National Chemistry Week - Experiments - Electrolysis of Water

4H₂O  +  4e^-  -->  4OH^-  +  2H₂  cathode
2H₂O  -->  O₂  +  4H⁺  +  4e^-  anode
net reaction:  6H₂O  -->  4OH^-  +  4H⁺  +  2H₂  +  O₂
equation simplified to:  2H₂O  -->  2H₂  +  O₂
  
The cathode will be recognized by a pink colour from the phenolphthalein indicator due to hydroxide production. Phenolphthalein turns pink in the presence of base and clear in the presence of acid. Both electrodes will produce bubbles; however, the cathode will be recognized due to the greater production of gas. Two molecules of hydrogen are produced for every molecule of oxygen produced. The gases produced at the electrodes can also be collected and tested. A positive test for the presence of hydrogen is a soft pop sound when a burning match is placed in the mouth of the container. The test for the presence of oxygen is to place a smoldering match in the mouth of the container. If the match glows oxygen is present.

Materials:

• 9V battery
• matches
• lid or cardboard
• tongs
• electrical wire (insulated) with alligator clips
• two pencils sharpened at both ends
• small deep bowl (white or glass)
• two tall narrow jars (try to have them the same size) and one small jar
• washing soda (sodium carbonate)
• phenolphthalein indicator (see the Make Your Own pH Indicator experiment for the procedure)

  Procedure:
  

  1. Clip a wire to one tip of each pencil (these are the electrodes).
  2. Fill bowl with water and add a little washing soda.
  3. Fill tall jars with water/washing soda solution and invert into the bowl (make sure no air is inside the jars). The jars can be rested on the side of the bowl or taped if they roll around.
  4. Place an electrode inside each jar and attach the other ends of the wires to the battery.
  5. Observe the gas collecting at the top of the jars.
  6. After 30 min. disconnect the electrodes and test for the presence of H₂ and 0₂ (procedure below).
  7. The addition of some phenolphthalein indicator to the solution will determine which electrode is the cathode. A pink colour around the cathode will be seen due to the production of hydroxide and hydrogen. This test can be used instead of the tests for H₂ and 0₂.

  Test for H₂
  
  1. Remove the jar from the water and keep it inverted.
  2. While holding a match in the tongs light it and place it in the mouth of the jar containing the hydrogen.
  3. A soft pop sound indicates hydrogen is present.

  Test for 0₂
  
  1. Remove the jar and cover with the lid (cardboard) quickly.
  2. Turn the jar right side up (a small amount of water in the jar is O.K.).
  3. While holding a match in tongs, light it and then blow it out.
  4. Remove the lid from the jar and place the smoldering match in the jar.
  5. If the toothpick glows then oxygen is present.