Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Schrödinger on November 07, 2009, 01:44:45 AM
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Hey guys. I am in desperate need of an explanation regarding the salt bridge. Please do not post links of other posts related to the topic. I want someone to clearly explain this to me.
Why is a salt bridge used? - Age old question, yet I need a clear cut explanation.
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To provide a path for movement of ions to complete the circuit, the ions compensate for the increasing positive and negative charges in the anode and cathode.
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Okay, will be asking a series of questions... please bear with me.
Let me tell you what i know.
Since Zn becomes Zn2+, this beaker has excess positive charge in the form of Zn2+ ions.
Since Cu2+ from solution becomes Cu, this beaker has an excess negative charge in the form of SO4 2- ions.
To neutralize this charge, a salt bridge is used.
Is this right?
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I guess you could say that it neutralises the charges. But not having a salt bridge is like having a gap between your wires in a circuit, of course the light bulb will not light.
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Now, what does a salt bridge actually do to the solutions/ions in the beakers? i.e, How does it neutralize those charges? Is there any chemical change involved?
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No chemical change (you mean like precipitation or something?), if we have a KNO3 salt bridge, NO3- will move into the Zn2+/Zn half cell and K+ ions into the other one.
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So, you mean to say that the ions of the electrolyte inside the salt bridge will align themselves/move into the respective beakers so as to neutralize the opposite charges?
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YES!
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What happens to the ions of the ZnSO4 and CuSO4 solutions? Do they move into the salt bridge as well, or is the movement a one-way process?
i.e., only from the salt bridge and not into the salt bridge
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i.e., only from the salt bridge and not into the salt bridge
Haha, well there is no way that you can just make this one way flow of ions happen so I presume that it is a two way thing.
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As a result of this 2-way movement, will we be able to notice any ZnCl2 well within the salt bridge and K2SO4 well within the salt bridge, presuming the electrolyte in the salt bridge to be KCl?
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Are we talking about an inverted U-tube here? The ions are all there in solution, isolated ones like as Zn2+ or Cl- and SO42-, so yes, you will find Zn2+ and Cl- and SO42- and K+ ions in the salt bridge. By the way, I'm trying to eat baked beans and toast while typing this and it's a bit messy so if my thoughts are a bit messy, apologies.
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Now that we are OK with the neutralization part, here is another question:
Although there is some neutralization of the charges due to the ions of the salt bridge, will these ions not dissociate and move towards the oppositely charged electrode?
i.e., why should the neutralized molecules remain neutral?
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Sorry to interrupt.
See this:
http://en.wikipedia.org/wiki/Daniell_cell
"In such a cell, the sulphate ions play an important role. Having a negative charge, these anions build up around the anode to maintain a neutral charge. Conversely, at the cathode the zinc (II) cations accumulate to maintain this neutral charge."
Where does the role of salt bridge ions come here.
What I thought of this line in Wikipedia is that the Zn2+ ions move to the Cu electrode and neutralize the SO42- ions and the SO42- ions move to the Zn electrode and neutralize the Zn2+ ions.
Is this right?
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Although there is some neutralization of the charges due to the ions of the salt bridge, will these ions not dissociate and move towards the oppositely charged electrode?
i.e., why should the neutralized molecules remain neutral?
Err.... what? I'm not sure I understand what you are asking
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Where does the role of salt bridge ions come here.
What I thought of this line in Wikipedia is that the Zn2+ ions move to the Cu electrode and neutralize the SO42- ions and the SO42- ions move to the Zn electrode and neutralize the Zn2+ ions.
Is this right?
Yes, well initially, you would still need the salt bridge to complete the circuit or else the reaction won't even get started, after a while, I suppose the different ions can move all over the place and keep the half-cells neutral. That's all I've got I'm afraid :-\
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Since the Zn rod is negatively charged with respect to the solution, will there not be a layer of Zn2+ around this rod due to opposite charge attraction?
This is quite an absurd question.
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Since the Zn rod is negatively charged with respect to the solution, will there not be a layer of Zn2+ around this rod due to opposite charge attraction?
This is quite an absurd question.
Huh? Sorry mate, you're just confusing me even more about what you are asking ???
Of course there will be Zn2+ ions around the rod because they are constantly being produced by the oxidation of zinc
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It's alright. Let's drop the question.
About your reply to arjunarul's post....
So, you mean we can find some ZnSO4 in CuSO4 beaker?
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Since the Zn rod is negatively charged with respect to the solution, will there not be a layer of Zn2+ around this rod due to opposite charge attraction?
This is quite an absurd question.
Huh? Sorry mate, you're just confusing me even more about what you are asking ???
Of course there will be Zn2+ ions around the rod because they are constantly being produced by the oxidation of zinc
So, do we use the salt bridge to prevent this build up of Zn2+ layer around the Zn rod?
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So, you mean we can find some ZnSO4 in CuSO4 beaker?
Well Wikipedia basically said it. Zn2+ ions by the way, sulfate ions were already there to start off with.
So, do we use the salt bridge to prevent this build up of Zn2+ layer around the Zn rod?
Yes, the nitrate ions takes care of this
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Got it!
Thanx mate...I'm giving you a mole snack... :)
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Thanks man ;D
Good luck!
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I'll tell what I understood now. Please tell if it is right.
Both the processes take place:
1. K+ and Cl- ions go to the opposite ends of the salt bridge ( K+ near the oxidative part and Cl- near the reductive part).
So, the Zn2+ ions come near the salt bridge and so, it does not accumulate near the electrode, thus allowing free electron flow. Similarly for the the other part.
2. The Zn 2+ ions move to the reductive half and the SO42- ions go to the oxidative part. This neutralizes the excess charge.
Some Zn2+ ions undergo first process while others undergo second process.
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I'll tell what I understood now. Please tell if it is right.
Both the processes take place:
1. K+ and Cl- ions go to the opposite ends of the salt bridge ( K+ near the oxidative part and Cl- near the reductive part). The other way round, K+ moves to the cathode
So, the Zn2+ ions come near the salt bridge and so, it does not accumulate near the electrode, thus allowing free electron flow. Similarly for the the other part. Yes
2. The Zn 2+ ions move to the reductive half and the SO42- ions go to the oxidative part. This neutralizes the excess charge. I am not sure about SO42-, the concentration should be 1 mol L-1 in each half-cell so there shouldn't be any net movement
Some Zn2+ ions undergo first process while others undergo second process.
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1. K+ and Cl- ions go to the opposite ends of the salt bridge ( K+ near the oxidative part and Cl- near the reductive part). The other way round, K+ moves to the cathode
Sorry, it was silly of me.
2. The Zn 2+ ions move to the reductive half and the SO42- ions go to the oxidative part. This neutralizes the excess charge. I am not sure about SO42-, the concentration should be 1 mol L-1 in each half-cell so there shouldn't be any net movement
Ok. But it was given like that in Wikipedia right??
So, according to you, it's something like diffusion of ions due to difference in concentrations right? So, if we use ZnCl2 and CuSO4, concentration of SO4 is zero in the other half. So, will they start diffusing through the salt bridge till they become evenly distributed???
I guess that I am terribly wrong somewhere.