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Topic: Here we go again: Galvanic cells  (Read 1251 times)

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Offline hnes

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Here we go again: Galvanic cells
« on: February 17, 2019, 08:43:10 AM »
Ok, I know that there are hundreds of other threads like this, but I really can't find a good answer.

I have tried to understand galvanic cells a bit better than high school (and even some enginering) text books presents it. Let's take a galvanic cell with Zn and Cu as electrodes, each dipped in say ZnSO4 and CuSO4. The beakers are connected with a Na2SO4 salt bridge.

1. Text books tends to get you to believe that you HAVE to have Cu as one of  the electrodes. But isn't the point that copper is beiing made AT the electrode? So the electrode could be made of a carbon rod, for example? The reason I want to get this clear, is that it makes it much easier to see the connection between the typical "dip a zinc rod in copper -solution" experiment and a galvanic cell.

2. In the beaker where the Zn - rod is, can't you just start with water there? Since Zn will be reduced to Zn2+ here and SO42- will wander to the ZN2+? If not, please give good explaination on this! Don just say "you need an electrolyte". I understand that you have to have an electrolyte in the saltbridge, but not in the Zn-beaker.

3. Does the Na+ ions from the salt bridge react with the SO42- in the Cu-beaker? How does the ions flow in the salt bridge? Different sources states different answears. Some say that Zn2+ wanders from Zn-beaker to Cu-beaker, while others say that only the ions in the salt bridge moves.




Really angry at this whole galvanic cell thing, looks like no one could give a good explaination on it!

Offline chenbeier

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Re: Here we go again: Galvanic cells
« Reply #1 on: February 17, 2019, 02:37:36 PM »
Answer zour questions

1.You can also use different material for the rod, but then you get different potentials and additional a reduction of copper. Ifzou use copper then you get the real copper potential.

2. Water would have a very bad conductivity and Zinc would get zinc hydroxide precipitation, what will block the electrode. For this reason zinc sulfate is used.

3. In short therm the ions in the salt bridge moving. In long term because there is a potential gradient the yinc will move to the copper beaker. But the salt bridges are equpped with diaphragma to prevent the mixing of the metal ions.

Offline hnes

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Re: Here we go again: Galvanic cells
« Reply #2 on: February 17, 2019, 03:18:12 PM »
Answer zour questions

1.You can also use different material for the rod, but then you get different potentials and additional a reduction of copper. Ifzou use copper then you get the real copper potential.

2. Water would have a very bad conductivity and Zinc would get zinc hydroxide precipitation, what will block the electrode. For this reason zinc sulfate is used.

3. In short therm the ions in the salt bridge moving. In long term because there is a potential gradient the yinc will move to the copper beaker. But the salt bridges are equpped with diaphragma to prevent the mixing of the metal ions.

Thanks for reply!

1. Why different potentials? The Nernst equation does not take the rod material into account, only the concentration of Cu2+ ..?

2. I don't understand that. I know that water has bad conductivity, but does this lack of conductivity mean that Zn2+ can be made at the Zn-electrode?

3. Ok.

Online Borek

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Re: Here we go again: Galvanic cells
« Reply #3 on: February 17, 2019, 04:55:52 PM »
1. Why different potentials? The Nernst equation does not take the rod material into account, only the concentration of Cu2+ ..?

Actually it does, just we typically assume the activity of the solid copper to equal 1 and we ignore it.
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Online Borek

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Re: Here we go again: Galvanic cells
« Reply #4 on: February 17, 2019, 04:57:05 PM »
2. Water would have a very bad conductivity

While it is not incorrect, it is also not entirely relevant to the question about zinc salt. You can add an inert salt to make the solution conductive, you can buffer it to avoid hydroxide precipitation.
« Last Edit: February 17, 2019, 06:14:58 PM by Borek »
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Re: Here we go again: Galvanic cells
« Reply #5 on: February 17, 2019, 04:59:15 PM »
I understand that you have to have an electrolyte in the saltbridge, but not in the Zn-beaker.

Actually you need the electrolyte in both places, for the same reason - conductivity. It doesn't have to be Zn salt solution, using just Zn rod will work (although as long as the concentration of Zn2+ is zero potential will depnd not on Zn/Zn2+ system, but on other redox system present (they are always present in water).

Quote
Since Zn will be reduced to Zn2+

Oxidized.
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Offline hnes

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Re: Here we go again: Galvanic cells
« Reply #6 on: February 18, 2019, 03:36:59 AM »
Thanks for the reply!

I'm not getting much wiser, though.

Maybe my starting point is wrong..?

