Junk_Enginerd
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How is an electrolyte different from a conductor?
I can't quite wrap my head around this. In an electrogalvanic cell, you use some type of salt bridge to ensure charge neutrality. But why can't it be
a conductive wire? That also allows charge to move between the cells.
I also understand that for example when using an Agar salt bridge, ions cannot move between the cells. This sounds even more like you could just use a
wire instead? I know you can't, but why?
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B(a)P
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A salt bridge moves ions to maintain neutrality, which a wire cannot do.
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Junk_Enginerd
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I'm interested in this not so much because of galvanic cells, but for separated electrolysis. There are many examples of where you don't want the
oxidized and the reduced parts to come in contact.
When you say that "ions" move across the bridge, what exactly does this mean? Is it actual physical ions, or more like "virtual" ions in that it's a
"wave" of ionization that moves across the salt bridge? Because surely things like an Fe ion can't "jump" across a salt bridge to plate out in the
other container in which for example there's only NaOH or some other electrolyte not containing Fe?
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DraconicAcid
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The salt bridge is full of ions. If you've got one that packed with, say, sodium chloride, it's full of sodium ions and chloride ions.
In the cathodic half-cell, you have reduction going on. This is either producing anions, or taking away cations. Either way, you need to add more
positive charge to that half-cell, so sodium cations will flow out of the salt bridge and into the solution.
In the anodic half-cell, you have oxidation- either you're neutralizing anions, or you're generating cations. Either way, we need more anions in
solution, so chloride anions flow out of the salt bridge and into the solution.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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unionised
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If you use a wire then you have two cells, not one.
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Junk_Enginerd
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So...
Let's say we have container C1, bridge B and container C2.
C1 contains ZnSO4, B is Agar+KNO3, C2 is FeSO4.
Then we add a power supply and place the positive lead in C1 and the negative lead in C2.
Am I eventually going to plate out zinc on the electrode in container C2? Will C1 be entirely devoid of metal ions? And will the salt bridge have lost
all the KNO3? All this despite the Agar preventing any actual flow from happening?
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DraconicAcid
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If you put the cathode in C1, then zinc will plate out, and potassium ions will diffuse from the salt bridge into C1 so that the zinc sulphate is
slowly replaced by potassium sulphate. The ions will flow, but the solution itself won't because of the agar.
At the anode in C2, iron(II) gets oxidized to iron(III), and nitration ions flow into that solution, so you get a mixture of iron(III) sulphate and
iron(III) nitrate.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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macckone
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Electrolytes as others have described move ions.
BUT, and this is a major BUT, they will also move electrons.
This is why electrolytic cells and batteries are never 100% efficient.
Some current will flow as a conductor through the electrolyte.
The amount that is wasted depends on voltage, current and ion mobility.
The resistance of such a cell is non-linear which leads to interesting effects.
Below a certain voltage threshold, ion movement is minimal.
Then you get a range where ion movement increases rapidly.
Finally you get a range where the current increases mostly due to electrical resistance of the solution.
Maximum cell efficiency is always at the cusp between minimal ion movement and the increasing ion movement. And above the second cusp is always
wasting electricity (unless needed to keep the cell hot).
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clearly_not_atara
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In a metallic conductor, electrons form a Fermi gas, a quantum mechanical phenomenon with no analogue in classical mechanics, and they follow
the Fermi-Dirac law for the energy distribution, from which we can derive the approximate values for the heat capacity and electrical conductivity of
a metal. In a liquid electrolyte, the ions behave as essentially classical particles subject to Maxwell-Boltzmann statistics. In other words, the
macroscopic properties of electrolytes can be estimated based on classical models, while metals are intrinsically quantum mechanical.
So they really are completely different things, despite some apparent similarities.
[Edited on 04-20-1969 by clearly_not_atara]
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woelen
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There is a very simple difference. In a wire, it is only electrons, which can move around to keep charge balanced at both ends. In a salt bridge, it
is ions which can move around to keep charge balanced.
At the interface of the liquid in the cell the difference is made. Electrons cannot enter into the liquid, the only thing which is possible is a
chemical reaction (something from which electrons are extracted, or something on which electrons are pushed). With the ions in a salt bridge, there is
no such barrier.
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