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Differing voltages in Electrolysis?


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Hello, I was wondering if there was an online resource that would tell me at what voltages certain ionic bonds are broken. I was reading something woelen said in a different thread and apparently when the voltage was at a precise level electroplating began. So is there some resource out there I could use for like if I had a solution of NaCl and water, and I only wanted one of the compounds to be broken down. Is this even possible? Thanks!

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If you are trying to get sodium metal from salt and water, you won't get it. You'll only get hydrogen gas, chlorine gas, and after a while some oxygen gas too. What you need to find is a table of standard reduction potentials. That should help you out.

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I realize you couldn't get sodium Metal, it was a quick example that obviously wasn't well thought out ;-) Anyhow, I have a question: So if you have copper and Nickel together and you put it through electrolysis. The Nickel has a more positive oxidation potential than copper, so it is prefferred. But the Copper is also Turned into an Ion. But then The copper has a more positive reduction potential and so is the only one Reduced onto the Cathode. So Why do both the Ni and Cu Oxidize but only the Cu Reduce? You'd think it'd either work the same one way or the other.. but I don't know. BTW, a table of reduction potentials is EXACTLY what I wanted, thanks!

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No problem. But make sure you're looking at the equation in the right direction. If you are looking at reduction potentials, you'll see equations like this;

 

Cu2+ + 2e- => Cu(s) +0.34

 

The oxidation potential is then Cu(s) => Cu2+ + 2e- -0.34

 

Also remember that copper can exist in multiple oxidation states while Nickel really doesn't do that.

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Yeah, all that makes sense; but doesnt really answer my question.

 

Cu ---> Cu+ + e- -0.52

Cu ---> Cu2+ + 2e- -0.34

Ni ---> Ni2+ + 2e- 0.25

 

All that's right, correct?

So Nickel has the most positive Oxidation potential. But BOTH Cu and Ni are Oxidized. Both Oxidation states are below Ni, but it is still Oxidized, Whereas..

 

 

Cu+ + e- ---> Cu 0.52

Cu2+ + 2e- ---> Cu 0.34

Ni2+ + 2e- ---> Ni -0.25

 

Now both Cu Ions are favorable for reduction. And ONLY the Cu is Reduced. So..what's going on?

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Hello, I was wondering if there was an online resource that would tell me at what voltages certain ionic bonds are broken. I was reading something woelen said in a different thread and apparently when the voltage was at a precise level electroplating began. So is there some resource out there I could use for like if I had a solution of NaCl and water, and I only wanted one of the compounds to be broken down. Is this even possible? Thanks!

Heard that Voltage affect the rate only but not the kind.

So Nickel has the most positive Oxidation potential. But BOTH Cu and Ni are Oxidized. Both Oxidation states are below Ni, but it is still Oxidized, Whereas..

I found one SP list is telling Ni is above H2.

And, I found my book saying that it is reduced during electroplating.

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The impurities in the copper anode include lead, zinc, nickel, arsenic, selenium, tellurium, and several precious metals including gold and silver. Metallic impurities that are more active than copper are readily oxidized at the anode but do not plate out at the cathode because their reduction potentials are more negative than that for Cu2+. However, less active metals are not oxidized at the anode. Instead, they collect below the anode as a sludge that is collected and processed to recover the valuable metals. The anode sludges from copper-refining cells provide one fourth of U.S. silver production and about one eighth of U.S. gold production.

 

Sample Exercise 23.1

Nickel is one of the chief impurities in the crude copper that is subjected to electrorefining. What happens to this nickel in the course of the electrolytic process?

 

SOLUTION The standard reduction potential for Ni2+ is more negative than that for Cu2+:

 

 

 

 

 

As a result, nickel is more readily oxidized than copper, assuming standard conditions. Although we do not have standard conditions in the electrolytic cell, we nevertheless expect that nickel is preferentially oxidized at the anode. Because the reduction of Ni2+ occurs less readily than the reduction of Cu2+, the Ni2+ accumulates in the electrolyte solution, while the Cu2+ is reduced at the cathode. After a time it is necessary to recycle the electrolyte solution to remove the accumulated metal ion impurities, such as Ni2+.

 

This says otherwise, so who's right? It'd make sense for you to be.. But who knows.

 

EDIT: I think I have a solution. Please tell me if I'm right. The way I see it, theres a lot mre Cu than Ni in the crude metal. So Nickel if oxidized. When the Nickel Runs out, the Cu is then oxidized. Once all the Ions are in the solution, there is no lack of Cu2+ Ions, therefor the Nickel is never reduced. Is this correct? If it is, it looks liek I jsut answered my own question, and that feels good. Of course it proll isn't right.

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Standard potentials are just standard potentials. What you observe in solution is described by Nernst equation which shows how potential depends on the concentrations. As concentrations of free ions are often modified by the presence of other ions (by complexation and precipitation reactions for example) you have to know much more than just which metals are pesent in the solution.

 

Best,

Borek

--

Chemical calculators at www.chembuddy.com

pH calculator

concentration conversion

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This says otherwise' date=' so who's right? It'd make sense for you to be.. But who knows.

 

EDIT: I think I have a solution. Please tell me if I'm right. The way I see it, theres a lot mre Cu than Ni in the crude metal. So Nickel if oxidized. When the Nickel Runs out, the Cu is then oxidized. Once all the Ions are in the solution, there is no lack of Cu2+ Ions, therefor the Nickel is never reduced. Is this correct? If it is, it looks liek I jsut answered my own question, and that feels good. Of course it proll isn't right.[/quote']

 

 

 

Yep, that's about right.

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