[adamcross]

Hi.  I'm just trying to understand how voltaic cells work.  Suppose we have the usual textbook example: copper and copper ions in one beaker, zinc and zinc ions in another beaker, and some kind of salt bridge.  All the descriptions I've read are quite vague about exactly what happens when you connect the two electrodes with a wire.  They talk like this puts the copper in direct contact with the zinc, and they completely neglect the material of the wire itself. 

As I understand, what drives the reaction is the difference in electronegativity between the two metals.  So then, what happens if the wire connecting them has higher electronegativity than both zinc and copper? 

More generally, what happens in the wire, and how can the copper pull electrons through it? 

Thanks. 

[408]

The problem in your reasoning is that it is not the electronegativity diff between metals.  It is the difference in potentials between oxidizing and reducing agent. 

In the Cu/Cu2+/Zn2+/Zn cell you need to identify the oxidation and reduction agent.

Ox is Cu2+, reduce is Zn. 

During this process Cu2+ + 2e- --> Cu and
Zn-->Zn2+ + 2e-

and electrons flow through whatever connects the two electrodes. 

When you consider it like this you can remove the unnecessary parts.  It does not matter that the copper electrode is copper, nor that the Zn solution contains Zn2+.  The former could easily be replaced by silver, or graphite, and the latter any solution that will not react with zinc that conducts electricity. 

[adamcross]

Thanks for the response.  I still don't understand.  In the copper ion solution, for the copper ions to be reduced, they have to be able to pull electrons from the electrode.  So, if the electrode is graphite, then the copper ions have to pull an electron off of the graphite.  How are they able to do that?

[Borek]

Graphite surface is charged, it is not like a reaction with pure graphite itself.

[408]

Thanks for the response.  I still don't understand.  In the copper ion solution, for the copper ions to be reduced, they have to be able to pull electrons from the electrode.  So, if the electrode is graphite, then the copper ions have to pull an electron off of the graphite.  How are they able to do that?

The electron is not coming from the graphite.  It is coming off the zinc on the other electrode as it turns into zinc cation.  The graphite and whatever connects the electrodes merely serves as conductor. 

[adamcross]

But that is exactly my point of confusion.  You talk as though the copper is acting in direct contact with the zinc and the conductor in the middle is irrelevant.  I guess you are right, but I don't understand why.  Whatever it does, it has to do it to the conductor, because it cannot touch the zinc.

[Borek]

You talk as though the copper is acting in direct contact with the zinc and the conductor in the middle is irrelevant.

Exactly. Conductor is inert and it only transfers the charge, it doesn't do anything else.

Redox reactions are about charge transfer - it can be transferred directly, it can be transferred through the inert conductor. It doesn't matter how.

[adamcross]

You're still just leaving the darkness unobscured.  You're merely repeating the unsubstantiated claim rather than explaining how or why.  I will probably need to ask a professor of chemistry.

[Borek]

Please keep us posted if you will get better explanation. Some of our members are professors of chemistry, so I believe lack of better answers means the answer given so far is correct.

Think about it this way: when the copper gets in the contact with a solution, and the solution doesn't have any copper dissolved, the potential of Cu/Cu²⁺ system is infinite (think about Nernst equation). That means copper is able to react with water, reducing it. It doesn't react for long - just till the concentration of free Cu²⁺ gets high enough for the copper to become in equilibrium with the solution. But if copper has some non-zero potential, it means it is charged (that's a simple conclusion of the potential definition). If it is charged, and you connect it to some conductor (say graphite) the charge flows, and all connected surfaces become charged - and this charge is ready to react with zinc in the other cell.