Chemistry for Everyone edited by
Tested Demonstrations
Ed Vitz
Entertaining Chemistry—Two Colorful Reactions
Kutztown University Kutztown, PA 19530
submitted by:
John F. Elsworth Chemistry Department, University of Cape Town, Rondebosch 7700, South Africa;
[email protected] checked by:
Jeannine Eddleton Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0212
Reactions which result in color change always have a magical appeal in “fun” lectures. Numerous excellent examples are to be found (1–3). Here are described two reactions, which are related and are relatively easy and safe to demonstrate. The first I have named “The Sunday School Reaction” for the reason that a Sunday School teacher many years ago asked me to prepare two solutions, one black and one red, which on mixing, would produce a white mixture or suspension. The second occurred to me in 1994 when the political situation in South Africa changed. We were named a “Rainbow Nation”, comprising four races: black, brown, red, and white. Why not produce a series of solutions, black, brown, red, and white, which on mixing would produce a clear, colorless solution, thus demonstrating that a clear solution to South Africa’s problems is to be seen when our four races work together! For this reason I have named the second the “New South Africa Reaction”. The Sunday School Reaction
Materials (concentrations need only be approximate) 0.02 M potassium permanganate 3 M sulfuric acid 0.10 M sodium thiosulfate 5 M sodium hydroxide 0.10 M barium chloride 10% potassium iodide Phenolphthalein indicator 1% starch solution Distilled water 250-mL beaker and 400-mL beaker. Graduated pipets and 100-mL measuring cylinders
Method BLACK SOLUTION. Into the 400-mL beaker pour ca. 100 mL of distilled water, ca. 1 mL of potassium permanganate solution, and 30 mL of sulfuric acid. Now add 10 mL of potassium iodide solution followed by starch solution and set aside. RED SOLUTION. Into the 250-mL beaker pour ca 100 mL of distilled water, ca 3 mL of sodium thiosulfate, 1 mL of barium chloride solution, 0.1 mL of NaOH, and a few drops of phenolphthalein indicator to give a deep pink color. DEMONSTRATION. Add the red solution to the black solution and swirl once or twice. The colors disappear and a white precipitate remains!
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Equations The black solution is the color formed when starch is added to iodine. The iodine is the result of oxidation of acidified iodide solution by permanganate: 2MnO4᎑ + 10I ᎑ + 16H+ → 2Mn2+ + 5I2 + 8H2O The red solution results when phenolphthalein is added to sodium hydroxide. When the solutions are mixed, the excess acid will eliminate the pink color and the excess sodium thiosulfate will eliminate the black of the iodine solution. The iodine oxidizes the thiosulfate ion to the tetrathionate ion according to the equation I2 + 2S2O32᎑ → 2I ᎑ + S4O62᎑ The barium(II) ions in the red solution are precipitated as white insoluble barium sulfate, owing to the sulfuric acid in the original black solution: Ba2+ + SO42᎑ → BaSO4(s) NOTES. In the black solution, one requires (i) excess of sulfuric acid to eliminate the red color and form a precipitate of barium sulfate, and (ii) sufficient permanganate solution to produce iodine. The red solution should not have an excess of sodium hydroxide but requires sufficient thiosulfate to react with all the iodine in the black solution. If too much thiosulfate is added, however, an undesirable precipitate of barium(II) thiosulfate may occur in the red solution. Using the given dilutions, no precipitation of barium(II) tetrathionate has been observed upon mixing the solutions. It is advisable, however, to do a trial run before the demonstration. The New South Africa Reaction
Materials The same solutions as above, omitting the barium chloride solution. In addition, one requires: Magnesium sulfate (satd aq) 5 M ammonia (aq) Four 250-mL beakers One 800-mL beaker Preparation of Solutions BROWN SOLUTION. Prepare 200 mL following the procedure described for the black solution above, omitting the addition of starch. Pour half the solution into a 250-mL beaker.
Journal of Chemical Education • Vol. 77 No. 4 April 2000 • JChemEd.chem.wisc.edu
Chemistry for Everyone
BLACK SOLUTION. To the remaining half add starch solution. RED SOLUTION. Prepare as described above but omitting the addition of barium chloride. WHITE SUSPENSION. To a beaker add 100 mL of the magnesium sulfate solution and then add ammonia from a measuring cylinder until a white precipitate forms. It is important that the total volume of 3 M sulfuric acid added in the brown and black solutions be well in excess of the volume of ammonia added.
Demonstration It is better to ask a member of the audience to assist in the mixing of the four solutions. The contents of the four beakers are poured, together if possible, into the 800-mL beaker. A clear, colorless solution will result! Equations The brown, black, and red colors disappear as a result of the reactions shown in the equations given earlier. The
white suspension is formed when magnesium hydroxide precipitates on addition of sufficient aqueous ammonia: Mg2+ + 2NH3 + 2H2O → 2NH4+ + Mg(OH)2(s) In the presence of sufficient sulfuric acid, the magnesium hydroxide redissolves: Mg(OH)2 + 2H+ → Mg2+ + 2H2O NOTE. One should avoid too much sodium thiosulfate, as the final mixture on standing may deposit colloidal sulfur through the acidic decomposition of thiosulfate: S2O32᎑ + H+ → S(s) + HSO3᎑ Literature Cited 1. Elsworth, J. F. J. Chem. Educ. 1995, 72, 1128–1130. 2. Alyea, H. N.; Dutton, F. B. Tested Demonstrations in Chemistry, 6th ed.; Division of Chemical Education of the American Chemical Society: Easton, PA, 1965. 3. Humphreys, D. A. Demonstrating Chemistry; McMaster University: Hamilton, ON, Canada, 1983.
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