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Do volts matter in plating pennies with zinc?

November 10, 2009

Dear Sirs,

I am 12 years old and my 7th grade science experiment is to electroplate a penny. I set up the experiment using a 9 volt battery, clipped to a penny and a piece of zinc in the solution (vinegar and salts). I waited for over and hour but nothing happened. Then I hooked up a 6 volt battery and the electroplating started happening after about 15 minutes.

My question is why would a lower 6 volt battery more effectively plate the penny than the 9 volt did? I'm guessing it has something to do with the number of ions or the speed of ions. I need to be able to explain why it worked for my science poster.

Caleb W
student - Stanardsville, Virginia, USA

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November 12, 2009

Hi, Caleb. A 1-1/2 volt battery would be even better! This will be a bit hard to understand, but here goes. . .

When you connect the battery and wires to your piece of zinc, you start pulling electrons away from it. For each 2 electrons that you pull away through the wire, you convert one atom of zinc from zinc metal to a zinc+2 ion. These positively charged ions dissolve into the solution. Meanwhile, the electrons that the battery pulled away from the zinc pass through the wiring and get to the penny. The zinc+2 ions float their way through the solution, attracted by the negatively charged penny. When the zinc ions reach the penny, 2 electrons combine with one zinc+2 ion to reform metallic zinc. And so the process continues.

The problem with higher voltages is that only a small amount of zinc at a time can dissolve into solution and float over to the penny, due to a number of limiting factors like how fast they will float (migrate is the more scientific term). But a lot of electrons are being transferred because of your high voltage battery, and something has to give to deal with all those electrons piling up on the penny. Actually, two things give. First, the very microsecond that an ion of zinc reaches the penny, it is converted to zinc as a microscopic powder; no chance for slow growth of a nice zinc crystal, just millions of tiny specs, so fine that they look like a black smut instead of metal. Second, with water all around the penny, but not enough zinc ions, the electricity starts splitting the water into hydrogen gas and oxygen gas, causing hydrogen bubbles to fizz away on the penny, blowing away the zinc dust ⇦ this on eBay or Amazon [affil links] .

1-1/2 volts is enough. 3 volts is a bit high but maybe still workable. 6 or 9 volts is just making a mess.

P.S. I own an undeveloped lot in Stanardsville for which they tax me a pittance. Glad to see they're putting it to good use :-)

Regards,

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