Halloween Reaction or Old Nassau Reaction
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Halloween Reaction or Old Nassau Reaction
Orange and Black Clock Reaction
By Anne Marie Helmenstine, Ph.D., About.com
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
1. Mix 50 ml solution A with 50 ml of solution B.
2. 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
1. Sodium metabisulfite and water react to form sodium hydrogen sulfite:
Na2S2O5 + H2O --> 2 NaHSO3
2. Iodate(V) ions are reduced to iodide ions by the hydrogen sulfite ions:
IO3- + 3 HSO3- --> I- + 3 SO42- + 3 H+
3. 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)
4. If I- and IO3- ions remain, then an iodide-iodate reaction takes place:
IO3- + 5 I- + 6 H+ --> 3 I2 + 3 H2O
5. The resulting statch-iodine complex is black to blue-black:
I2 + starch --> a blue/black complex
Orange and Black Clock Reaction
By Anne Marie Helmenstine, Ph.D., About.com
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
1. Mix 50 ml solution A with 50 ml of solution B.
2. 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
1. Sodium metabisulfite and water react to form sodium hydrogen sulfite:
Na2S2O5 + H2O --> 2 NaHSO3
2. Iodate(V) ions are reduced to iodide ions by the hydrogen sulfite ions:
IO3- + 3 HSO3- --> I- + 3 SO42- + 3 H+
3. 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)
4. If I- and IO3- ions remain, then an iodide-iodate reaction takes place:
IO3- + 5 I- + 6 H+ --> 3 I2 + 3 H2O
5. The resulting statch-iodine complex is black to blue-black:
I2 + starch --> a blue/black complex
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chemistry education
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