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Me12 Electrolysis
of salt water using a Cola can closure as anode
In the experiment for testing
the electric conductivity of sea water chlorine
was formed at a carbon anode.
In the following experiment
you will replace carbon by the closure of a Cola can. Universal indicator
will be added to the water.
What you
also need: 2 insulated copper wires with 4 crocodile clips,
9-Volt battery, blister, salt water, universalindicator solution (UI).
Experiment
Left:
Transfer 1 ml of salt water into a blister, add two drops of UI, sandpaper
a piece of tin from a Cola can (Fe), connect it with the
negative terminal of the battery. The closure of the can (Al) is connected
with the positive terminal. *
Middle:
Dip the metals into the salt solution. * Right: Stir the solution after
electrolysis.
Observations
Middle:
1.
Gas
bubbles at both electrodes. 2.
Violet colour around Fe, 3. Red
colour around Al.
4. No
smell of chlorine at the anode.
Right:5.
When
the liquid is mixed the green colour of of UI reappears. The liquid is
not turbid.
Explanation
Right:
In
this Electrolytical Cell the flow
of electrons is different from the flow in a Galvanic
Cell: It does not move spontaneously but it is forced into the opposite
direction by a battery.
*
The
Fe cathode attracts the
positive sodium ions and the H2O molecules surrounding them.
1.2.
At
Fe: Cathodic reduction, hydroxide ions + hydrogen formed: 4
H2O + 4 e ---reduction---> 4 OH- + 2 H2(g)
* The Al anode attracts
a flow of the negative chloride ions and the H2O molecules surrounding
them. Hydroxide ions are also attracted..
1.3.
At Al: Anodic oxidation of H2O molecules
release oxygen: 4 H2O -
4 e- --oxidation--> 2 H++ 2 H2O
+ O2(g)
4. Negative
chloride ions are screened by the water molecules around them.
* The battery ("electron
pump") transports electrone (e- )
from the anode to the cathode.
5. By
mixing the liquid the positive H+(aq) ions and negative
hydroxide [OH-(aq)] form water molecules.
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modification: 14.07.2003