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In pretty much all introductory chemistry courses, the galvanic cell (i.e., battery) is introduced as a system of two half-cells with a salt bridge converting chemical energy to electrical energy. From what I understand, this is what is called a Daniell cell. For example, $\text{Zn}_{(\text{s})}|\text{Zn}^{2+}_{(\text{aq})}||\text{Cu}^{2+}_{(\text{aq})}|\text{Cu}_{(\text{s})}|$ with an $\text{NaCl}_{(\text{aq})}$ salt bridge.

However, we can design a much simpler battery using only one cell: $\text{Zn}_{(\text{s})}$ and $\text{Cu}_{(\text{s})}$ electrodes both dipped in the same $\text{H}_2\text{SO}_{4\text{(aq)}}$ solution (similar to a lemon battery). Both designs would produce the same voltage (~$1.10 \text{V}$). But since the latter design is simpler/more practical/cheaper/etc., why do most resources exclusively talk about the Daniell cell? Are there flaws in the single-celled system?

I tried looking for an answer in Why can't a galvanic cell be a single cell? and Why can't an electrochemical cell be a single cell?. They both point towards the idea that single-celled batteries would not do useful work on the external circuit, but I'm not convinced. In the single-celled design, when $\text{Zn}_{(\text{s})}$ dissolves to $\text{Zn}^{2+}_{(\text{aq})}$, it won't react with $\text{Cu}_{(\text{s})}$. Also, free electrons can't flow in an aqueous solution. Hence the only path for electrons is through the wire. Am I missing something?

scrap
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  • Read about the history of the battery: https://en.wikipedia.org/wiki/History_of_the_battery. The single reservoir of sulfuric acid works, but only on short time scales because the zinc reacts with the sulfuric acid. The Grove cell and Volta’s pile were better and the Daniell cell, of course. – Ed V Jul 24 '23 at 14:33
  • Zn + Cu in H2SO4 would not produce 1.1 V, but about 0.77 V or less, being a zinc–hydrogen cell. – Poutnik Jul 25 '23 at 01:36
  • It seems you confuse a cell like a compartment (that may contain a half-cell) versus a cell as a complete galvanic system, consisting of 2 half-cells, that may or may not be physically separated. – Poutnik Jul 25 '23 at 02:01
  • States of aggregation should not be subscripted, it is not wrong, but the recommendations (Sec. 2.1.) are different. – Martin - マーチン Aug 08 '23 at 19:51

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Yes, you're missing something. The overall reaction in a Daniel cell is: $$\ce{Zn + Cu^{2+} -> Zn^{2+} + Cu }$$ The salt bridge isolates the $\ce{Cu^{2+}}$ ions so that they do not react directly on the zinc electrode. If the $\ce{Cu^{2+}}$ ions do react directly on the zinc electrode then two counterproductive things happen. First of all no electrons flow through the external electrical circuit since the copper is reacting directly at the zinc electrode not the copper electrode. Secondly the zinc electrode itself gets coated with copper which eventually stops the reaction altogether.

Now the other concept to grasp is that in a working battery electrons flow through the external electrical circuit and ions migrate in the battery itself. Both half cells in the battery however must remain charge neutral. So for the Daniel cell cations from the salt bridge have to flow into the copper half cell and anions from the salt bridge have to flow into the zinc half cell.

MaxW
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  • Thanks, your answer justifies why we need two cells (two beakers) for a battery with two electrolytes (e.g., Zn and Cu dipped in ZnSO4 and CuSO4 respectively). But, regarding the single-celled system with an H2SO4 electrolyte, there won't be any Cu2+ ions in solution, so we won't run into the two problems you outlined. There will only be Zn2+ ions, which won't react directly on the Cu electrode. So, it seems like the single-celled system works just as fine as a Daniell cell, right? Or is there a flaw in the single-celled battery that explains why most textbooks only discuss the Daniell cell? – scrap Jul 24 '23 at 05:27
  • I can't conceptualize how a single cell for Cu/Zn would work. You can't just have solid Cu and Zn electrodes in an electrolyte. You need $\ce{Cu^{2+}}$ ions in solution which can be reduced. – MaxW Jul 24 '23 at 10:58