SPLITTING THE WATER MOLECULE
This demonstration requires a Hoffman Apparatus (Flinn: AP5879).
Electrolysis of an aqueous solution of 1.0M sodium sulfate is followed
xat 9 volts supplies the electric current for the electrolysis.
Solutions needed:
1. Sodium sulfate, 1 molar solution prepared as follows:
●71g of Na2SO4 (Anhydrous) is dissolved in enough
distilled water to make 500mL of the solution.
2. Bromothymol blue indicator (0.04%) solution (blue basic,
yellow acidic, and green neutral)
3. Dilute solutions (0.1 molar) of sulfuric acid an sodium
hydroxide prepared as follows:
●2mL of concentrated (18M) H2SO4 are carefully poured
into a 500mL flask containing 250mL of distilled water (exothermic
reaction). Dilute the solution to 500mL.
●2.0g of NaOH are dissolved in 250mL of distilled water
and the resulting solution diluted to 500mL.
Procedure:
1. Set up 2 clamps about 6 inches apart on a support stand and
secure the clamps to the center of the Hoffman apparatus.
2. Insert the rubber stoppers containing the electrodes tightly
into the bottom of the graduated side arms of the apparatus.
3. Pour about 100mL of the Na2SO4 solution in a small beaker
and add 5mL of the bromothymol blue indicator. If the solution
turns blue, add 0.1M H2SO4 until the solution turns green (the color
for a neutral solution, pH=7). If the solution is yellow add drops of
NaOH until the solution turns green.
4. Close the stopcocks on the arms of the Hoffman apparatus.
5. Slowly fill the bulb on the apparatus with the colored Na2SO4
solution to about half full. The bulb holds approximately 60mL.
6. Open the stopcock on one arm of the apparatus a small
amount until the arm is filled with the solution. Caution: Do not allow
liquid to flow into the glass tips above the stopcocks. Add more
solution to the bulb if necessary. Close the stopcock
7. Use the same procedure to fill the other arm. Close the
stopcock. The bulb should be no more than ½ full.
8. Connect the wire leads from the dc power supply to the
platinum electrodes of the Hoffman apparatus. Label the electrode
connected to the negative terminal of the power supply “cathode”.
Label the other electrode connected to the positive terminal, “anode”.
9. Place a piece of white poster board ath the back of the
Hoffman apparatus.
10. Turn on the power supply (set at 9 Volts). Bubbles of gas
will form rapidly in both arms of the apparatus.
Discussion:
●The gas will collect at the top of each arm after it has
displaced the liquid, which flows from the arms into the central tube
and bulb. The volume of gas above the cathode is twice that above
the anode. The solution in the anode arm will turn yellow (acidic), and
that in the cathode arm will turn blue (basic).
●The over all reaction is: 2H2O  2H2 + O2
Note: The sodium sulfate does not enter into the reaction: the ions
act as to transport the electrons from the power source to the
electrodes and keep electrical neutrality around the electrodes.
●To explain what is happening at each electrode, it is best to
investigate the half reactions at each electrode.
Cathode (reduction or gain of electrons)
[also negative terminal].
2{2H2O + 2e-  H2 + 2OH-} (blue indicator color)
Anode (oxidation or lose of electrons)
[also positive terminal]
2H2O  O2 + 4H+ + 4e- (yellow indicator color)
Addition of these two ½ reactions with the same number of electrons
(4) exchanged gives the overall reaction above when you consider
4H+ + 4OH-  4H2O
Final Analysis:
●When the cathode arm is about 2/3 full of gas, turn off the
power supply and disconnect the wire leads.
●Hold a test tube inverted over the stopcock on he tube holding
the smaller volume of gas (the anode tube). Seal the test tube with
the thumb.
●Light a wooden splint, blow out any flame on the splint leaving
glowing embers. Insert the splint into the test tube. The splint will
burst into flame (oxygen).
● Following the procedure above, fill another test tube with the
gas from the other arm of the apparatus (largest volume of gas: the
cathode tube). Seal the test tube with the thumb.
●Insert the glowing splint into the test tube: The gas
(hydrogen) will explode with a loud “POP”.
●Examine both test tubes for traces of water. Explain the
formation of water by the reaction of hydrogen with oxygen.
The Cleanup:
Drain the solution from the Hoffman apparatus into a large
beaker. After the solution has mixed, its color will return to the
neutral green color and can be used again. Remove the electrodes and
rinse them with distilled water. Rinse the apparatus with distilled
water, drain, and store in the original carton.