Electrolysis

Introduction

Chemical reactions are sometimes conducted by converting electrical energy into chemical potential energy. The generic name for these reactions is electrolysis. Whenever a direct current passes through an electrolyte solution for a sustained period of time, electrolysis reactions take place.

In this experiment, several electrolysis reactions are studied.

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1. Build the apparatus shown. Sharpen both ends of 2 pencils. Clip the alligator clips to one end of each pencil. (Some workers prefer to tape the battery between the pencils.)

   

2. Place an acetate sheet on a piece of white paper. Place enough KI/starch solution on the acetate to form a puddle 1.5 cm in diameter. Place the electrodes of the electrolysis apparatus into the solution. Note any evidence for reaction. Add 1 drop of 1% phenolphthalein to the solution. Stir with a toothpick. Note evidence for reaction.
3. Rinse the electrodes. Wipe with a paper towel.
4. Place a puddle of freshly prepared 0.1 M stannous chloride solution on the acetate. Place the electrodes in the solution on the acetate. Note any evidence for reaction.
5. Rinse the electrodes. Wipe with a paper towel.
6. Place a puddle of 0.1 M Na₂SO₄ on the acetate. Place the electrodes in the solution on the acetate. Note any evidence for reaction. Add a drop or two of blue food coloring. Stir with a toothpick. Place the electrodes in the solution. Note any evidence for reaction.
7. Rinse the electrodes. Wipe with a paper towel.
8. Place a puddle of 0.1 M NaCl solution on the acetate. Place the electrodes in the solution, and note any evidence for reaction. Add a drop or two of blue food coloring. Stir with a toothpick. Place the electrodes in the solution. Note any evidence for reaction. After a few moments, use a wafting technique to sniff the gas produced.
9. Rinse the electrodes. Wipe with a paper towel.
10. Place a puddle of 0.1 M NaCl solution on the acetate. Place the electrodes in the solution, and note any evidence for reaction. Add a drop or two of 1% phenolphthalein. Stir with a toothpick. Place the electrodes in the solution. Note any evidence for reaction.
11. Rinse the electrodes. Wipe with a paper towel.
12. Place a puddle of 0.1 M silver nitrate solution on the acetate. Place the electrodes in the solution on the acetate. Note any evidence for reaction.
13. Rinse the electrodes. Wipe with a paper towel.

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Account for the loss of color at one electrode during the electrolysis of sodium chloride.

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Potassium iodide, stannous chloride, silver nitrate and phenolphthalein are toxic. Do not ingest toxic chemicals. Silver nitrate causes black stains on skin. Wash all spilled chemicals immediately with large amounts of water. Wash hands before leaving the laboratory.

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The bubbles result from the oxidation of chloride ions to produce elemental chlorine. The chlorine is detectable by its odor even though present in very small amount. The chlorine bleaches the color from the dye.

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• Electrolysis is a technique for converting electrical energy into chemical potential energy. The products of an electrolysis have greater chemical potential energy than do the reactants. In a sense, electrical energy provides a way of undoing a chemical reaction.
• In order for electrolysis to occur, two conditions must prevail. First, the solution (or system) electrolyzed must conduct electric current. In water solutions this is accomplished by having ions move through the solution toward electrodes.
• Second, at the electrode, electrons must pass from the electrode into the solution or vice versa. Nearly always this process is accompanied by a chemical reaction. At one of the electrodes, called the cathode, electrons enter the solution. An example of a cathode reduction is:

  2 H₂O + 2e^- --> H₂(g) + 2OH^-

• At the other electrode, electrons are removed from the solution and enter the electrode. This electrode is called the anode, and an example of an anode oxidation is:

  2 H₂O --> O₂ + 4H⁺ + 4e^-

• Water will always pass an electric current and support these reactions. Adding a soluble ionic compound, a salt such as sodium sulfate, greatly enhances the extent of reaction, however, since it allows more current to flow. This happens even though neither of the ions (sodium cation or sulfate anion) is involved in an electrode reaction.
• Some compounds provide species, usually cations or anions, that react at the electrode in preference to water. These are used as examples here. Stannous ion is reduced to tin metal; iodide ion is oxidized to iodine.
• Gas bubbles observed during electrolysis are usually hydrogen at cathodes or oxygen at anodes.
• Several other solutions, such as Na₂SO₄ and NaCl, may be tried. For example, electrolysis of solutions of ordinary table salt lead to the production of chlorine. The chlorine is concentrated enough to detect by odor, and may be used to bleach selected colored fibers that may be projected on the overhead projector.

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The solutions shown here may be disposed of safely at the sink.

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Potassium iodide, stannous chloride, silver nitrate and phenolphthalein are toxic.

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• Construct the portable electrolysis device for this lesson.
• Store the device disconnected so as to preserve the battery. When used for overhead demonstrations, the electrodes may be taped to the battery. When used for experimentation by students, placing the battery on the work surface and moving the electrode freely, one in each hand, seems to provide the best set of conditions. Expect to change batteries every semester.
• The electrode tips need to be cleaned (sharpen the pencils) during the experiment to prevent build up of food colors, plated matters, stains, etc.

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• knife or razor blade
• two pencils
• tape
• 1 9-volt battery
• battery clip with 2 alligator clips
• acetate sheet
• toothpicks
• 1.5 % KI / 0.5% starch (Boil 100 mL of distilled water. Cautiously and carefully add spray starch from a can (sold with laundry products at a grocery store), with stirring, until a bluish grey tinge appears in the liquid. Cool to room temperature. Add 1.5 g KI.)
• 1% phenolphthalein indicator solution (dissolve 1 g phenolphthalein in 60 mL of ethanol, and dilute to 100 mL with distilled water.)
• 0.1 M AgNO₃ (dissolve 1.7 g AgNO₃ in enough distilled water to make 100 mL of solution)
• 0.1 M SnCl₂ (dissolve 1.1 g of stannous chloride in 100 mL of 0.1M HCl. Keep some metallic tin in contact with the solution. Use fresh.)
• 0.1 M Na₂SO₄ (dissolve 3.2 g Na₂SO₄•12H₂O in enough distilled water to make 100 mL of solution)
• 0.1 M NaCl (dissolve 0.59 g NaCl in enough distilled water to make 100 mL of solution)
• blue food coloring
• paper towels

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Do not ingest toxic chemicals.

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To illustrate electrolysis.

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30 minutes
Teacher preparation: 15 minutes
Presentation: 15 minutes

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