The Naked Scientists

The Naked Scientists Forum

Author Topic: Can a double oxidation phenomenon generate an electric current?  (Read 7158 times)

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
An unusual battery is proposed in this experiment. The particularity of this cell is the fact that both electrodes (formal cathode and anode) undergo oxidation phenomenon.
The experiment is very simple: put a piece of Fe and a piece of Zn in a sulfuric acid solution and connect these pieces of metal to an ammeter.
Although both metals are oxidized, an electric current is generated in an external circuit.
You can perform the experiment in the kitchen using same metals and a acetic acid solution (vinegar).
How can the experiment be interpreted in the frame o actual scince?
Regards,


 

Offline peppercorn

  • Neilep Level Member
  • ******
  • Posts: 1466
    • View Profile
    • solar
Homework?
 

Offline SimpleEngineer

  • Sr. Member
  • ****
  • Posts: 117
    • View Profile
Is it worth considering this through the application of potential difference? I recal once a long time ago during metallurgy something about something wrt galvanic action..
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile
At the iron electrode two reactions take place at the same time.
Iron dissolves in the acid to give hydrogen and Fe++ ions
Those Fe++ions are reduced back to iron by the electrons arriving from the zinc.
 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
Are you dreaming or fabulating?
Even with naked eyes anyone can see that both electrodes are consumed and no Fe deposition takes place.
Why should Fe deposit again and not Zn? Have you replicated the experiment and saw the direction of the electric current?
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4726
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
In a primary cell the external current is driven by the difference in electronegativity of the two electrodes. The electrolyte is irrelevant to the potential difference*, but the more conductive the electrolyte the greater the prospective external current. The first trick of battery design is to find an electrolyte that doesn't corrode the electrodes.   

*which is why "potato power" and "lemon batteries" is nonsense. The power comes from the copper and zinc electrodes, not the vegetable between them.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile
Are you dreaming or fabulating?
Even with naked eyes anyone can see that both electrodes are consumed and no Fe deposition takes place.
Why should Fe deposit again and not Zn? Have you replicated the experiment and saw the direction of the electric current?

No, I'm explaining.
Eventually the metals dissolve, just as they would without the circuit. The connection slows down the rate of corrosion of iron
A bit like this
http://en.wikipedia.org/wiki/Galvanization
Iron redeposits because it's less reactive than zinc.
"Have you replicated the experiment and saw the direction of the electric current? "
No, but if my idea is right then I can predict the direction of the current.
The zinc will  become negatively charged and electrons will flow from it to the iron.The conventional current, measured with a meter will flow the other way- from the iron to the zinc.

If my prediction is right then it validates my model.
So, which way does the current flow?
 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
alancalverd

You missed the point. I did not discuss about the ,,power delivered by such models”. I am talking about what happen at atomic level at cathode and anode. In any case what you have written here can be very easily dismissed with the Volta battery revisited discussion ( it will be posted today). Electrolyte may seem to have a more then crucial importance …

Bored chemist

so the iron dissolve (like alcohol in the water or how?)
If a metal dissolves because reacts with acid it means both metals are oxidized and both metals go into solution as cations. It means on the metallic part electrons remains both at Fe and Zn electrodes. As Coulomb law is still valid, these electrons should repel each other… and they do not have any place to go …
In these conditions, to say that an electric current (charge displacement) goes from Zn to Fe or opposite is completely outrageous …
Supposing this happen, what is happen with electrons which travel from Fe to Zn ? or in case of opposite circulation?
I will post today to new experiment one with Volta battery and another with Isotopes and electrode potential and anyone can see the situation seems more simple than we imagine … but more complex to be explained…
Reagrding the effective electric current direction … do the experiment … and you ll see… ;)

 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile
If you put a piece of iron in acid it reacts and dissolves.
The electrons are transferred from the metal to hydrogen ions. You get H2 gas and a solution of the metal salt.
That reaction is quite fast.
If the conductivity of the solution is low then the "battery" won't deliver much current.
It won't be able to remove the electrons from the iron fast enough to stop then reacting with hydrogen ions and producing hydrogen gas.

