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Oxidation-Reduction

I am doing a science fair project on how will acid affect dirty pennies and clean the nails. I really want to know what is going on in the experiment and the use of data. I know this experiment has something to do with Oxidation-Reduction, but I really don't get it.

Thank you.

Linda L.
Student - Ottawa, Ontario, Canada
2004

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2003

That's okay, your science teacher probably doesn't get it either, and neither do I--the oxidation states and the oxidation-reduction reactions of copper are tricky.

The basic idea is that copper can exist at several oxidation states. It can be metallic copper at an oxidation state of zero, it can be a cuprous compound or salt like Cu2O at an oxidation state of plus 1, or it can be a cupric compound or salt like CuO at an oxidation state of plus 2. When it goes from a lower oxidation state to a higher one it is being oxidized. When it goes from a higher oxidation state to a lower one it is being reduced. For it to be reduced, some other element in the reaction has to be oxidized. For it to be oxidized, some other element in the reaction must be reduced. If you try to balance an equation where copper is at a different oxidation state on one side than the other, you'll see that some other element in the equation has to change oxidation states too to offset it.

Now back to what happens when you are 'cleaning' pennies with acid--

Mineral acids (non oxidizing acids) like hydrochloric acid have the ability to take cupric oxide, CuO, the brown tarnish on pennies, and dissolve it as a soluble salt.

2HCl + CuO => CuCl2 +H2O

But the oxidation state of the copper from the tarnish is Cu++ before and after the reaction so there is no oxidation-reduction taking place. If you use student-friendly acids like vinegar [in bulk on eBay or Amazon] plus salt, in place of hydrochloric acid, the reaction is similar, although it's another challenge to explain why vinegar or salt alone won't remove heavy tarnish yet the combination will. I haven't tried to balance that equation.

Then, look at the rag you cleaned it with and observe the green copper salts, verdigris-like, which must be composed of something similar to the CuCl2 the reaction predicted.

But another issue is that the 'clean' penny comes out rather salmon colored, not copper colored. This seems to indicate the presence of at least a little Cu2O, rather than just exposure of the underlying Cu metal. If that is the case, has a portion of the copper in the CuO tarnish been reduced to Cu2O, or has a portion of the metallic copper been oxidized? If the supposition is true, but I'm not sure it is, what other element was reduced or oxidized to make that possible. Got me! Is the explanation that some of the metallic copper was oxidized and some of the tarnish reduced, and that maintained the balance? If so, isn't it a little weird that the same reaction would cause both reduction and oxidation of copper?

I can't understand why you don't get it, Linda :-)

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey