Objective:To electrolyze water. Electrolysis is a process by which a chemical reaction is carried out by means of the passage of an electric current. In the electrolysis of water, water is oxidized at the anode (negative) and reduced at the cathode (positive).
4H2O + 4e- --> 4OH- + 2H2 cathode2H2O --> O2 + 4H+ + 4e- anodenet reaction: 6H2O --> 4OH- + 4H+ + 2H2 + O2equation 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:
Clip a wire to one tip of each pencil (these are the electrodes).
Fill bowl with water and add a little washing soda.
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.
Place an electrode inside each jar and attach the other ends of the wires to the battery.
Observe the gas collecting at the top of the jars.
After 30 min. disconnect the electrodes and test for the presence of H2 and 02 (procedure below).
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.