DEMONSTRATION pubs.acs.org/jchemeduc
The Spark(L)ing Vinaigrette: A Demonstration That Produces Flames Jorge G. Ibanez,*,† Jorge Luis Vazquez-Olavarrieta,† Max Edgardo Moran-Orozco,† Elizabeth Garcia-Pintor,† Angela K€ohler-Kr€utzfeldt,‡ Michael P. Anderson,§ and Bruce Mattson§ †
Centro Mexicano de Quimica Verde y Microescala, Departamento de Ingenieria y Ciencias Quimicas, Universidad Iberoamericana, Prolongacion Reforma 880, 01219 D.F. Mexico ‡ Romain Rolland High School, Reinickendorf, Place Moliere 4, Berlin, Germany § Department of Chemistry, Creighton University, Omaha, Nebraska 68178, United States ABSTRACT: A laboratory demonstration is described where preparation of a salad vinaigrette is mimicked when a table salt-looking substance is added to a clear vinegar-smelling liquid. Then, when adding a pepper-looking solid, the whole preparation either sparks or bursts into flames. This phenomenon takes advantage of the well-known gas reaction between ethyne and chlorine. KEYWORDS: First-Year Undergraduate/General, General Public, High School/ Introductory Chemistry, Demonstrations, Hands-On Learning/Manipulatives, Acids/Bases, Aqueous Solution Chemistry, Descriptive Chemistry, Oxidation/ Reduction, Reactions FEATURE: Tested Demonstration
I
magine yourself preparing a delicious vinaigrette for your salad, with vinegar, salt, and pepper... and getting a fizzling solution that suddenly bursts into flames! The well-known gas reaction between ethyne and chlorine1,2 is used to develop this spectacular demonstration (Figure 1), in which a table salt-looking substance is added to a clear vinegarsmelling liquid to obtain a fizzling solution. Then, when adding a pepper-looking solid, the whole preparation either sparks or bursts into flames. Acidbase, redox, and thermodynamics concepts are involved and can be discussed with students.
shaker with CaC2 powder (preferably new); if not already a powder, grind a few grams of CaC2 into a fine powder and place inside the empty pepper shaker. Caution: CaC2 grinding must be done away from flames and sparks. Add enough 2040% CH3COOH solution to the beaker (or transparent kitchen Pyrex bowl) as to fill at least one-third of it. Ask one of the students to tell the audience what is in the liquid by smelling its vapors. (Be sure that the student uses a correct wafting procedure for smelling vapors. She or he will immediately say: it is vinegar!) With the saltshaker add some Ca(ClO)2 (shake out the salt several times). A fizzling is immediately obvious as a result of the production of chlorine from the acidification of the hypochlorite (see eqs 1 and 2).3 Now, with the peppershaker carefully add some CaC2 until you get a burst, some sparks, or a sudden flame (Figure 2). If sparks or flames are not observed, it may be because the holes in the shaker tops are too small to deliver sufficient quantities of reagents. Try enlarging their holes as described above or using a spoon. Caution: Hazardous chlorine and acetylene gases are produced. It is best to do this demonstration under a fume hood. If this is not possible, the bowl should be removed to a fume hood immediately after performing the demonstration. Alternative procedures include (i) adding the Ca(ClO)2 first, then the CH3COOH, and last the CaC2; (ii) adding to the CH3COOH, 1/4 or 1/2 teaspoon of Ca(ClO)2, almost immediately followed by a similar quantity of CaC2; (iii) instead of the 200 mL beaker or the Pyrex bowl, a 100 mL Erlenmeyer flask can be used. Caution: the gases become more concentrated and a
’ REAGENTS AND MATERIALS • Glacial acetic acid, CH3COOH • Solid calcium hypochlorite, Ca(ClO)2 • Solid calcium carbide, CaC2 • One vinegar container • Two small transparent salt-and-pepper shakers (make their holes bigger with a hot wire if they are made of plastic or with a drill if they are metallic) • One 200 mL beaker or a small transparent kitchen gelatin dessert Pyrex bowl • One 100 mL graduated cylinder • Mortar and pestle ’ PROCEDURE Prepare 100 mL of a 2040% (v/v) CH3COOH solution by slowly and carefully adding 2040 mL of glacial CH3COOH to 8060 mL of H2O and stirring with a glass rod. Fill approximately one-half of a transparent saltshaker with Ca(ClO)2 powder. Fill approximately one-half of a transparent pepper Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Published: August 01, 2011 1404
dx.doi.org/10.1021/ed100583g | J. Chem. Educ. 2011, 88, 1404–1405
Journal of Chemical Education
DEMONSTRATION
thus, 1.611 1.229 = +0.382 V. Interestingly, singlet dioxygen is produced in the process.4 A plausible alternative reaction involves the oxidation of water by hypochlorite, which produces chloride ions that reduce hypochlorite ion to yield Cl2. When the “pepper” is added, protons replace Ca2+ in the calcium carbide to yield ethyne: CaC2 ðsÞ þ 2Hþ ðaqÞ f C2 H2 ðgÞ þ Ca2þ ðaqÞ
ð5Þ
The reaction of ethyne with chlorine produces the soot: C2 H2 ðgÞ þ Cl2 ðgÞ f 2HClðgÞ þ 2CðsÞ Figure 1. The reagents and material used in the demonstration. Picture taken by Lucia Ibanez-Velasco.
