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Author: Subject: Why can metals above Hydrogen in the reactovity series be electroplated?
Junk_Enginerd
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[*] posted on 8-8-2020 at 03:33
Why can metals above Hydrogen in the reactovity series be electroplated?


I have very little formal training in chemistry. Just the regular high school stuff, then I went and became an electrical engineer instead. I assume this is a very basic question, but I haven't been able to find the answer I'm looking for.

Anyway, I thought about electroplating. I recently needed some zinc powder. I only had it in shapes and forms that were impure or annoyingly massive to be ground efficiently. So I thought, I can just make it through electrolysis. With a high current, I should get either dendrites or spongy zinc, both which are somewhat more easily made into a powder. So I made a sodium bisulfate solution, threw some zinc in, filled a stainless steel sieve with more zinc as the anode and a random nail as cathode to gather the zinc. It worked great and gave me lots of spongy zinc. But...

I don't really understand why. I had a quick look and saw that zinc is above hydrogen in the reactivity series. I know the reactivity series is relevant to electrochemical reactions but not exactly how it applies. Something like if a metal is above hydrogen, hydrogen should be the one being reduced. Or maybe it's the other way around? But then I shouldn't be able to plate out copper, which is on the other side of hydrogen?

I'd appreciate if someone could explain or point me to a source which explains the relevance of hydrogens reactivity vs others when it comes to electrochemical reduction. Thanks.
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[*] posted on 8-8-2020 at 03:48


The complicated answer involves this
https://en.wikipedia.org/wiki/Nernst_equation

The simple answer is that, in your solution, you have a lot of zinc ions and not so many hydrogen ions, and that means the zinc gets a chance to deposit at the electrode.
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[*] posted on 8-8-2020 at 04:01


Quote: Originally posted by unionised  
The complicated answer involves this
https://en.wikipedia.org/wiki/Nernst_equation

The simple answer is that, in your solution, you have a lot of zinc ions and not so many hydrogen ions, and that means the zinc gets a chance to deposit at the electrode.


Ah. So in a way, I'm not wrong, I was just missing that important part? So it's a bit like partial pressure in gasses then?
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[*] posted on 8-8-2020 at 08:14


Yes, the activity series assumes everything is at 1 M. So, you wouldn't be able to plate zinc from a 1 M acid solution, but in your case you didn't have that.



As below, so above.
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[*] posted on 8-8-2020 at 23:05


Nernst equation is one part. Another is called overpotential:

https://en.wikipedia.org/wiki/Overpotential

Spot the table. Unfortunately, zinc is absent.

Platinum is an excellent catalyst for reactions involving molecular hydrogen - both reducing something else with hydrogen and reducing hydrogen with something else.

Put hydrogen in air. Oxygen in air has oxidation potential of 1,23 V. Hydrogen should burst into fire. It won´t... not unless you set it on fire. Or put platinum catalyst in - which also sets hydrogen on fire.

Catalysts are not Maxwell demons. If platinum catalyzes destruction of hydrogen, it also catalyzes production of hydrogen. Try plating zinc on a platinum cathode, and you should not be able to - only hydrogen should form.
Whereas cadmium is a very poor catalyst for hydrogen, so you might be able to plate zinc on cadmium cathode.
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