MadSci Network: Chemistry

Re: Where will the products of the electrolysis of water form?

Date: Sat Dec 5 17:05:35 1998
Posted By: Tye Morancy, MadSci Admin
Area of science: Chemistry
ID: 912887160.Ch

    Hello Monica,

      That's a good question to ask.  I was a little puzzled at how you asked 
the question, but I think I know what you are asking.  If I am understanding 
you correctly, you are asking a question which has to do with a piece of 
equipment which is made of lots of glass.  Inside the glass tubing is where the 
water is and electricity is passed through the water somehow, right?

      Well, that special piece of glassware is called an electrolysis tube.  It 
has three long tubes which stand upright and are sealed.  The two tubes on the 
right and left each have a wire which is attached to their bottom.  These two 
wires pass into each of the tubes and are attached to square platinum 
electrodes.  At the top of each of these tubes there is something that looks 
like a switch, which we call a stopcock.  It is a special valve which we can 
open and close.  In the middle, there is a tube which is connected to the other 
tubes and at it's top there is a bulb, which is called a reservoir (usually 
half-filled with water). I have included a cool link which has a great picture 
of the apparatus if you'd like to see it for yourself :

Click here for a 
picture of the experiment: 

     When we attach those two wires to a power supply, like a battery or other 
source, we get a current which is generated and a reaction begins.  You can 
actually see the reaction happening because on each of those little electrodes 
you will see lots of little bubbles appearing and floating towards the top of 
each tube.  These bubbles are oxygen and hydrogen gas from the water being 
broken down.  This type of reaction is called a REDOX reaction.  REDOX means 
that on one of the electrodes there is a reduction reaction happening and on 
the other there is an oxidation reaction occurring.
     On the negative electrode, which we sometimes call the anode, the 
oxidation reaction occurs.  Oxidation means a reaction in which we are giving 
off electrons as a product of the reaction.  The oxidation reaction is written 
like this :
                        2H2O  --->  O2 + 4H+ + 4e-

     , which means that two water molecules get broken up and we form a 
molecule of oxygen, 4 hydrogen ions, and 4 electrons.  The O2 is the gas that 
bubbles upwards on that side.

     The other electrode is positive, which is sometimes called a cathode, 
where the reduction reaction occurs.  Reduction means a reaction where 
we put in electrons to produce something.  The reaction for this one is written 
like this :

                          4H2O + 4e-  --->  2H2 + 4OH-

      , which means that we took 4 water molecules and 4 electrons and produced 
2 hydrogen molecules and 4 hydroxide ions.  The electrons come from our power 
supply and the hydrogen is gaseous and bubbles upwards on that side.
      We can write these two equations as one overall reaction, which we would 
write like this :

                            2H2O  --->  2H2 + O2

      , this shows that for every 2 water molecules we can make 2 hydrogen 
molecules and one oxygen molecule.  These two molecules on the right side of 
the equation are gases, so they form the bubbles that we see on the electrode 
and float up to the top of each tube.  So we would see oxygen floating to the 
top of the tube on one side and on the other side we would see hydrogen 
floating to the top.  But you might ask me, "how can I tell which gas is which?"
      The answer is simple, because, as I said in my description of the 
equipment, the tubes are sealed up on the top where the valves are.  As the 
gases on each side bubble to the top, they pusk down on the water and you will 
see a gap or pocket form at the top of each tube.  One of those tubes will have 
a gap twice as big as the other.  The one which is twice as big as the other is 
the one where the hydrogen gas is collecting, because, remember, this reaction 
forms two hydrogens for every oxygen that it makes.
     Once you've done all this you can remove each gas by itself to use for 
something else.  For instance, there are people in NASA who want to use this 
idea to help people to live on the Moon or Mars.  If you can go to a planet 
where there is lots of water or ice, you can use this neat little trick to make 
oxygen and hydrogen.  The oxygen gives the astronauts air to breathe and the 
hydrogen is great stuff to make rocket fuel so they can get back home or get 
around the universe. We actually know that there is ice on the moon so this 
might be a great idea to help us live on the moon!

    I hope this helped you Monica...

Tye Morancy

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