Figure 2. The reaction mixture in flames.
much more vigorous (even violent) combustion may occur, especially upon shaking of the reaction vessel. If a more theatrical effect were desired, consider using a commercial salad dressing vessel; or (iv) small chunks of carbide can also be used instead of the powdered version. One student can be asked to confirm that soot particles have formed on top of the remaining solution and on the walls of the reaction vessel. Because only small quantities of the “salt” and the “pepper” are used, this is truly a small-scale experiment.
’ REACTIONS The observed effects are due to the following reactions. The “salt” and “vinegar” react to produce chlorine gas. This results from the combination of acid and base (eq 1) and redox (eq 2) effects: CaðClOÞ2 ðsÞ þ 2Hþ ðaqÞ / 2HClOðaqÞ þ Ca2þ ðaqÞ ð1Þ 4HClOðaqÞ / 2Cl2 ðgÞ þ O2 ðgÞ þ 2H2 OðlÞ
ð2Þ
The hypochlorite ion, ClO, is protonated in acetic acid because pKa(HOCl) = 7.5 is higher than pKa(HOAc) = 4.8, and thus, ClO is more basic than OAc. The redox reaction is spontaneous because the sum of the standard reduction potentials of the reduction and oxidation half-reactions is positive: 2HClOðaqÞ þ 2e þ 2Hþ ðaqÞ / Cl2 ðgÞ þ 2H2 OðlÞ
E0 ðreductionÞ ¼ þ 1:611 V
ð3Þ
2H2 OðlÞ / O2 ðgÞ þ 4e þ 4Hþ ðaqÞ E0 ðoxidationÞ ¼ 1:229 V
ð4Þ
ð6Þ
From the corresponding enthalpies of formation of reactants and products in eq 6, the reaction enthalpy is calculated as ΔH0 = 412 kJ/molC2H2.5 The soot that is observed is from the incomplete oxidation of ethyne. The highly exothermic nature of this last reaction is responsible for the fire and sparks observed that are highly visible probably due to bits of carbon being heated to incandescence. The last reaction occurs readily because the ClCl bond is relatively weak and because Cl2 and C2H2 are strong oxidizing and reducing agents, respectively. ’ HAZARDS Glacial acetic acid is extremely irritating. Handle with care; prepare the solution under a fume hood and if necessary, smell it by cupping your hand near your nose to recover the vapors. Calcium hypochlorite is a poison and must be handled accordingly. Chlorine gas produced during the experiment is toxic and very irritating. Ethyne is irritating as well. Either perform this demonstration under a fume hood or a very well-ventilated area, or take the entire reaction setup under a fume hood immediately after obtaining the desired effect on a benchtop. Neutralize the resulting solution with a base and handle the residues according to local regulations. Make sure to label the shakers as to their contents and appropriate warnings and clean them out after performing the demonstration; do NOT store the chemicals in them. Handle the residues according to local regulations.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
Michael P. Anderson, (Department of Chemistry, Creighton University, Omaha, NE 68178) and Terry A. Bunde (Division of Natural Science, Maryville College Maryville, TN 37804-5907) tested this demonstration.
’ REFERENCES (1) Mattson, B. Creighton University. Experiments with Ethyne. http://mattson.creighton.edu/C2H2/index.html (accessed Jul 2011). (2) Mattson, B.; Anderson, M.; Nguyen, J.; Harrison, B. Chem13 News April 1997, 257. (3) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; National Association of Corrosion Engineers: Houston, TX, 1974; p 601. (4) Jacobsen, J.; Knak Jensen, S. J. Chem. Phys. Lett. 2007, 449, 135–137. (5) CRC Handbook of Chemistry and Physics, 85th. ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 20042005; pp 529.
’ NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on August 1, 2011, with a typographical error in eq 1. The corrected version was posted on August 8, 2011. 1405
dx.doi.org/10.1021/ed100583g |J. Chem. Educ. 2011, 88, 1404–1405
