Electrolysis

Description

A variety of solutions are electrolyzed using an apparatus constructed from pencils and a 9-volt battery.

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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 + 2 e^- --> H₂(g) + 2 OH^-(aq)
• 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(g) + 4 H⁺(aq) + 4 e^-
• 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. For example, stannous ion is reduced to tin metal; iodide ion is oxidized to iodine.
  • Sn²⁺ + 2 e^- --> Sn
  • 2 I^- --> I₂ + 2 e^-
• Gas bubbles observed during electrolyses are usually hydrogen at cathodes or oxygen at anodes.

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Potassium iodide, stannous chloride, and phenolphthalein are toxic.

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Do not ingest toxic chemicals.

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1. Build the apparatus shown. Solder alligator clips to the ends of a 9-volt battery clip. Sharpen both ends of 2 pencils. Tape the battery between the pencils. Clip the alligator clips to one end of each pencil. Or pick up an apparatus from your instructor.
2. Place a clean Petri dish on the stage of an overhead projector. Adjust the projector so all students can see clearly.
3. Add water and 0.5 g solid potassium iodide to the dish.
4. Place the electrodes in the solution in the dish. Note any evidence for reaction.
5. Add 2-3 drops of 1% phenolphthalein to the dish and stir with a rubber policeman. Rinse the electrodes.
6. Place a clean Petri dish on the stand of an overhead projector. Fill the bottom of the dish with freshly prepared 0.1 M stannous chloride (Tin(II)chloride, SnCl₂) solution. Place the electrodes in the solution in the dish. Note any evidence for reaction.
7. Rinse the electrodes. Place 10 mL of 0.1 M Na₂SO₄ in a fresh Petri dish on the stage of the overhead projector. Add a drop or two of blue food coloring. Stir with a rubber policeman. Place the electrodes in the solution in the dish. Note any evidence for reaction.
8. Use a fresh Petri dish. Place 10 mL of 0.1 M NaCl solution in the dish. Place on an overhead projector stage. Rinse the electrodes. Place the electrodes in the solution, and note any evidence for reaction.
9. Add 1 or 2 drops of blue food coloring. Mix with a rubber policeman. Rinse the electrodes. Immerse the electrodes in the solution. Note that there is an effect on the blue dye around only one electrode.
10. Immerse the electrodes in a Petri dish containing 10 mL of 0.1 M NaCl. After a few moments, use a wafting technique to sniff the gas produced.
11. Note the technique.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 047 Electrolysis

• 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 + 2 e^- --> H₂(g) + 2 OH^-(aq)
• 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(g) + 4 H⁺(aq) + 4 e^-
• 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.

Watch the movie and answer these questions.

1. Record the colors observed when the KI solution with phenolphthalein is electrolyzed. Which ions produced by the electrolysis are responsible of each color?
2. Record observations for the SnCl₂ solution.

   Write a half-reaction for the reaction taking place where the tin crystals form.

   Is this electrode + or -?

3. Account for the loss of blue color at one electrode during the NaCl electrolysis, but not the Na₂SO₄ electrolysis.

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Purpose

To illustrate electrolysis.

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• knife or razor blade
• two pencils
• tape
• 1 9-volt battery
• battery clip with alligator clips
• overhead projector and screen
• 5 Petri dishes
• rubber policeman (a glass stirring rod fitted with a flexible rubber tip that provides efficient stirring in Petri dishes)
• 1-2 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 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
• 0.1 M NaCl
• blue food coloring

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• Construct the portable electrolysis device demonstrated for this lesson. Its performance exceeds that of other electronic devices.
• Store the device disconnected so as to preserve the battery. Expect to change batteries every semester.
• Several other solutions (Na₂SO₄, 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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Construction: 30 minutes

Teacher preparation: 15 minutes

Presentation: 15 minutes

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

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Presentation Question:

• Account for the loss of color at one electrode.
  • 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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1. Pink from the phenolpthalein indicates that OH^- is produced at one electrode.

   Yellow at the other electrode if iodine. Brown I₃^- is formed in dilute concentrations.

2. Sn²⁺ + e^- --> Sn

   The tin is formed at the negative or - electrode.

3. Chlorine is formed in the NaCl solution but not in the Na₂SO₄ solution. Chlorine bleaches the food coloring.

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• electrolysis
• oxidation
• reduction

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