National Chemistry Week - Experiments - Electrolysis of Water
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 cathode
2H2O --> O2
+ 4H+ + 4e- anode
net reaction: 6H2O --> 4OH- +
4H+ + 2H2 + O2
equation simplified to: 2H2O --> 2H2
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.
- 9V battery
- lid or cardboard
- 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)
Test for H2
- 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.
Test for 02
- Remove the jar from the water and keep it inverted.
- While holding a
match in the tongs light it and place it in the mouth of the jar containing
- A soft pop sound indicates hydrogen is present.
- Remove the jar and cover with the lid (cardboard) quickly.
- Turn the jar
right side up (a small amount of water in the jar is O.K.).
- While holding a
match in tongs, light it and then blow it out.
- Remove the lid from the jar
and place the smoldering match in the jar.
- If the toothpick glows then
oxygen is present.