A Penny for Your Thoughts - Pt I

 (Disclaimer: This post is for purposes of creating fine artwork! We are not attempting to create bullion for profit.)

Everyone knows the "Liberty Penny," the one that looks like it is made from copper and has Lincoln on one side. Due to the cost of copper far exceeding a one cent, in 1982, the US government modified the penny to to have a zinc core (97.5%) with copper plating (2.5%). A penny weighs 2.5g, so about 2.44g is Zn and the remaining .06g Cu. It is not an alloy (mixture), but rather one metal over the other.

So naturally, I wondered, can I get the zinc out of the middle? 

Yes, it turns out there are several ways to do this. The easiest is to simply hold the penny over a gas flame (with tongs), and when it starts to bloat, give it a little shake over a container full of water. If you do it correctly, the copper shell will be left in the tongs and the molten zinc will fall out to cool in the water. This is easy, but maybe only 99% pure or so. It is easy to accidentally break a little copper off in the process. Another problem is your zinc will be shaped like a meteorite or tektite, losing the penny shape/details. Finally, you will not get all 2.44g out with this method. Some molten zinc will undoubtedly remain attached to the copper shell.

Here is a picture of zinc obtained by this method. The flow lines where it entered the water, radiating from what is called the stagnation point, are easy to see at the bottom:

Here is what the remaining shell looks like (this one broke up):

You may ask why the zinc melts but not the copper. Easy. The melting point for zinc is 787F and for copper it is 1983F. You average gas stove, Bunsen burner, or propane torch will easily melt the zinc, but can't get hot enough for the copper.

So let's say we want a perfect zinc 2.44g penny, not a flying projectile cooled by water. We need chemistry!

We need something that will eat away at the copper, but leave the zinc alone. Sulfur is our answer. We need to turn our copper into CuS (copper (ii) monosulfide, which in nature is the highly collectible mineral covellite).

The exact reactions we will use start with preparing a solution of what is known as "lime sulfur," which is used as a pesticide:

Ca(OH)2 + S -> CaS + CaS2O3 + H2O 

(calcium hydroxide and sulfur react to make calcium sulfide, calcium thiosulfate, and water)

When we balance the equation we get...

3Ca(OH)2 + 4S -> 2CaS + CaS2O3 + 3H2O

I admit this one took me awhile: 3 calciums, 4 sulfurs, 6 oxygens, and 6 hydrogens on each side! The internet had varying equations here, but I think mine is correct.

In this early experiment, we won't calculate the exact weight of reactants needed... In a beaker, add a gram of calcium hydroxide and 2g of sulfur and some water so we can boil. The mixture will not dissolve as the sulfur floats. We need what is called a wetting agent. So add a drop or two of dishwashing soap and mix.

Incidentally, soap is a substance that makes greases, oils, and dirt dissolve in water. That is exactly why it exists, although usually most useful for washing your hands and body. In this case we are "connecting" water and sulfur into our solution.

Now, add some heat by putting this under a burner until it comes to a boil. We need energy to make the reaction occur. It will change colors from a light yellow to a dark orange as it reacts.

The gas released is *theoretically* steam from water boiling, but best to do this outside or under a vent. See below for more detail on this.

Now, add a few pennies (I used four), dated 1982 or later, and wait 30 minutes or until the pennies are fully coated with a black substance. This is copper (ii) monosulfide (or may I say lab created covellite?). The copper has been tarnished fully through. Note that excess reactants and by-products of the first reaction may remain in the solution. In my case I used too much sulfur.

 

I believe the balanced reaction should be:

2Cu + CaS2O3 -> 2CuS + CaO + O2

I may be wrong, the internet had no information on this reaction that I could find. Note we produce oxygen gas, and steam is released from our boil again. Again, do this in a ventilated area. I have made mistakes before and created hydrogen gas and carbon dioxide. Assume the worst. With water and excess sulfur, it is likely to product harmful hydrogen sulfide gas.

Remove the pennies and scrub them with a soft scrub, like ceramic cooktop cleaner. They should look a very shiny silver color and weigh 2.44g after a rinse. If you have bits of copper still attached, cook them some more in the solution and repeat. If you have black deposits remaining, soak and scrub again. It should not be too hard to clean the penny completely of its copper plating.

Warning: the black CuS and soft scrub can make a bit of a mess!

The final product(s) look like this:

You can weigh to prove the theory:

Thanks for reading, 

Paul

p.s. It has come to my attention that some people would like to repeat these experiments without the hassle of finding the chemicals and/or having to buy too much, etc. I will try to keep an Ebay store alive (seller: cinnabarminerals) that offers low cost and related material for the purpose of enabling education. I will title offerings "Brave New Chemist Science Pack - (related Blog post name)." You can always comment and ask where to buy certain equipment too.