Saturday, January 26, 2008

Penny chemistry - Verdigris and copper plating

Lesson: Copper plus oxygen creates copper oxide (2Cu+O2-->2CuO )
What Happened: Changing the chemistry of pennies and nails changed their color

Anthony (perhaps inspired by a discussion of the construction of the Statue of Liberty in history this week) wanted to see if he could make a penny turn green by soaking it in water. I suggested we also try vinegar. Nothing very dramatic happened, so I looked it up and found this great demonstration on About.com's Chemistry page:
  1. First we mixed 1/4 cup vinegar with 1 teaspoon salt.
  2. Next we dipped one dull penny in for 10 seconds, making it half shiny.
  3. Then we dumped about 20 dull pennies in the solution for 5 minutes.
  4. When the pennies were removed, some were rinsed with water, the rest left to dry as is. Within a couple of hours, the vinegary pennies had developed a nice verdigris finish.
But here's the best part. Once we took out the pennies, we put some steel nails (I couldn't quite tell if they were galvanized -- zinc-coated -- or not) in the vinegar. Over the next few hours, the nails got a beautiful copper coating. Here's Anne Marie Helmenstine's explanation of what happened: The pennies react with the salt/vinegar solution, releasing positively charged copper ions. The vinegar also dissolves the iron and oxides on the surface of the steel nail, giving it a negative charge. The positively charged copper ions are more strongly attracted to the nail than the iron ions, so a copper coating forms on the nail.
During the process, the hydrogen ions from the vinegar (acetic acid) and the metal/oxides produced bubbles of hydrogen gas.

Although it took longer than advertised, the end result was impressive. Most impressive was finding a way to do copper plating without using CuSO4 (one of the caustic copper sulfate experiments from Joy of Chemistry that I had been avoiding doing).



Update: Works on paperclips, too!


Here's a site that gives all the chemical equations involved.

5 comments:

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Jimmy said...

I believe what's really going on is a standard redox reaction: Cu+2(aq) + Fe(s) --> Cu (s) + Fe+2(aq). This reaction is definitely spontaneous (the reduction potential of Cu+2 in acidic solution is +0.16, for Fe+2 is -0.44 for an overall reduction potential of the reaction of -0.28), so would happen in solution without requiring any electrical charge to push it through.

It's not simply positively charged copper ions being attracted to the negatively charged nail -- the ions have to be reduced in order to plate out onto the iron.

Jimmy said...

A small correction to my previous post: The reduction potential of Cu+2 -> Cu(s) is +0.34 (not +0.16), giving the overall reaction a reduction potential of -0.10. Still spontaneous, but slightly less so.

Anonymous said...

Love this post! It really helped A LOT with my science experiment on copper plating. Thanks!

Anonymous said...

Thank you for this formula. However, I found that distilled vinegar at 5% does not work. Had to check with several neighbors to get the right one. I am making copper jewelry and want the verdigris instead of the black from liver of sulfur.