Orange-To-Black Halloween Clock Reaction

Orange-To-Black Halloween Clock Reaction The Old Nassau or Halloween reaction is a clock reaction in which the color of a chemical solution changes from orange to black. Heres how you can do this reaction as a chemistry demonstration and a look at the chemical reactions that are involved. Halloween Chemical Reaction Materials WaterSoluble starchSodium metabisulphite (Na2S2O5)Mercury(II) chloridePotassium iodate (KIO3) Prepare the Solutions Solution A: Mix 4 g soluble starch in a couple milliliters of water. Stir the starch paste into 500 ml boiling water. Allow the mixture to cool to room temperature. Add 13.7 g of sodium metabisulphite. Add water to make 1 liter of solution.Solution B: Dissolve 3 g mercury(II) chloride in water. Add water to make 1 liter of solution.Solution C: Dissolve 15 g potassium iodate in water. Add water to make 1 liter of solution. Perform the Halloween Chemistry Demonstration Mix 50 ml solution A with 50 ml of solution B.Pour this mixture into 50 ml of solution C. The color of the mixture will change to an opaque orange color after a few seconds as the mercury iodide precipitates. After another few seconds, the mixture will turn blue-black as the starch-iodine complex forms. If you dilute the solutions by a factor of two then it takes longer for the color changes to occur. If you use a smaller volume of solution B the reaction will proceed more rapidly. Chemical Reactions Sodium metabisulfite and water react to form sodium hydrogen sulfite:Na2S2O5 H2O → 2 NaHSO3 Iodate(V) ions are reduced to iodide ions by the hydrogen sulfite ions:IO3- 3 HSO3- → I- 3 SO42- 3 H When the concentration of iodide ions becomes sufficient for the solubility product of the HgI2 to exceed 4.5 x 10-29 mol3 dm-9, then orange mercury(II) iodide precipitates until the Hg2 ions are consumed (assuming an excess of I- ions):Hg2 2 I- → HgI2 (orange or yellow)If I- and IO3- ions remain, then an iodide-iodate reaction takes place:IO3- 5 I- 6 H → 3 I2 3 H2OThe resulting statch-iodine complex is black to blue-black:I2 starch → a blue/black complex