 10 and 30 mg) on the sulphuric acid film. A colour change – from dark violet (MnO4- ion) to intense green (formation of the dimer, Mn2O7) – will be observed. Place the beaker with Mn2O7 into the polystyrene container. If the beaker floats onto the cold water then fix it with a clamp. Pay much attention not to pour any water into the reaction vessel. Wait till the beaker and its content are cooled to water temperature. At this moment, two alternative procedures can be followed: either ethanol can be added dropwise from another pipette or it could be gently sprayed. With the latter choice, little crackling sparkles confined into a small environment will form. Spraying ethanol can continue until no more green compound turns into brownish-black (Mn2O7 reduced to MnO2). Dropping ethanol with a pipette will create a crackling fire (this is the most suitable way for rapidly exhausting Mn2O7).
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Handling and disposal. Chemicals used in this demonstration must be handled with great care although the very small used quantities. Goggle, latex gloves and safety glasses must be worn to prevent accidental and painful contacts with skin, hands and eyes. Furthermore, the demonstration must not be scaled up for any reason due to the extreme reactivity of the produced Mn2O7. A fire extinguisher must be on hand. After completely exhausting dimanganese heptaoxide with ethanol, reaction products and excess of reagents (mainly MnO2 and H2SO4) should be disposed of according to local requirements. The relevant MSDSs should be carefully read before performing this experiment [17–18].
References and Notes 1.
Shakhashiri, B. Chemical Demonstrations; University of Wisconsin Press: Madison, WI, 1992; Vol. 1, pp. 83–84.
2.
Hennis, A. D.; Highberger, C. S., Schreiner, S. J. Chem. Ed. 1997, 74, 1340.
3.
Yu, Q.; Gao, H. J. Chem. Ed. 1997, 74, 233.
4.
Allen, M.; Joyner, C.; Kubler, P. G.; Wilcox, P. J. Chem. Ed. 1976, 53, 175.
5.
Butler, J. N. Ionic Equilibrium, A Mathematical Approach; AddisonWesley: Reading, MA, 1964; pp. 360–362.
6.
Brauer, G. ed. Handbook of Preparative Inorganic Chemistry, 2nd edition; Academic Press: New York, NY, 1965; p. 1459.
7.
Chambers, C.; Holliday, A. K. Modern Inorganic Chemistry, An Intermediate Text; Butterworth & Co.: London, UK, 1975; p. 385.
8.
Roesky, H. W.; Möckel, K. Chemical Curiosities: Spectacular Experiments and Inspired Quotes, VCH, Weinheim, D, 1996; p. 33 (translated by Mitchell, T. N. and Russey, W. E.), ISBN 3-52729414-7.
9.
Lide, D. R. et al. CRC Handbook of Chemistry and Physics. 85th edition; CRC Press: Boca Raton, FL, 2005; p. 4–69.
10.
Oxidation von Ethanol: “Blitze unter Wasser”. http://www.phheidelberg.de/wp/schallie/minilab/blitze.htm (accessed Mar 2008).
11. Blitze unter Wasser. (accessed Mar 2008).
http://www.tibicen.de/chemie/blitze.htm
12.
Gewitter. http://www.chemieexperimente.de/zauber/gewitter.html (accessed Mar 2008).
13.
Versuch – Nr.031 Blitze unter Wasser. http://www.experiment alchemie.de/versuch-031.htm (accessed Mar 2008).
14.
Koch, K. R. J. Chem. Educ. 1982, 59, 973.
15. Lide, D. R. et al. CRC Handbook of Chemistry and Physics. 85th edition; CRC Press: Boca Raton, FL, 2005; p. 3–256. 16. Lide, D. R. et al. CRC Handbook of Chemistry and Physics. 85th edition; CRC Press: Boca Raton, FL, 2005; p. 4–87. 17. Manganese dioxide. http://www.jtbaker.com/msds/englishhtml/ m0715.htm (accessed Mar 2008). 18. Safety (MSDS) data for sulfuric acid. http://ptcl.chem.ox.ac.uk/ MSDS/SU/sulfuric_acid_concentrated.html (accessed Mar 2008).
© 2010 The Chemical Educator, S1430-4171(10)12220-1, Published 01/27/2010, 10.1333/s00897102220a, 15100108.pdf
