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Alternative
Hofmann Experiment 3: Horizontal glass tube
Take
safety precautions concerned with electricity. Work on a tray
Left:
Brine coloured blue by cabbage indicator is electrolyzed in a horizontal
glass tube closed by two rubber stoppers with paper clip electrodes and
by an empty air balloon.Observations
:
* Photo 2 and 3:
1. Gas bubbles at both electrodes. 2. A green layer spreads on the surface
from the cathode. 3. A red liquid (shaped like a christmas-tree) liquid
around the anode.
* Right: A gas mixture
blows the balloon up.
Explanation:
Red:
4 H2O(l) + 4 e- --> 2 H2(g) + 4
OH-(aq)
Ox:
2 H 2O(l) --> O2(g) + 4 H+(aq)
+ 4 e -
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) phenolphthalein.
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last modification: 05.03.2002