On Electrochemical Cells and Salt Bridges

Question

We're studying the topic Electrochemistry at school now, and I've got a few doubts regarding electrochemical cells and salt bridges.

What I am going to do is listing out (point-wise) what I understood from class, and then pose my questions (in bold). The logic behind listing out what I understood about the topic beforehand? I've a very strong feeling I'm harboring some serious misconceptions regarding the topic, so if I list out what I do know, then hopefully someone can alert me to an incorrect point, and also correct it.

• LHC - Left half cell (contains the anode, which is negatively charged in electrochemical cells)
• RHC - Right half cell (Contains the cathode, which is positively charged in electrochemical cells)

During the course of the reaction,

1. In the LHC, metal atoms of the electrode release their electrons to the anode and enter the solution as positive ions.

2. The positive metal ions, under the influence of the negatively charged anode, accumulate around the anode, resulting in a Solid-Liquid Junction. The Solid-Liquid Junction is also known as the Helmholtz Double Layer.

3. Similarly in the RHC, negatively charged ions (formed from the electrolyte solution) accumulate around the cathode, resulting in another Solid-Liquid Junction.

4. It is the potential difference between the LHC and RHC Solid-Liquid Junctions potentials that gives rise to the emf which results in the flow of electrons (electric current) from the LHC to the RHC via the metal wire that connects their corresponding electrodes.

5. However, as the reaction proceeds, the quantity of positive and negative ions builds up (in the LHC and RHC respectively). So much, that these ions under their mutual attraction, override the force of attraction that binds them to the electrodes and accumulate at the porous asbestos partition that separates the two half cells. This is called the Liquid-Liquid Junction. The formation of the Liquid-Liquid Junction comes at the cost of the Solid-Liquid Junctions, which disintegrate.

   

6. As the Liquid-Liquid Junction potential doesn't involve the electrodes, it cannot set up a potential difference across them, therefore no current flows along the wire. This is why an electrochemical cell stops generating current after some time has elapsed.

7. This problem can be overcome by use of a salt bridge, which in effect, neutralizes the net charges on either side of the asbestos partition, thus releasing the ions back into the solution. These ions are once again attracted by the electrodes, and the two Solid-Liquid Junctions are re-established. So current once again begins flowing through the wire.

   

I'd appreciate it if anyone could proof-read those points. Also I have doubts whether the Solid-Liquid Junction can also be called the Helmholtz Double Layer. I made these points in class, and I let my teacher go through them, and he's in agreement with them, so I don't think this is a case of talking down the points wrong, at least not with respect to what was mentioned in class.

Now I asked my teacher, Q1: Screw the partition. Why not simply have the two half cells in separate compartments kept at a distance from each other, instead of using a 'partial' barrier like the porous asbestos partition? Wouldn't that eliminate the possibility of a Liquid-Liquid Junction being formed?

His answer: Well they're ions buddy, so they'll attract each other anyway and accumulate at the sides of the compartment, after all....they're charged. So even though that wouldn't result in a Liquid-Liquid Junction, there still wouldn't be any potential difference across the electrodes, so nil current. Plus, if you separate the compartments like that, it'll just take up space; compact electrochemical cells are more convenient to use.

Q2: Is my teacher entirely right here?

And yet again, Q3: Are all the points I've mentioned correct?

Answer 1

You have it all backwards. Liquid-liquid junction does not exist at all in your supposed meaning. Solid-liquid junction does, but it is very much not what you think it is, to the point that you might just as well assume it does not exist either.

The complete story would hardly fit in an answer (and, moreover, it is explained in any decent textbook on the subject), so I'll focus on the details that are especially wrong. First of all, the double electric layer, being electrically charged, creates an electric field of its own, and thus shields the rest of the solution from the electrodes and their field. Second, the electric forces are really, really strong (like a thousand times stronger than you seem to think, and even stronger than that), and therefore the said layer at any point contains but a very tiny portion of metal ions. That portion suffices for creating an electric field which would completely negate the field of electrodes. The vast majority of metal ions float freely inside the solution, having no idea at all that they are being electrolyzed.

As the ions build up, the shielding becomes ever more perfect. Dissolve some $\ce{NaCl}$ in a glass of water without any electrodes or anything; the ions would float freely in the solution. Do they feel mutual attraction? Well, in a way, they do. Do they assemble into strange walls across the glass? I've never seen anything like that. Now, the ions in you cell (the majority that does not participate in the double layer on either side) feel exactly the same way as ions in that glass.

Electrochemical cell, if set up properly, will give current until one of the electrodes is completely eaten up. The need for a salt bridge has absolutely nothing to do with electric effects. Rather, it serves to prevent the mixing of solutions, which are presumably different and better left that way, for fear that otherwise they might react with the electrodes directly.

As for the cell with separate spaces and no bridge, it will stop working right away, much like the light bulb connected with a single wire to one pole of a battery.