Electrolysis
of sodium sulfate solution on a film canister
Take
safety precautions concerned with electricity. Work on a tray
Left:
On a film canister mix 5 drops of sodium sulfate solution and 2 drops of
a red cabbage extract in methylated spirit.
Connect paper
clip with the negative terminal of a 9-Volt battery and 0.7 mm pencil
lead with the positive one.
Dip these two
electrodes into the blue solution.
Observations:
Right:
Gas bubbles appear at both electrodes, more bubbles can be seen at the
cathode (-). The colours of the indicator around the
electrodes change to green (cathode ) and to red (anaode).
Explanation:
Red:
4 H2O(l) + 4 e- --> 2 H2(g) + 4
OH-(aq)
(-->green colour at the cathode)
Ox:
2 H 2O(l) --> O2(g) + 4 H+(aq)
+ 4 e - (-->red colour
at the anode)
Redox:
6 H 2O(l) --> 2 H2 (g) + O2
(g)
+ 4 OH-(aq) + 4 H+(aq)
Colour
change : The different
colours of the red cabbage juice result from acid base reactions of coloured
compounds called anthocyans (anthos = flower, kyanos = blue). Their cations
"are especially interesting because they behave much like the pH indicator..."
(S.S.Zumdahl, Chemistry).
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last modification: 18.03.2002