I'm not going to bother doing the experiment, but I suspect that you would have gleefully told me if I had been wrong.
In any event, it may depend on the details of the experiment.

Alan's comment "The first trick of battery design is to find an electrolyte that doesn't corrode the electrodes. " is a significant part of the answer here.

 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1879
  • Thanked: 145 times
    • View Profile
Sorin, it's not just electrons that are moving. The electrons are not repelled because there is no significant buildup of charge.Ions flow through the solution to balance the charge. If you just connect the electrodes to each other in the air there won't be any sustained current. But as soon as you put them in an ionic medium--the circuit is completed.
 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1879
  • Thanked: 145 times
    • View Profile
Also, the electrons do have to go somewhere. If there is oxidation, there must be reduction (or a cathode ray...). You cannot run a battery with only oxidation half reactions.
 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
Of course there is a double oxidation and a double reduction in system ...
The problem is it cannot be interpreted in ,,actual science”...
To be more clear, because I see the discution is going toward fabulation...

Both electrodes are consumed, hidrogen develops at both electrodes...
So there is a ,,simple guess” with no solution for actual chemsitry:

Fe = Fe 2+ plus 2e which remains on the metalic electrode

Zn = Zn2+ plus 2e which remains on the metalic electrod.

According to actual chemistry electrones from Fe must travel at Zn electrode to develop hydrogen and electrons from Zn must travel to Fe electrode to generate hydrogen. How these electrons travel in opposite directions?
No answer...


So ... there remains the posibility that electrons from Fe react at the Fe electrode with hidrogen cation and form molecular hydrogen and electrons from Zn react at surface of Zn electrode and form molecular hydrogen ...
But in this case no charge is going into external circuit so no electric current ...
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile
"The problem is it cannot be interpreted in ,,actual science”..."
Wrong, science can do a perfectly good job of explaining it.
Unfortunately, you can't understand the science.


"Both electrodes are consumed, hydrogen develops at both electrodes..."
Which is exactly what you would expect if there was no wire connecting them.
It is also what you would expect if there were a very high resistance in series with the wire.
Do you agree so far?

 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1879
  • Thanked: 145 times
    • View Profile
H+ from the acid will react with either electrode with no wires needed:

Fe + 2H+ --> Fe2+ + H2

Zn + 2H+ --> Zn2+ + H2

The fact that you measure current in the wire between the electrodes suggests that there is another reaction going on. This is not unusual in chemistry--in addition to the desired reaction, there are often dozens or hundreds of different chemical reactions ongoing in any flask, but most are either reversible and in equilibrium, or negligible.

It is quite possible (in fact I would bet on it) that Fe2+ is getting reduced on the Fe electrode, using electrons coming from the oxidation of the Zn electrode. Using standard reduction potentials, it is apparent that this is a very favorable reaction, so if given the opportunity (Fe and Zn electrodes in same solution and connected by wire) this reaction will happen--even if there is another reaction (or 10,000) going on simultaneously in the same apparatus.
« Last Edit: 24/10/2013 19:15:36 by chiralSPO »
 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile

"Both electrodes are consumed, hydrogen develops at both electrodes..."
Which is exactly what you would expect if there was no wire connecting them.
It is also what you would expect if there were a very high resistance in series with the wire.
Do you agree so far?

With or without a resistance between electrodes the hidrogen evolves at both electrodes because there are two independent reactions.
I am not here to discuss how can we extract the maximum power from this system, this is completely other discussion and bringing in discusion resistance, impedance and so on ... it is not useful for the topic.
The topic is the origin of the electric current in shortcircuit if you want; insted of getting an explanation, you are interested on my level of science...

H+ from the acid will react with either electrode with no wires needed:

Fe + 2H+ --> Fe2+ + H2

Zn + 2H+ --> Zn2+ + H2

The fact that you measure current in the wire between the electrodes suggests that there is another reaction going on. This is not unusual in chemistry--in addition to the desired reaction, there are often dozens or hundreds of different chemical reactions ongoing in any flask, but most are either reversible and in equilibrium, or negligible.

