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Author Topic: What happens when performing electrolysis on a copper coin and a carbon rod in?  (Read 2063 times)

Offline Noah Håkansson

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Hello,

I would like to know what happend here:

I performed an experiment where i used electrolysis on a copper coin and and a carbon rod in normal tap water with salt (saltwater). I cant remember what was on the negative/positive side of the electrolysis but the coin got really dirty covered in black, and the water went to a darkish green color.

Would be really happy if womeone could explain the reaction and logic behind whats going on here!

Thank you!


 

Offline chiralSPO

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Hello Noah,

I'm happy to help out, but you will see I also have a lot of questions that will aid us to figure out what happened and why...

The greenish color produced in the solution is almost certainly dissolved copper chloride.

The simplest way this could have formed is if the copper electrode was the positive electrode (anode), where the following half reaction could have occurred:

Cu → Cu2+ + 2 e

The Cu2+ ions would then react with chloride in the solution (from the salt):

Cu2+ + 4 Cl → CuCl42–

It is the CuCl42– ion that is so green.

The final green compound could also be formed if the copper was the cathode (negative electrode). The chloride ions could be oxidized at the carbon (graphite?) anode:

2 Cl → Cl2 + 2 e

Then the produced chlorine could react with copper and chloride:

Cl2 + Cu + 2 Cl → CuCl42–


I don't have a good answer about the coin turning black. When I have done similar experiments in the past, my copper anodes always ended up looking really clean and shiny because the copper dissolved off exposing fresh copper metal. However, it is possible that, due to different experimental conditions, you observed a reaction in which the copper anode dissolved unevenly, producing a very rough structure that would appear dark despite being pure copper.

Or, if the copper were the cathode, it could be reducing dissolved copper ions back to metallic copper, which could form a very rough surface on the coin, that would appear dark.

Alternatively, the black surface could be a copper (I) compound (unlikely) or have nothing to do with copper. Was the coin very clean to begin with? Do you know that it was copper? (For instance US pennies are mostly zinc now, and only have a thin skin of copper.) Was there black material coming off of the other electrode? Was there also a black solid present in the water? Did the "darkish" part of the darkish green water settle if left alone for a while?

What was your electric source? Was is at AC or DC? Do you know the voltage or current? I am assuming the electrodes were attached with alligator clips (did either of the clips corrode?)

Also, what happened at the carbon electrode? Did is fizz or bubble?
 
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Offline Noah Håkansson

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Hallo and thanks (:

Ye, Im from Sweden so im not to good with the chemistry terms in english but yes it was graphite i meant.

The swedish 50-öring coin consists of an alloy of 97 percent copper, 2.5 percent zinc and 0.5 percent tin. It weighs 3.70 grams.

The graphite rod did fizzle and bubble, and the black darkish part of the water was like little particles/bits in the water.

The black did come of the graphite rod, yes.

im not really sure what you mean about AC or DC but the voltage was at 10volts.

The coin was attatched to an alligator clip, dont know if they corroded.

Thanks again!

 

Offline chiralSPO

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Hej Noah :-) (and that's all the Swedish I know)

There are multiple types of carbon. Graphite is most common, but there are also vitreous carbon, and boron-doped diamond electrodes.

I think 2.5 % zinc and 0.5 % tin shouldn't change the chemistry too much. The zinc probably gets consumed first, and then the copper begins to oxidize.

Was the bubbling at the graphite electrode particularly violent (enough to physically break the electrode apart)? Otherwise the graphite was probably corroded electrochemically. If very violent, I would guess that you were producing hydrogen at a graphite cathode:

2 H2O + 2 e → H2 + 2 OH

If not particularly vigorous, it could be oxidation of a graphite anode.
C + 2 H2O → CO2 + 4 H+ + 4 e

This would generate carbon dioxide gas (some of which would dissolve in the water) and would corrode the graphite electrode significantly.

AC means alternating current (each electrode is positive and negative at different times), and DC means direct current (one electrode is positive and one is negative, for the whole time).

10 V is very high for this type of reaction. I am not surprised that you see decomposition of one or more of your electrodes. Voltages between 1.5 V and 3.0 V should offer more controlled (and slower) reactions.

I would also recommend against using sodium chloride (table salt) as the electrolyte because chloride can be oxidized to chlorine or hypochlorite, which will corrode your electrodes, and could be dangerous if you make enough. Sodium bicarbonate (baking soda) can serve as a weak electrolyte. You can also use salts of sodium phosphate (which you should be able to find at a gardening store. Also dilute sulfuric acid or sodium hydroxide are excellent electrolytes, but you have to be careful with such strong acids and bases, even when they are dilute.
 
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Offline chiralSPO

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You can try the experiment again, keeping track of which one was + and which was – to see what is going on.

But it sounds like oxidation of the coin and reaction with chloride caused the green color, and the black stuff came from the graphite electrode.
 
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Offline Noah Håkansson

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Hej igen (: (hey again)

The bubbling was not particularly vigorous.

Can confirm it was DC now that i know what it is (:

Have no possibility of redoing the experiment, it was done in school in the beggnining of december.

I would also recommend against using sodium chloride (table salt) as the electrolyte because chloride can be oxidized to chlorine or hypochlorite, which will corrode your electrodes, and could be dangerous if you make enough.

I guess this is why our teacher, a bit concerned asked us to stop the electrolysis |:


 

Offline Noah Håkansson

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Hey, finally got my notes back and the Copper coin was indeed the positive electrode.

I understand from your first answer what happend to the water and the copper, but what reaction is behind the black stuff from the electrode going onto the coin?

Thanks again!

 

Offline chiralSPO

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I'm not exactly sure what is causing the black stuff on the copper coin, but perhaps it is just black material from the graphite rod that gets stuck to the copper as it corrodes, or a very rough copper surface (as suggested above)...
 

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