Expt 017 -- Electrolysis

Several solutions are electrolyzed using an apparatus constructed from a 9 volt battery and pencils.

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.

Background

• 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.
• An electrode usually is a metallic conductor that serves as an interface in the electric circuit between where current is carried by metallic conduction (electrons carry current) and electrolytic conduction (ions carry current). Electrons enter or leave the metallic circuit at an electrode.
• 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) + 2 OH^-

• The chemical reaction taking place at the cathode is called a reduction - a gaining of electrons.
• 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^-

• The chemical reaction taking place at the anode is called an oxidation -- a giving up of electrons.
• Given an adequate applied voltage, 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.

Safety

Potassium iodide, stannous chloride, copper chloride, and phenolphthalein are toxic. Wear goggles and apron. Do not ingest toxic chemicals. Wash all spilled chemicals immediately with large amounts of water. Wash hands before leaving the laboratory.

Procedure

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

   !!!Click here to See Picture.

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.

   !!!Click here to See Movie.

3. Add 1 drop of 0.5% phenolphthalein to the solution. Note evidence for reaction.

   !!!Click here to See Movie.

4. To prevent mixing problems, you may wish to mix the phenolphthalein with the KI/starch and electrolyze the mixture.

   -!!!Click here to See Movie. Note the + and ^- electrodes switched position.

5. Rinse the electrodes. Wipe with a paper towel.

6. 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.

   !!!Click here to See Movie.

   !!!Click here to See Picture.

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.

   !!!Click here to See Movie.

9. Add a drop or two of blue or green food coloring. Stir with a toothpick. Place the electrodes in the solution. Note any evidence for reaction.

   !!!Click here to See Movie.

10. After a few moments, use a wafting technique to sniff the gas produced.

11. Rinse the electrodes. Wipe with a paper towel.

12. 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.

13. Add a drop or two of blue or green food coloring. Stir with a toothpick. Place the electrodes in the solution. Note any evidence for reaction.

    !!!Click here to See Movie.

14. Rinse the electrodes. Wipe with a paper towel.

15. Place a second puddle of 0.1 M NaCl solution on the acetate. Add a drop or two of 1% phenolphthalein. Stir with a toothpick. Place the electrodes in the solution. Note any evidence for reaction.

16. Rinse the electrodes. Wipe with a paper towel.

17. Place a second puddle of 0.1 M Na₂SO₄ solution on the acetate. Add a drop or two of 1% phenolphthalein. Stir with a toothpick. Place the electrodes in the solution. Note any evidence for reaction.

18. Rinse the electrodes. Wipe with a paper towel.

19. 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.

20. Rinse the electrodes. Wipe with a paper towel.

21. Place a puddle of 0.1 M copper(II) chloride solution on the acetate. Place the electrodes in the solution on the acetate.

22. Use a magnifying glass to examine carefully the electrodes.

23. Note any evidence for reaction.

24. Rinse the electrodes. Wipe with a paper towel.

Questions

1. Predict the oxidation product (anode product) of an aqueous solution when chloride salts are electrolyzed.

2. Predict the reduction product (cathode product) of an aqueous solution when salts of readily reduced metals (copper, tin) are electrolyzed.

3. The electrolysis products are the same, independent of the nature of the salt, when aqueous solutions of salts with neither electroactive cations nor anions present are electrolyzed. Identify these products, and write equations for their production.

4. Household laundry bleaches usually contain chlorine. Identify observations to suggest how these bleaches work.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 017 Electrolysis

Record observations for each reaction on the movies. Describe any changes at both electrodes. Identify the positive and negative electrodes from the reactions observed.

Negative Positive

Use the movies and pictures to answer the questions.

Curriculum^-

Perform this experiment when studying oxidation^-reduction, activity series, or electrochemistry. The experiment also provides clean examples of chemical reaction, and gives insights about obtaining elemental metals from their compounds.

Safety^-

Potassium iodide, stannous chloride, copper chloride, and phenolphthalein are toxic. Wear goggles and apron. Do not ingest toxic chemicals. Wash all spilled chemicals immediately with large amounts of water. Wash hands before leaving the laboratory.

Time^-

Teacher Preparation: 30 minutes (The tin solution does not store well. Prepare the tin solution within a few days of use.)

Class Time: 40 minutes

Materials^-

• two pencils -- each about 10 cm in length and sharpened at both ends^-- are used as the electrodes

• 9-volt battery

• battery clip with 2 alligator clips

• acetate sheet

• toothpicks

• paper towels, tissue papers, or toilet tissue

• 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 gray tinge appears in the liquid. Cool to room temperature. Add 1.5 g KI.)

• 0.5% phenolphthalein indicator solution (dissolve 0.5 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 1.420 g of Na₂SO₄, sodium sulfate in water. Dilute to 100 mL with water.)

• 0.1 M NaCl (dissolve 0.59 g NaCl in enough distilled water to make 100 mL of solution)

• 0.1 M CuCl₂ (Dissolve 1.70 g of CuCl₂¥2H₂O in enough water to make 100 mL of solution.)

• food coloring

Disposal^-

The solutions shown here may be disposed of safely at the sink. Discard soiled papers with ordinary trash.

Lab Hints^-

Sharpen the pencils and have them available for the students.

Background^-

• 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) + 2 OH^-

• 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.

  • Sn²⁺ + 2 e^- --> Sn

  • 2 I^- --> I₂ + 2 e^-

• Gas bubbles observed during electrolysis are usually hydrogen at cathodes or oxygen at anodes.

Answers^-

Q1. Predict the oxidation product (anode product) of an aqueous solution when chloride salts are electrolyzed.

A1. Chlorine is produced at the anode when aqueous chloride salts are electrolyzed:

2 Cl^- --> Cl₂ + 2 e^-

Q2. Predict the reduction product (cathode product) of an aqueous solution when salts of readily reduced metals (copper, tin) are electrolyzed.

A2. The elemental metals are produced when salts of readily reduced metals are electrolyzed:

Cu²⁺ --> Cu + 2 e^-

Sn²⁺ --> Sn + 2 e^-

Q3. The electrolysis products are the same, independent of the nature of the salt, when aqueous solutions of salts with neither electroactive cations nor anions present are electrolyzed. Identify these products, and write equations for their production.

A3. Oxygen is produced at the anode:

2 H₂O --> O₂ + 4H⁺ + 4e^-

Hydrogen is produced at the cathode:

2 H₂O + 2e^- --> H₂(g) + 2 OH^-

Q4. Household laundry bleaches usually contain chlorine. Identify observations to suggest how these bleaches work.

A4. When food coloring is added to solutions in which chloride salts are being electrolyzed, the colors fade (are bleached) near the anode. This suggests that chlorine is oxidizing the dye material.

CoopLearn^-

This activity lends itself well to sharing and comparing. Have groups work on several solutions. Use more solutions than listed here. Give each solution to two or three groups so that results are confirmed. Then, as a group of the whole, try to create an activity series.

Reference^-

Kolb, K. E. and Kolb, D. K., J. Chem. Ed., 63, No.6, S17 (1986)

Key Words 1^-

electrolysis, oxidation, reduction, redox, electrode, cathode, anode

Elements^-

Na Cu Sn Cl I H O K