It is quite possible (in fact I would bet on it) that Fe2+ is getting reduced on the Fe electrode, using electrons coming from the oxidation of the Zn electrode. Using standard reduction potentials, it is apparent that this is a very favorable reaction, so if given the opportunity (Fe and Zn electrodes in same solution and connected by wire) this reaction will happen--even if there is another reaction (or 10,000) going on simultaneously in the same apparatus.

As I told upstairs the experiment can be made at home in order to show the simplicity of the experiment. But I made the experiement many times using reagent grade chemicals ... and please stop telling stories...
We are not in the Lavoisier time ....
If I can measure 1 part per trilion species in a solution ... you think I could believe that another reaction or other reactions take place ?

On the other hand your circulation of electrons can be ruled out simple...
If electrons from Zn travels toward Fe, no Hidrogen evolves at Zn electrode.
So look at the movie attached... I made it few days ago and a current of about 80 mA is measuread.

I cannot attach files greater then 200 kb.. so ask for a derogation and I will post the movie here...
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile
Here
"I am not here to discuss how can we extract the maximum power from this system, this is completely other discussion"
You make a mistake about why I'm asking about the resistance.
which is why you are wrong about this
"bringing in discusion resistance, impedance and so on ... it is not useful for the topic."

I take it you agree that the reaction will take place anyway.
If, on the other hand, the cell worked as you might expect and was short circuited , all the hydrogen would be evolved at the iron electrode (and the zinc would just dissolve)

If you increased the resistance of the wire then the system would be more like the case with no wire- the hydrogen would come off at both electrodes.
Do you agree so far?
In an idealised system if the cell was short circuited you would only get hydrogen from the iron electrode but with a high resistance you would get hydrogen at both electrodes because both would dissolve.
Please don't argue about my asking that question- just answer it yes or no.
(By the way, you might want to learn how to spell hydrogen correctly)

 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
Bored chemist, I presented only an experiment ... and even the cost of replication is less then a pound, you try to invent all sort of strange explanations.

The hidrogen come off at electrodes with or without an resistance between them, with a low resistance or with a high resistance.
This does not mean the speed of reaction is not an a certain way correlated with the resistance in the circuit. But this is another topic ...and as far I have more interesting things to do ... other people can do  this research ..
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile
Re.
"This does not mean the speed of reaction is not an a certain way correlated with the resistance in the circuit."
I have news for you.
Yes it is, and real scientists have known this for a long time.


http://en.wikipedia.org/wiki/Faraday's_laws_of_electrolysis

That's the point I was making, so perhaps you can stop behaving like some sort of prima donna and answer my question.
 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
I will post in the afternoon a new experiment and I will ask you to explain me the experiment and how can I apply the laws of Faraday ...
Maybe I am already outdated ...
The post it will be in physics section because it is related more to electric current definition...
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile
"I will post in the afternoon a new experiment"
Possibly, but in accordance with the rules of the forum, you should answer the question.
 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
Bored chemist,
as I pointed out I agree that speed of chemical reactions is correlated with some factors and I will outline for our topic.
At beginning, with fresh acid there is a bigger speed for both reactions (Zn and Fe with acid). As far the acid get exhaused, the ionic strength of solution increase, it is normal that those reactions change their speeds and this can be observed in the current powered into external circuit.
I do not see the relevance of Faradays laws ...
As I pointed out in the cut off experiment, the Faraday laws are respected when a electric current generate chemical reaction in a circuit . ...
As far a current can pass through a solution without chemical effects, there will be necessary at least to adjust the faradays laws to this reality ....

 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile

"I do not see the relevance of Faradays laws"
Really, You don't see how the laws that link current flow to rates of chemical reaction are important here?
This could be slow going.
Anyway,  in accordance with the rules of the forum, you should answer the question.
Then we might make progress when I explain something to you .
 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
I do not perform an electrolysis process ... so there is no relevance to claim the Faraday s laws...
This is what I was pointing...
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8670
  • Thanked: 42 times
    • View Profile
There is none so blind as him who will not see.
 

Offline sorin cezar

  • Full Member
  • ***
  • Posts: 56
    • View Profile
I am sorry fot those who cannot see...
There is none so blind as those  who don't want to see.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums