Colorful Electrolysis

Introduction

Demonstrate simple electrolysis in a very colorful and dramatic way on an overhead projector.

Concepts

• Electrolysis
• Anode
• Cathode
• pH

Materials

Safety Precautions

To extend the life of the battery, avoid touching the positive and negative terminals to each other. Universal indicator is an alcohol-based solution and is flammable; do not use near an open flame. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Wash hands thoroughly with soap and water before leaving the laboratory.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All materials may be disposed of according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

To prepare the demonstration solution, add 10 mL of universal indicator to 40 mL of 0.5 M sodium sulfate solution and stir.

Procedure

1. Place the two halves of a Petri dish on the projection stage of an overhead projector.
2. Pour enough demonstration solution into each half of the Petri dish to just cover the bottom of each half dish. Adjust the overhead so that the dishes are in clear focus. Each half dish should appear to be a rich, transparent green color.
3. Break a pencil lead in half. Attach the leads to opposite sides of the Petri dish with the alligator clips. Make sure the tip of each lead is submerged in the green solution and the alligator clips remain out of the solution.
4. To start the demonstration, clip the 9-volt battery into the snaps on the battery clip (see Figure 1).
   {13921_Procedure_Figure_1_Demonstration setup}
5. Let the demonstration run for 5–10 minutes and note the changing colors over time. (A purple color will appear at the cathode very quickly. An orange color at the anode will appear more slowly. Over time, the entire spectrum of universal indicator colors will appear.)
6. Discuss the results as the demonstration continues. Discuss the various colors as well as why the “extra” dish was included in the demonstration.

Student Worksheet PDF

13921_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration at least seven times: 9-V battery, battery clip with alligator clips, 100 mL universal indicator, 300 mL of 0.5 M sodium sulfate solution, 12 pencil leads and a reusable Petri dish.
• Concepts of pH and electrolysis should be discussed prior to this demonstration. Universal indicator colors, as they relate to pH values, should also be discussed. The Flinn Overhead Color Chart (Catalog No. AP5367) would be a nice visual supplement during this discussion.
• The reaction at the anode and cathode seem to proceed at the same pace. There is bubbling at both the anode and the cathode. A deep purple forms immediately at the cathode and an obvious red/orange at the anode. Both of these color changes occur with similar intensity. Purple, blue, green, yellow and red/orange are all visible after five minutes.
• The demonstration can be repeated using a pH 7 buffer to show the effects of buffering a solution. (When the solution is buffered, no color changes result.)
• The sulfate ion is an extremely weak base (the pK_a for its conjugate acid, HSO₄^–, is 2.0). The initial indicator color for the electrolysis solution may be more blue-green rather than green. According to the Merck Index, the pH of a sodium sulfate solution is 6.0–7.6. Test a small amount of the sodium sulfate stock solution with bromthymol blue indicator before class—the solution should turn green. If the solution is blue, add one drop of 1 M hydrochloric acid to the stock solution. If the solution is yellow, add one drop of 1 M sodium hydroxide to the stock solution.
• Try the same demonstration procedure using different solutions in the Petri dish as follows:
  1. 20 g potassium iodide

     500 mL distilled water
     30 drops phenolphthalein

  2. 20 g potassium chloride

     500 mL distilled water
     30 drops bromthymol blue (adjusted green)

• The odor of chlorine may be observed at the anode if sodium chloride is substituted for sodium sulfate as the electrolyte in the electrolysis solution. Based on their standard reduction potentials, oxidation of chloride ion to chlorine (Eº = –1.36 V) is less favorable than oxidation of water to oxygen (Eº = –1.23 V). However, there is a significant overvoltage for oxidation of water, and oxidation of chloride competes with oxidation of water under typical electrolysis conditions. Although the cause of the overvoltage is poorly understood, it is generally believed to be due to a kinetically slow reaction at the anode.

Answers to Questions

1. Draw a diagram of the set-up for this demonstration.
   {13921_Answers_Figure_2}
2. Describe the color changes that occurred.

   The solution around the cathode quickly turned purple. Slowly, the solution around the anode turned orange. A variety of other colors eventually appeared at both ends.

3. When an electrical current is passed through a solution containing an electrolyte, water molecules are broken down into hydrogen and oxygen. Write a chemical equation for each of independent half-reactions that occurred.
   1. At the cathode: water breaks down into elemental hydrogen

      4e^– + 4H₂O → 2H₂(g) + 4OH^–

   2. At the anode: water breaks down into elemental oxygen

      2H₂O → O₂(g) + 4H⁺ + 4e^–

4. In which of the two reactions did oxidation occur? In which did reduction occur? Explain.

   Oxidation occurred at the anode, where the oxygen atoms in the water released electrons. Reduction, therefore, occurred at the cathode, where the hydrogen atoms in the water gained electrons and became elemental hydrogen.

5. Which reaction caused the solution around it to become more acidic? Which caused the solution around it to become more basic?

   The pH of the solution around the cathode increased, becoming more basic due to the excess OH^–. Around the anode, the pH of the solution dropped and became more acidic, due to the excess H⁺.

Discussion

When an electric current is passed through an aqueous solution containing an electrolyte (Na₂SO₄), the water molecules break apart or decompose into their constituent elements, hydrogen and oxygen. The overall reaction occurs as two separate, independent half-reactions. Reduction of the hydrogen atoms to elemental hydrogen (H₂) occurs at the cathode (–) while oxidation of the oxygen atoms in water to elemental oxygen (O₂) occurs at the anode (+). Each half-reaction is accompanied by the production of OH^– or H⁺ ions as shown:

At the cathode the excess OH^– will cause the pH to increase, resulting in a color change of the universal indicator solution from green (neutral, pH 7) to purple (basic, pH ≥ 10).

At the anode the excess H⁺ will cause the pH to decrease, resulting in a color change of the universal indicator solution from green to an orange/red color (acidic, pH ≤ 4). The electrolysis half-reactions can also be followed by observing the production of gas bubbles at the cathode (H₂) and anode (O₂).

Universal indicator is an acid–base indicator that is different colors at different pH values. All colors will be visible in the Petri dish as electrolysis progresses and as the pH conditions continually change due to diffusion and neutralization.

References

Special thanks to Mike Shaw, West Stokes High School, King, NC, for this demonstration idea.