A second demonstration -- Copper plating a key or a quarter

Another slightly harder demonstration is plating a quarter or a brass key with copper. The key on the left was copper plated from a solution of vinegar with a pinch of table salt and a pinch of sugar, again using a 1-1/2 volt flashlight battery for power.

Understanding why this experiment is a little harder to do than the first one is a good science lesson: You can't plate a metal out of a solution until you can get that metal dissolved into the solution as ions (actually, this is why we didn't do a student demo of nickel, silver, gold, or chrome plating; you won't be able to dissolve these metals in vinegar, you would need a stronger and more dangerous acid).

Copper will not dissolve in vinegar without electricity to help it along, so it's best to get started with a small piece of scrap as your cathode and a large coil of copper wire as the anode. I stripped and scrunched together about 2 foot of 14 gauge wire to use as the anode (wire is very pure copper) and used a 1/2 inch length of stripped copper wire as the scrap cathode. After I ran it this way for a couple of hours the solution acquired a faint blue tinge to it -- indicating that a little copper was dissolved in it. Then I cut off the scrap length of cathode wire, attached the key and plated it for several hours. Vinegar is too weak an acid to hold much copper in solution, so there is no rushing it, you have to plate slow and for a long time so copper can slowly dissolve into solution to replace what you plate out. I found that just a pinch of table salt (maybe two shakes) was enough. If you use more, what happens is you make a more conductive solution, so more electricity flows, but since there is not enough copper dissolved in the solution to support that current flow, you generate a lot of hydrogen gas and deposit a lot of black "smut" -- you can't plate copper out of solution faster than it goes into solution!

What happens if your solution is too conductive due to too much table salt? The electricity is flowing through the solution, so electrons are flowing into it from the cathode. But if there are no copper ions there to pull out of solution, the electricity will pull hydrogen ions out of the water per this equation:

2H₂0 --> H₂⁺⁺ + 2OH^-

This will cause bubbles of hydrogen gas to accumulate on your key or quarter, and the OH, the hydroxide, will neutralize your vinegar so you'll have no acid left.

What is "smut", or as some students call it, "black glop that coats the coin"? When you have too much current flow, what also happens is the moment that an ion of copper gets to the cathode, it is "reduced" instantaneously with no opportunity for proper crystal growth, and it forms a powder of tiny, non adherent individual specs of metal which appear black because they scatter all the light, reflecting none. So use 1-1/2 volts maximum, very little salt, and take your time!

Date: 2016-04-22; view: 584