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Physical details of the operation of galvanic cells

  1. May 21, 2017 #1
    I'm trying to understand the physical details of the mechanism, by which galvanic cells work, instead of more abstract descriptions of the half reactions that take place and I find it hard to piece together concrete information on this. Below is a description of my basic understanding of the process and I'd be grateful if anyonce could comment on it, either confirming the validity of my assumptions, correcting them, or filling in the blanks, as necessary. I understand that my description is simplified, restricted to the most important aspects of the process and that other details, such as the interaction of H+ and OH- ions from the dissociation of water, with the electrodes and electrolyte have been ignored, but, as I've said, I'm trying to get a grasp of the basics. Hopefully, if what follows is not entirely off the track, it'll also help others with similar questions. So, here goes:

    Let us consider an electrode made of Zn, suspended in a solution of ZnSO4. The Zn atoms in the electrode tend to break away from the surface, leaving electrons behind and becoming suspended in the solution as ions. This happens presumably due to their own thermal kinetic energy, facilitated by molecules from the electrolyte attracting them, or bumping into them and knocking them free and depends therefore, on the ionization energy of the metal and the temperature. This process then leaves an excess of free electrons in the cathode, charging it negatively, which in turn attracts the suspended Zn ions back onto it. Since the electrolyte has ZnSO4 molecules dissolved in it, which dissosiate into Zn++ and SO4-- ions, they too interact, either capturing Zn++ ions which ejected from the surface, or providing Zn++ ions to be attracted to and deposited onto the surface. In any case, a dynamic equilibrium develops, with an excess of Zn++ ions in the electrolyte and an excess of e- in the cathode, which depends mostly on the temperature I suspect, but perhaps also on the electrolyte conecentration, and the process stops there.

    In the above, I'm unsure about what determines whether the Zn ions will be Zn+, or Zn++ ions, but it seems that the Zn in the electrode, will more readily ionize to Zn++, which, I suspect, has to do with its electron configuration, and/or the details of the metallic bond between Zn ions. I'd be greatly interested in any more detail on this.

    Now the same process develops with a Cu electrode immersed in a CuSO4 solution, but here, since Cu has a higher sum of first and second ionization energies, than Zn, the process will achieve equilibrium with fewer Cu++ ions suspended in the electrolyte and fewer excess e- in the electrode, hence a smaller negative charge (assuming the same temperature as before). If we then proceed to connect these two electrodes electrically, via a wire, electrons will flow from the more negatively charged Zn electrode to the less negatively charged Cu electrode until the charge is equalized, shifting the equilibrium at both electrodes. In the case of the Zn electrode, the excess of free e- will have been reduced, which will reduce the rate at which Zn++ ions from the solution deposit onto it and allow more Zn++ ions to escape. Conversly, at the Cu electrode, more excess e- will arrive, attracting more suspended Cu++ ions out of the solution. This will lead to a new equilibrium being established, with equal charge at both electrodes, somewhere inbetween their former charges, and the process will stop once more. It should be noted, that the shift is such at both sites, that the relative difference in excess metal ions suspended at each electrolytes has increased, further increasing their positive charge difference.

    If we now connect the two electrolytes in such a way, that ions can migrate between them, then the negative SO4-- ions in the less positively charged Cu half-cell are attracted to the more positvely charged Zn half-cell. This creates an excess of SO4-- ions in the Zn half-cell's electrolyte, binding more freely suspended Zn++ ions and allowing yet more to escape. Similarly, positve Zn++ ions will be attracted to the less positive Cu half-cell, where they will, presumably, either bind with the SO4-- ions floating around, thus forming ZnSO4, or be plated onto the negatively charged Cu electrode. In any case, the additional Zn++ ions escaping the Zn electrode and leaving more excess e- behind, coupled with the Zn++ ions, either depositing onto the Cu electrode, or binding SO4-- ions around it and causing Cu++ to deposit onto it, in any case, reducing excess e- there, will once again create charge imbalance and a flow of e- from the former to the latter. Thus the reaction will be sustained, until the Zn electrode has fully dissolved.

    I'm sorry in advance for the verbosity of the above, but, as I've said, it's the details of the physical mechanism, that drives the whole process, that I'm unclear about, so I wanted to make these details explicit. Any comments are welcome.
     
  2. jcsd
  3. May 21, 2017 #2

    Borek

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    Staff: Mentor

    Long and a bit wordy, chances are I missed something, so don't treat these comments as final.

    In general you are right about the first phase of the process (electrodes becoming charged when ions get into the solution), but you are wrong on several accounts later.

    Equilibrium - yes, but it is not about charge, but about potential being equal.

    Not sure if "binding" makes sense to me here - migration of SO42- just makes the solution electrically neutral, there is no "binding" between these ions. (Well, they can create ion-pairs, but that's way above the level we are talking about for now).

    No binging as said above, no way Zn is going to be plated on the Cu surface (reactivity series, which is more or less the driving force behind the way battery works).

    Zn2+ getting into the copper electrode department is not something you want in the battery, assume salt bridge to contain some inert cation that is being transferred.
     
  4. May 22, 2017 #3
    Thanks for taking the time to read through and comment on my post. I know it was long, but I was interested in discussing the details of how the process is initiated and sustained. Most textbook accounts (that I've come across at least), simply assert that "electrons flow through the wire" and "ions migrate through the salt bridge", etc. without really giving any details on the forces that cause them to do so. This doesn't promote a real understanding of the process.

    Regarding your comments:

    Yes, you're right, of course; I didn't think that one through properly.

    The word "binding" is probably incorrect; perhaps I should have said "combining". My understanding is that the ZnSO4 in the electrolyte, will dissociate according to the reversible reaction

    ZnSO4 ⇌ Zn2+ + SO42-

    and that this means that ZnSO4 molecules will be broken apart into the ions and they will recombine back, eventually at constant and equal rates, so that an equilibrium is reached. Then as more SO42- ions migrate into the electrolyte, there will be more of them to combine with the Zn2+ ions, shifting the equilibrium towards the formation of the reactant. This will in turn rob the other reversible reaction

    Zn(s) ⇌ Zn2+(aq) + 2e-,

    which takes place at the electrode, of Zn2+ ions, shifting the equilibrium towards products.

    Is this view incorrect?

    Ah. Should I take that to mean that plating of Cu on Zn is not going to be favored, in the sense that it won't be happening at any significant rate, compared to Zn being deposited back onto the electrode, or is there some physical reason preventing the Cu from plating onto the Zn electrode?
     
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