AP Physics/Chemistry electroplating experiment

A. Hello Chris. Hello Jim.

Jim, please tell us what grade your son is in because I will try to answer Chris, but the reply is directed to him as an AP Chemistry / AP Physics student. The internet is a giant one-room schoolhouse and it unfortunately isn't possible to reply in a way that is informative to someone at Chris' level while understandable to 3rd grade students who may stumble upon it. We probably can explain it to 3rd grade students as well, but such a response would waste Chris' time. It's hard to help without knowing grade level.

Chris: The idea behind your experiment appears to be a proof of Faraday's Law and it's a great idea for an experiment because it allows you to see and understand the rules of chemistry from a very informative angle that takes into account atomic weight, valence, and a lot of other stuff.

Faraday's Law says that 96,485 coulombs (ampere-seconds) of electricity will deposit one gram molecular weight of metal. And it would seem that by trying 3 different metals (which would each have different atomic weights and valences, and therefore different gram molecular weights) and by plating at constant current, you should be able to demonstrate it. Faraday's Law is a very cool aid to understanding chemistry because you are counting the electrons moving from one pole to the other, and measuring that the weight of atoms moved corresponds the way it's supposed to. However, I would plate for a lot more than 2 minutes... more like two hours so you have a fighting chance of measuring weight differences.

While Faraday's Law is universal, there are two interfering factors which you must avoid.

A). you must avoid immersion reactions. An example of an immersion reaction is how immersing an iron nail in copper sulphate causes copper to deposit on the nail even with no electricity applied. Obviously if some of the metal is depositing without application of electricity, the amount of metal deposited will not be directly proportional to the amount of electricity applied. So you must always plate more noble metals onto less noble cathodes.

B). if you try to electroplate too fast, some of those electrons (some of the current) will go towards separating water into hydrogen and oxygen instead of depositing metal. This isn't difficult to conceptualize: you are forcing electrons to the cathode where they will reduce dissolved metal ions to metal if they are there; but if no metal ions are there but water is, those electrons will liberate hydrogen. So keep the voltage and current very low.

1. As an industrial process, the cathode does not need to be made of the metal you are trying to deposit, and often isn't. But it may make your calculations and your understanding of them difficult if it isn't. Because the ideal situation is where one Faraday of electricity deposits one gram molecular weight of metal onto the cathode while dissolving one gram molecular weight from the anode.

2. I think your copper sulphate and silver nitrate should work as long as the cathode is a silverplate spoon (or even a costly Sterling spoon). They would not work if your cathode were steel or zinc or brass (see interference 'A' above). Nickel sulphate would probably be a good third solution to try, and zinc sulphate ⇦ this on eBay or Amazon [affil links] is another possibility.

3). The solutions should be as concentrated as possible. See interference 'B' above. In industrial use, concentrated solutions are used, and sometimes anions like fluoborate and sulfamate are used because they support higher concentrations in solution. But sulphates are fine.

In the 2nd part of the experiment, when you vary the voltage, you will automatically be varying the current proportionally (Ohm's Law), and you can watch Faraday's Law in action once again.

The only way to vary the voltage without varying the current is to move the anode further from the cathode to increase the solution resistance. If your experiment offers sufficient resolution, you will see that the voltage has no effect (apart from it's role in varying the current).

Good luck, and compliment your teacher or advisor for me: this is a great experiment.

A. Hi Karl.

We appended your inquiry to a similar thread, and you may be interested in doing Chris' experiment. But if you feel that your own, about the effects of temperature, is more interesting... well, that's fine too. The best experiments are the ones that interest you.

If you do go with an experiment on the effects of temperature, you can see that it won't effect how much plating occurs at a given current. But you may find that higher temperatures allow you to do decent plating at higher currents (the increased mobility of ions at higher temperatures). I would suggest copper sulphate as your plating solution rather than silver plating solutions though. Good luck.

Regards,