The Old Nassau or Halloween reaction is a clock reaction in which the color of a chemical solution changes from orange to black. Here's how you can do this reaction as a chemistry demonstration and a look at the chemical reactions that are involved.
-Halloween Reaction Materials:
- water
- soluble starch
- sodium metabisulphite (Na2S2O5)
- mercury(II) chloride
- potassium 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 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 sufficent 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 H2O
The resulting statch-iodine complex is black to blue-black:I2 + starch --> a blue/black complex
For the Halloween reaction the materials that we need are water, mercury chloride, soluble starch, potassium iodite and sodium metabisulphite. We start off lining up three different beakers for each of the solutions. In Solution A, we mix 4grams of starch in 2 milliliters of water until it's a paste. Then we pour the paste into 500 milliliters of boiling water. As it cools down to to room temperature, we add 13.7 grams of sodium metabisuphite; we add water to make 1 liter of the solution. In solution B, we just dissolve 3 g mercury(II) chloride in water then add water to make 1 liter of solution. Then for solution C, we dissolve 15 g potassium iodate in water and add water to make 1 liter of solution. In order to perform the experiment, we mis 50 milliliters of each solution together and watch as it change into an orange color as the mercury iodide precipitates. After another few seconds the solution will turn into a black as the starch-iodine comples forms. The speed reaction depends on how much of the solutions we put in. If we use a smaller volume of solution B the reaction will happen faster, but if we dilute the solutions then the solutions by a factor of two then it takes longer for the color changes to occur.
While making the solutions , we need to take serious precautions. We should really be careful with the mercury. When it is powder it isn't as serious as it is as a liquid. Mercury is not to be poured down the drian or else it will cause the water to be poisonous. While handling the mercury goggles need to be worn, not just for handling the mercury, but any lab. There is not eating or drinking of any kind NEAR the labs.
the solutions before the experiment.