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Assume you have a galvanic (voltaic) cell with E = +2V. If you wanted to reverse the current by inserting a 9V battery, which end of the battery should connect to the cathode of the galvanic cell?

The Kaplan answer key states that the "positive end of the battery should connect to the cathode."

Since electrons flow from anode to cathode in the galvanic cell, we want them to flow from cathode to anode to reverse the current. In a battery, electrons move from the negative to positive end. Therefore, why wouldn't the positive end of the battery be on the galvanic cell's anode to attract electrons in order to reverse the current?

Could someone point out the flaw in my reasoning?

Thanks :)

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  • $\begingroup$ Electrons always flow from anode to cathode. When you reverse the flow, you rename the electrodes. See chemistry.stackexchange.com/a/135974/72973 $\endgroup$
    – Karsten Theis
    Dec 29, 2020 at 3:28
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    $\begingroup$ Note that it is not specific for galvanic cells, the answer is general for any voltage sources. So the question belongs rather to electronics or similar than chemistry. $\endgroup$
    – Poutnik
    Dec 29, 2020 at 9:41
  • $\begingroup$ Poutnik, Please don't make chemistry so narrow that the public literally start thinking that chemistry is nothing but titrations. You are good at electrochemistry and this very well belongs to electrochemistry. $\endgroup$
    – AChem
    Dec 29, 2020 at 15:56

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The electrochemical signs and labels have confused several generations of students. You have to remember a couple of things about the labels for the electrolytic and galvanic cells.

Anode: The electrode at which oxidation takes place. Cathode: The electrode at which reduction takes places.

The signs of the electrodes of a galvanic cell are electrostatic signs (positive and negative) of the electrodes. This is an important point to keep in mind, which is often not emphasized during teaching. So (-) sign on the battery terminal indicates excess of electrons and (+) sign indicates deficiency in electrons.

In a galvanic cell, the cathode is positively charged and the anode is negatively charged. Work this out.

In order to reverse the flow of electronic current in the 2V galvanic cell, you need to connect the electron rich electrode of the 9V battery to the electron rich electrode of the galvanic cell i.e., you will connect the negative terminal of the 9 V battery with the negative terminal of the 2V galvanic cell and connect the positive terminal of the battery with the positive end of the galvanic cell.

Kaplan's wordings is quite bad!

Consider the analogy of water supply in your own house: When you open a tap, water spontaneously flows out because the actual source of water is at a higher gravitational potential than your tap. What do we need to do to reverse the direction of water flow in the tap. We need to connect the tap to a powerful water pump which overcomes the gravitational potential energy of the water source and pumps the water against the gravitational potential. This battery connection is exactly the same because the battery generates 9V and the galvanic cell produces 2V.

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    $\begingroup$ (+) Well explained! $\endgroup$
    – AJKOER
    Dec 29, 2020 at 12:52

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