EXTREMELY CONFUSED- electrochemical cells

Watch
HelloThere191817
Badges: 12
Rep:
?
#1
Report Thread starter 1 year ago
#1
I don’t understand the purpose of a salt bridge. It says it completes the circuit. How? Do electrons flow through it? Why are they necessary to complete the circuit if all the electrons are released at the anode and accepted at the cathode? Why can’t you mix the solutions? I’m so confused here I can’t move on. I’ve looked at everything and tsr is my last hope
0
reply
username3729202
Badges: 13
Rep:
?
#2
Report 1 year ago
#2
https://chemistry.stackexchange.com/...a-wire-be-used
2
reply
eznot
Badges: 12
Rep:
?
#3
Report 1 year ago
#3
TBH it's very hard to get your head around, all you will ever be asked is what is the function of the salt bridge you just need to remember this bog standard response: "To complete the circuit by allowing the movement of ions" most of the time so long as you say ions NOT electrons you will get the mark.

Don't stress over it!
(Original post by Anna191817)
I don’t understand the purpose of a salt bridge. It says it completes the circuit. How? Do electrons flow through it? Why are they necessary to complete the circuit if all the electrons are released at the anode and accepted at the cathode? Why can’t you mix the solutions? I’m so confused here I can’t move on. I’ve looked at everything and tsr is my last hope
0
reply
David Tan
Badges: 7
Rep:
?
#4
Report 1 year ago
#4
There are 2 purposes of a salt bridge. One of them is obvious and it is to get a closed circuit. The next one is less obvious and it is to maintain charge neutrality.

The salt bridge is typically made up of potassium nitrate because all potassium salts are soluble and all nitrate salts are soluble. I will use a simple analogy to explain what charge neutrality means. Let's imagine that an electrochemical cell comprises two half cells and each half cell has its designated electrolyte which comprises cations and anions + cations go to cathode & anions go to anode. For the reduction half cell (cathode), cations will be discharged and as a result, there will be an accumulation of anions in the reduction half cell. However, reduction half cell is the place where electrons are accepted and yet we have negatively charged anions accumulating in the electrolyte. What does it entail? It means after a while, there is repulsion between anions and electrons and the whole electrochemical cell would stop working. To prevent this from happening, the K(+) ions will "neutralise" the excess negative charge that is accumulating at the electrolyte of the reduction half cell, hence the term "charge neutrality". Since all K(+) salts are soluble, we do not have to worry about the formation of any precipitate which would disrupt the entire current flow. Current flows because there are mobile charge carriers (either electrons or ions). For the oxidation half cell (anode), anions will be discharged and after a while, there will be an accumulation of cations in the oxidation half cell. Oxidation half cell is the place where electrons are lost and yet we have positively charged cations accumulating in the electrolyte. To prevent this from happening, the nitrate ion will "neutralise" the excess positive charge that is accumulating at the electrolyte of the oxidation half cell.

Hope it helps.
(Original post by Anna191817)
I don’t understand the purpose of a salt briThere dge. It says it completes the circuit. How? Do electrons flow through it? Why are they necessary to complete the circuit if all the electrons are released at the anode and accepted at the cathode? Why can’t you mix the solutions? I’m so confused here I can’t move on. I’ve looked at everything and tsr is my last hope
1
reply
HelloThere191817
Badges: 12
Rep:
?
#5
Report Thread starter 1 year ago
#5
I totally understand the bit about charge neutrality 😂!! What I don’t understand is the simple one- how it completes the circuit. Charge neutrality aside, as far as I can tell, the circuit is already complete- electrons are given off by the electrode and accepted by the ions in the other half cell. Why does charge need to flow between the two solutions? Also- when using a high resistance voltmeter- apparently no current flows at all and the reaction does not occur. Is this correct? That is, in order to measure the potential difference between the two half cells?
0
reply
Adro789
Badges: 13
Rep:
?
#6
Report 1 year ago
#6
(Original post by Anna191817)
I totally understand the bit about charge neutrality 😂!! What I don’t understand is the simple one- how it completes the circuit. Charge neutrality aside, as far as I can tell, the circuit is already complete- electrons are given off by the electrode and accepted by the ions in the other half cell. Why does charge need to flow between the two solutions? Also- when using a high resistance voltmeter- apparently no current flows at all and the reaction does not occur. Is this correct? That is, in order to measure the potential difference between the two half cells?
If you have electrons flowing from one electrode to another (- to +) without the salt bridge the circuit is not complete because electrical neutrality would not be maintained and the reaction would stop...

Without the salt bridge the solution in the anode compartment would become positively charged and the solution in the cathode compartment would become negatively charged, because of the charge imbalance, the electrode reaction would come to a halt.
So, by enabling the flow of ions (e.g. K+ and NO3-) between the two solutions the salt bridge enables the flow of e- to continue and thus completes the cirucit.
1
reply
HelloThere191817
Badges: 12
Rep:
?
#7
Report Thread starter 1 year ago
#7
Thank you SO much! Our teacher skipped over it in class and I’ve been trying to understand it for the past two weeks!!
(Original post by Adro789)
If you have electrons flowing from one electrode to another (- to +) without the salt bridge the circuit is not complete because electrical neutrality would not be maintained and the reaction would stop...

Without the salt bridge the solution in the anode compartment would become positively charged and the solution in the cathode compartment would become negatively charged, because of the charge imbalance, the electrode reaction would come to a halt.
So, by enabling the flow of ions (e.g. K+ and NO3-) between the two solutions the salt bridge enables the flow of e- to continue and thus completes the cirucit.
0
reply
Adro789
Badges: 13
Rep:
?
#8
Report 1 year ago
#8
No worries! Took me a while to get my head around it when I did it last year as well.
(Original post by Anna191817)
Thank you SO much! Our teacher skipped over it in class and I’ve been trying to understand it for the past two weeks!!
0
reply
David Tan
Badges: 7
Rep:
?
#9
Report 1 year ago
#9
The electrons are traveling in the external circuit. The ions are traveling in the "internal circuit" e.g. from one electrolyte to another via the salt bridge. For current to flow, we need to have mobile charge carriers and the charge carriers can be either electrons or ions. By maintaining charge neutrality, the circuit remains a closed one. Hope it helps.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Should there be a new university admissions system that ditches predicted grades?

No, I think predicted grades should still be used to make offers (505)
33.64%
Yes, I like the idea of applying to uni after I received my grades (PQA) (622)
41.44%
Yes, I like the idea of receiving offers only after I receive my grades (PQO) (306)
20.39%
I think there is a better option than the ones suggested (let us know in the thread!) (68)
4.53%

Watched Threads

View All