Here's my thought process:

1. I know that if you put a Zn rod in Cu2+ solution, the Zn-rod gets oxidized to Zn2+ and Cu2+ gets
reduced to CU on the Zn-rod.

2. This is a redox-reaction, and I want the electrons to do usefull work for me.

3. If I start with Zn-rod in a beaker with some solution X, and connects it through a load to a carbon rod in Cu2+ solution, we will have a current through the load - if we manage to make the Zn2+ kations to flow to the anions in the Cu2+ beaker (salt bridge or "membrane").

4. I can buy that you need an electrolyte in the Zn2+ beaker (solution X)  "because it have to conduct current" (Zn2+ kations), but I dont understand why. Don't we get Zn2+ in that beaker anyway from the oxidation?

Would really like some depth answears. Come on, I can handle it! ;)


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Re: Here we go again: Galvanic cells
« Reply #7 on: February 18, 2019, 07:25:25 AM »
Don't we get Zn2+ in that beaker anyway from the oxidation?

What is the initial concentration of Zn2+ if you start with pure water?

Think about a difference between the battery that just starts to work and battery that already worked for some time.
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Offline hnes

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Re: Here we go again: Galvanic cells
« Reply #8 on: February 19, 2019, 02:49:20 AM »
Don't we get Zn2+ in that beaker anyway from the oxidation?

What is the initial concentration of Zn2+ if you start with pure water?

Think about a difference between the battery that just starts to work and battery that already worked for some time.

Borek, I really appreciate your explainations. I can feel that I understand the galvanic cell and redox reactions within a certain "understanding frame" , but I very often meet problems where my understanding is not good enough.

Take this picture to the left, for instance. The text on the page http://large.stanford.edu/courses/2010/ph240/nie1/
says that you wil get a voltage between the two electrodes if you have a electrolyte between them.
How is this related to the typical Daniell cell? The electrodes are in the same beaker. If you have NaCl (aq) as electrolyte and Zn and Cu elektrodes, what happends? It does not fit my "understanding frame" from the explaination of displacement reactions and galvanic cells.


Offline hnes

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Re: Here we go again: Galvanic cells
« Reply #9 on: February 19, 2019, 02:56:00 AM »
Here's an other one (refer to my last post)


Online Borek

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Re: Here we go again: Galvanic cells
« Reply #10 on: February 19, 2019, 08:52:06 AM »
Take this picture to the left, for instance.

Sadly, this is wrong. That is - it shows the general idea without getting into details, but to someone trying to learn it is confusing as it ignores the fact when the cells are not isolated things can react directly (I believe you have correctly mentioned that earlier in the thread).
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Offline hnes

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Re: Here we go again: Galvanic cells
« Reply #11 on: February 19, 2019, 01:12:18 PM »
Take this picture to the left, for instance.

Sadly, this is wrong. That is - it shows the general idea without getting into details, but to someone trying to learn it is confusing as it ignores the fact when the cells are not isolated things can react directly (I believe you have correctly mentioned that earlier in the thread).

Ok, thanks!

I did an experiment today. I took one strip of Zn and one strip of Cu, and put it in a beaker of 3,5 % acetic acid.

Here is what happend, along with my assertions and questions.

1. I got about 0,8V between the electrodes (old manual voltemeter). Zn connected to "ground". So Zn is negative pole, Cu is positive from the experiment.

2. Bubbles formed at both the Zn and Cu electrode, but much more on the Zn. I assume that H2 is formed at the Zn electrode. Zn is getting oxidized and H+ is getting reduced to make up H2.

3. What happends at the Cu electrode? First i thought that it is not part of the reaction, but when i removed the Cu and just
took the wire in the beaker (the wire clip was probably iron or something) the voltage went down. Is H2 gas formed at both the electrodes? How does this relate to the reduction potentials? Cu should not be oxidized by acetic acid? Is it oxygen from copperoxide?

4. Does electrons travel to both H+ at the Zn electrode AND to H+ at the Cu electrode, forming H2 at both places? If that is not happening, I dont understand how Zn can be the negative pole, since all the electrons then join the H+.

Hope to get some clarifications!





Offline hnes

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Re: Here we go again: Galvanic cells
« Reply #13 on: February 21, 2019, 11:52:48 AM »
I did an experiment today. I took one strip of Zn and one strip of Cu, and put it in a beaker of 3,5 % acetic acid.

You haven't said if they were connected to each other or not, those are two very different cases.

And yes, gas doesn't have to evolve in the same place Zn2+ gets into the solution, so the bubbling on the copper doesn't have to mean copper reacting.
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