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Electrolysis
of water using an electrical current
What are the products
when water decomposes electrolytically?
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Demonstrating
the electrolysis of water requires a specialized piece of glassware,
and a source of electrical current. Click here
or on the picture to examine this apparatus in more detail.
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The outer glass
tubes and the middle tube are filled with water and the reservoir
half filled. Food coloring is usually added so that it is easier
to see the liquid in the tubes. In this case, a drop or two of red
food color was added.
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Rectangular
platinum electrodes attached to a platinum wire are held in position
by the rubber stoppers. The platinum wire passes through holes in
the rubber stoppers. The platinum electrode are flattened into rectangles
to expose as much metal as possible to the reactant (water).
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The platinum
wires extending from the rubber stoppers are attached to the power
source with aligator clips. Either a dry cell battery or DC current
provides enough current to cause the decomposition of water.
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Once the current
is applied, you can see that a reaction is occurring in one of two
ways. Bubbles of gas rise from the surface of the electrodes as
water decomposes to H2 and O2 gas.
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Alternatively,
an acid base indicator can indicate that a reaction has occurred.
This example contains Universal Indicator, which turns green-blue
in base, and pink in acid.
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Why are H+
and OH- present since we know that the products of decomposition
of water are H2 and O2?
Decompostion of water
is a redox reaction. The oxidation reaction occurs at one electrode, and
the reduction reaction at the other electrode.
Water is oxidized
at the anode. The reaction is...
2H2O
---> O2 + 4H+ + 4e-
Water is reduced
at the cathode. The reaction is...
4H2O +
4e- ---> 2H2 + 4OH-
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We can tell
that the tube on the right has the cathode, since at the cathode
H2 is produced. In the decompostion reaction, the volume
of H2 produced is twice the volume of O2.
2H2O
---> 2H2 + O2
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