Re: Chemistry of a wet cell

Hi John,

You have been witness to an oxidation-reduction reaction between zinc and
copper.  As background, let me remind you that "oxidation" refers to the loss
of electrons, while "reduction" is the gain of electrons.  A handy mnemonic
is "OIL RIG": Oxidation Is Loss, Reduction Is Gain.  In this particular
reaction, the zinc is oxidized to form Zn2+, and Cu2+ is reduced to form
solid copper.  Here are the reactions:

			At the zinc electrode (ox): Zn (s) ---> Zn2+ (aq) + 2 e-
			At the copper electrode (red): Cu2+ (aq) + 2 e- ---> Cu (s).

Oxidation and reduction reactions are a matched set (they always exist as a
pair, like salt and pepper).  The reduction reaction produces a "driving force"
for the oxidation -- you can imagine the reduction electrode pulling the 
electrons towards it.

Now, how does this work as a lightbulb-illuminating device?  The wire 
connecting the oxidation electrode to the reduction electrode allows for the
movement of electrons.  The oxidation electrode loses electrons, which then 
travel through the wire.  That current travels through the filament of the bulb,
producing light, and moves on to the reduction electrode.  At the reduction
electrode, Cu2+ ions in solution combine with electrons to form solid copper.
In fact, if you allow the reaction to continue for some time, you will see
the oxidation electrode shrink as the reduction electrode grows. You may also
see the striking blue color of the CuSO4 solution fade over time, as blue Cu2+
ions are removed from the water, and are replaced by colorless Zn2+ ions.

Finally, the SO42- ions play a supporting role in the reaction.  These anions
(negatively charged ions) migrate to the anode (AKA the oxidation electrode).
Since the anode is losing electrons, it can build up a positive charge -- stopping
the reaction.  Anodes migrate to the anode and maintain a neutral region, allowing
the reaction to continue.  You can picture the same thing taking place at the
cathode (the reduction electrode): as electrons are gained, a positive charge
builds up.  Cu2+ cations migrate to the cathode, "neutralizing" the charge
difference and forming solid copper in the process.  I should add that if you
make a wet cell using 2 solutions in separate beakers, you need to provide a
"salt bridge" connecting the beakers and containing anions and cations.  Or,you
can use a porous ceramic disk to connect the two beakers, allowing the ions in
solution to flow back and forth.

Hope this helps,
Amanda Kahn