April 20, 2019, 10:47:14 AM
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Topic: Question about salt bridges  (Read 151 times)

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Offline JoeB

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Question about salt bridges
« on: April 17, 2019, 09:33:30 AM »
Hi,

I've come across an aspect of electrochemical cells that confuse me. From my understanding, if you have two half cells connected by a salt bridge and a wire, at the negative electrode the metal will be oxidised, for example Fe will be oxidised to Fe2+ and 2e-, and the electrons will flow through the wire to the positive half cell. To keep the charges balanced, since there are now more positively charged ions in the negative electrode, the negative ions in the salt bridge (perhaps Mn04-) will move into the negative electrode.

That's my current understanding of how this works, and applying that to this question I found (pictures of the question and mark scheme attached), I'm confused by the answer to part (eii). I thought that the Cl- ions would move into the negative electrode (the Fe2+/Fe half cell) and react with the Fe2+ ions, not go into the positive electrode and react with the Ag+ ions.

I don't know if this misunderstanding of the answer is due to some misconceptions I have, or if there's more to this than what I know, but I'd appreciate any help I could get answering this question. Thanks,

Joe
« Last Edit: April 17, 2019, 09:54:46 AM by JoeB »

Offline chenbeier

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Re: Question about salt bridges
« Reply #1 on: April 17, 2019, 10:48:09 AM »
Understanding of the salt bridge. You have two containers (half cells). Here a iron bar and a unknown iron salt (maybe sulfate or nitrate) and one with silver metal and silver nitrate. The two cells will be connected by an almost inert salt (suggestion was potassium chloride) for conductivitity.
The problem now is that silver ions can react with the chloride ions to precipitate AgCl and block the bridge. So better is to use potassium nitrate instead.

The voltage is calculate by E = E ox - E red,  E = 0,8 V -(-0,44 V) = 1,24 V

Offline JoeB

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Re: Question about salt bridges
« Reply #2 on: April 17, 2019, 11:03:10 AM »
Oh okay, that makes sense; so the problem is that the product of the reaction between Ag+ and Cl- produces a precipitate that would block the salt bridge. So would there still be a reaction between the Fe2+ ions and the Cl- ions, but it's not a problem because the product is soluble? Or is there no reaction at all between them?

Offline chenbeier

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Re: Question about salt bridges
« Reply #3 on: April 17, 2019, 11:40:11 AM »
The iron Fe2+ and Chloride is no problem. Iron Fe3+ would be a problem causing oxidising of chloride.

Offline JoeB

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Re: Question about salt bridges
« Reply #4 on: April 17, 2019, 11:55:55 AM »
Okay, thank you for your explanation! :)

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