The Initial Reaction of a Voltaic Cell

[student34]

I just want to start by saying that I have found many questions very similar to this one using the search bar, but I still don't understand exactly what happens initially in a simple voltaic cell with a salt bridge (Daniel cell).

For example, I am trying to figure out what initially happens in a cell with a zinc anode in zinc sulfate solution connected with an external circuit to a separate jar with a copper cathode in a copper sulfate solution. The salt bridge contains potassium chloride.

Because I am concerned about the very beginning of the flow of electrons in the voltaic cell, sources say that the salt bridge does not come into effect yet due to the lack of positive zinc ions. So the charges do not make it through the full circuit yet. Okay, that's fine.

So what is the initial reaction that starts this initial one-way flow of charges: zinc is oxidized, copper is reduced, both simultaneously or some other reason?

 

[Bystander]

both simultaneously

... , and, the salt bridge maintains the charge balance (zero) in both sides of the cell.

 

[student34]

... , and, the salt bridge maintains the charge balance (zero) in both sides of the cell.

But I am missing the reason why the electrons flow from the zinc to the copper in the first place. Even if they oxidize and reduce simultaneously, what initially causes the copper to be reduced and at the same time that the zinc to be oxidized. I don't understand the initial driving forces.

 

[Bystander]

Copper ions oxidize zinc metal; or, zinc metal reduces copper ions, either way you care to think about it. You have been introduced to free energies of chemical reactions?

 

[student34]

Copper ions oxidize zinc metal; or, zinc metal reduces copper ions, either way you care to think about it.

I think I understand. Just to be sure, if I touched solid copper to solid zinc instead of connecting them with a conducting wire, would the copper oxidize some of the zinc atoms?

You have been introduced to free energies of chemical reactions?

I don't think so.

 

[Bystander]

I don't think so.

Just looked at previous threads, and, you're being "eased" into the concept. It's pretty much the same thing as is going on with the plating out of silver on the copper strip, or this thread https://www.physicsforums.com/threads/oxidizing-and-reducing-agents.799828/ , and hopefully, they'll get to the "bottom line" and give you a few rules and a table of redox potentials to use for this type of problem fairly soon. Copper is a less reactive metal than zinc, and that means that copper ions are more reactive toward zinc metal (corrosive, likely to oxidize zinc, whatever) than zinc ions are toward copper metal.

 

[DrDu]

But I am missing the reason why the electrons flow from the zinc to the copper in the first place. Even if they oxidize and reduce simultaneously, what initially causes the copper to be reduced and at the same time that the zinc to be oxidized. I don't understand the initial driving forces.

Entropy

 

[Borek]

Entropy

To be honest, I don't see how it addresses the problem.

We remove ion on the copper side from the solution and put it on the electrode surface. Entropy goes down.

We remove an atom on the zinc side from the electrode and put it into the solution. Entropy goes up.

As a first approximation final entropy doesn't change, so it can't be a driving force.

 

[DrDu]

Sorry, I didn't have the possibility to write much. As soon as you bring an electrode into the corresponding solution, some of the metal will dissolve and some get deposited. Which of the two reactions predominates depends on the relative stability of the ions vs. metal. This stability changes then when the electrodes start to charge up, which is a very fast process. As long as there is no external current, there will also be no internal current through the salt bridge, as each half cell will stay overall neutral.

 

[student34]

I think I found the answer. My textbook seems to say that the copper atoms of the anode will actually oxidize the zinc atoms of the cathode with or without the solution.

Does anyone know if this is accurate?

 

[Borek]

My textbook seems to say that the copper atoms of the anode will actually oxidize the zinc atoms of the cathode with or without the solution.

I am trying to imagine what it means. I have two pieces of metal, I touch them - and what? Zinc cations jump out into the air?

Definitely there will be some potential difference created by the junction, that's the thermoelectric effect at work.

 

[student34]

I am trying to imagine what it means. I have two pieces of metal, I touch them - and what? Zinc cations jump out into the air?

Definitely there will be some potential difference created by the junction, that's the thermoelectric effect at work.

I am not sure. It seems to be the only way thing that makes sense to me. I don't know what else could cause the flow of electrons?

 

[Borek]

For the record: zinc cations will NOT jump into the air.

 

[student34]

For the record: zinc cations will NOT jump into the air.

So then what else would be the initial reaction?

 

[Bystander]

So then what else would be the initial reaction?

If you're asking what initiates the reaction, it's when the circuit is closed and electrons are able to flow. Open circuit, no flow, no reaction. Closed circuit, electrons flow, copper is reduced and zinc is oxidized.

 

[Borek]

So then what else would be the initial reaction?

No idea, that's why it caught my attention.

 

[Borek]

If you're asking what initiates the reaction, it's when the circuit is closed and electrons are able to flow. Open circuit, no flow, no reaction.

There is a potential difference even in the open circuit, so there is _some_ initial reaction involved. Electrodes become charged the moment they are immersed in the solution, even if the circuit is otherwise open.

 

[James Pelezo]

I just want to start by saying that I have found many questions very similar to this one using the search bar, but I still don't understand exactly what happens initially in a simple voltaic cell with a salt bridge (Daniel cell).

For example, I am trying to figure out what initially happens in a cell with a zinc anode in zinc sulfate solution connected with an external circuit to a separate jar with a copper cathode in a copper sulfate solution. The salt bridge contains potassium chloride.

Because I am concerned about the very beginning of the flow of electrons in the voltaic cell, sources say that the salt bridge does not come into effect yet due to the lack of positive zinc ions. So the charges do not make it through the full circuit yet. Okay, that's fine.

So what is the initial reaction that starts this initial one-way flow of charges: zinc is oxidized, copper is reduced, both simultaneously or some other reason?

Check out this post.
https://www.physicsforums.com/threads/galvanic-cells-and-salt-bridges.812134/

 

[tinska.h]

Hey,

a little bit of hijacking here, but anyways: In Daniell cell, why SO4 ion does not get bonded with Zinc at the electrode interface? I guess it has to do something with reduction potential or is it because its a soluble salt? But in that case, would that mean that solvent bondings are keeping SO4 ion so tightly? Or would SO4 ion get initially attached and then solvated again?

 

[Borek]

Solubility is the only thing that matters here.

 

[James Pelezo]

The SO₄ ^2- ion is a 'spectator ion' and does not enter into the Galvanic process. Also, the zinc electrode in, say a Zn/Cu cell,. the Zn^o us undergoing oxidation leaving and imposes excess of negative charge on that electrode. SO^{₄ 2-} is a negative ion whose oxidation potential is on the positive end of the Reduction Potential table and would function as a Furthermore, the SO₄ ^2- as a counter ion remains in solution throughout the Galvanic process. In the Zn/Cu cell Zn and Cu will be the dominate reactions of the Voltaic Cell.

Link edited out.

Solubility is the only thing that matters here.

Could you elaborate. Why are you saying 'solubility' is the only thing that matters here? There are several issues to consider beyond solubility when describing the Galvanic process and chemistry of Voltaic Cell. Thank you. jp

 

[Borek]

Could you elaborate. Why are you saying 'solubility' is the only thing that matters here? There are several issues to consider beyond solubility when describing the Galvanic process and chemistry of Voltaic Cell.

ZnSO₄ doesn't precipitate because it is well soluble.

This question was also asked today somewhere else and there behavior of ZnSO₄ was compared to the behavior of AgCl which covers Ag electrodes in the presence of chlorides. The only difference - or in other words, the only reason why Ag electrode becomes covered with AgCl, while Zn electrode doesn't get covered with ZnSO₄ - is the solubility of the salt involved. Galvanic process is not important here and not needed to explain the problem.

 

[James Pelezo]

Your reply suggests that Ag as an 'electrode' is in basic standard state [Ag^o(s)]. I can't find one citation that suggests that Ag-metal in basic standard state reacts with chloride ions (Cl^-(aq)). I find plenty that relate Ag⁺(aq) ions react with Cl-ions to produce AgCl, which precipitates as a solid because it has a K_sp = 1.8 x 10^-10. It's common understanding that unless Ag^o(s) is oxidized to Ag-ions in the presence of Cl^-(aq), AgCl will not form. Also, the positive reduction potential of [Cl₂(g)/2Cl^-(aq)] ( = 1.36 volts ) is greater than [Ag⁺(aq)/Ag^o(s)] ( = 0.80 volts ) which indicates that Cl₂(aq) would have to be present to induce oxidation of the silver. From your reply, I get the impression that you are talking about Ag-metal in the presence of chloride ions reacting to form the ionic compound AgCl without considering the oxidation-reduction process. Silver Chloride may be forming when Ag(s) and Cl^-(aq) are interfaced, but what is driving the oxidation of Ag(s)? I'm finding it difficult to find citations that support your statement that Galvanic Process in not important when oxidation-reduction reactions are required to generate the AgCl precipitate. Redox reactions are a basic tenant of Galvanic Processes.

 

[Borek]

James, you have a tendency to overcomplicate things. My bet is that the OP has no idea what you are talking about. To reiterate in the context of the original question:

In Daniell cell, why SO4 ion does not get bonded with Zinc at the electrode interface?

It doesn't happen because the solubility of ZnSO₄ is high enough.

Question doesn't ask about what and why gets oxidized.

 

[James Pelezo]

Borek, Being a new guy, I'm not totally fluent with this site. I want to ask, is there a private communication channel on this forum? I have a request for your review only and not for the public viewing. Thank you, jp

 

[epenguin]

But I am missing the reason why the electrons flow from the zinc to the copper in the first place. Even if they oxidize and reduce simultaneously, what initially causes the copper to be reduced and at the same time that the zinc to be oxidized. I don't understand the initial driving forces.

I think I understand. Just to be sure, if I touched solid copper to solid zinc instead of connecting them with a conducting wire, would the copper oxidize some of the zinc atoms?

I don't think so.

For the second case whatever happens there is between Cu and Zn, at the same oxidation level, a bIt different from what we are considering - Cu²⁺ and Zn.

Your first question is really what drives any chemical reaction, or any ionic one with simple electron transfer at least. That is part of general chemistry. Some atoms hold on to electrons or give them away more than others. All tied with the explanations of the systematics of the periodic table. The elements are all different so one will tend to donate electrons to another, the big question is which to which. You can study that even without electrochemistry. Zn will donate electrons to Cu²⁺ giving Cu and Zn²⁺. You can easily find many you-tube etc. videos showing this happen and red copper precipitating out of blue copper sulfate solutions in which there some plates of zinc. (I only don't quote anyone because those I saw were sufficient but not very good.)

The driving force you ask after is already there in thse reactions. Electrochemistry just has the nice addition that through diffusion of ions in the liqiid and the miracle of metallic conditivity you can couple one chemical reaction at one point to another at another, and also in the process extract useful work, like light up a lightbulb.

 

[student34]

For the second case whatever happens there is between Cu and Zn, at the same oxidation level, a bIt different from what we are considering - Cu²⁺ and Zn.

Your first question is really what drives any chemical reaction, or any ionic one with simple electron transfer at least. That is part of general chemistry. Some atoms hold on to electrons or give them away more than others. All tied with the explanations of the systematics of the periodic table. The elements are all different so one will tend to donate electrons to another, the big question is which to which. You can study that even without electrochemistry. Zn will donate electrons to Cu²⁺ giving Cu and Zn²⁺. You can easily find many you-tube etc. videos showing this happen and red copper precipitating out of blue copper sulfate solutions in which there some plates of zinc. (I only don't quote anyone because those I saw were sufficient but not very good.)

The driving force you ask after is already there in thse reactions. Electrochemistry just has the nice addition that through diffusion of ions in the liqiid and the miracle of metallic conditivity you can couple one chemical reaction at one point to another at another, and also in the process extract useful work, like light up a lightbulb.

Thank-you, I understand this now.