In Zemansky and Dittman's book on thermodynamics, pages 41–43, the authors discuss the Daniell cell as a thermodynamic system.

Their cell consists of two saturated $\ce{CuSO4}$ and $\ce{ZnSO4}$ solutions separated by a porous wall, with a copper strip dipped in the first and a zinc in the second, and with salt crystals in the bottom of each solution.

There are two problems:

The first is that the authors say that the cell is rechargeable, whereas this Wikipedia page says it not, so who is right?

The second is that the book states that in discharging mode:

If the electrolytes are saturated solutions, a current driven by the chemical reaction at constant temperature and pressure will not alter the concentrations of the electrolytes.

Well I can't understand how this happens. I can imagine that the $\ce{ZnSO4}$ solution will be over-saturated and the extra amount of it will deposit on the crystals, but how can the other solution keep a constant concentration?

Any help is appreciated.

  • 1
    $\begingroup$ Why would it change? It is a saturated solution. If some CuSO4 vanishes, more will dissolve and keep it saturated. $\endgroup$ – Ivan Neretin Oct 23 '16 at 16:01
  • $\begingroup$ @IvanNeretin so $\ce{CuSO4}$ dissolves spontaneously till it reaches saturation ? $\endgroup$ – Tofi Oct 23 '16 at 16:30
  • $\begingroup$ Of course it does, like all compounds do. $\endgroup$ – Ivan Neretin Oct 23 '16 at 17:35
  • $\begingroup$ @IvanNeretin forgive my ignorance, i clearly know nothing on this, but would the deposit dissolve even if the solution wasn't saturated at the beginning ? $\endgroup$ – Tofi Oct 23 '16 at 18:40
  • 1
    $\begingroup$ Of course it would. That's the meaning of the word "saturated". Unless the solution is saturated, more compound would dissolve. $\endgroup$ – Ivan Neretin Oct 23 '16 at 19:13

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