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Electrode potential is the potential difference between the electrode and the surrounding electrolyte.

If the potential of the cathode is positive this means that the potential of the electrode is greater than the potential of the electrolyte, which means that the electrons will flow from the electrolyte to the cathode.

Shouldn't it be the other way around? shouldn't electrons flow from the cathode to electrolyte meaning the electrode potential should be negative?

Similarly, we can say that the electrode potential for an anode is positive.

Can someone point out the faults in this reasoning?

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  • $\begingroup$ Electrode potential is NOT the potential difference between the electrode and the surrounding electrolyte. $\endgroup$
    – Poutnik
    Commented Jul 23, 2020 at 6:27
  • $\begingroup$ "An electrode potential is strictly the potential difference developed at the interface between the electrode and the solution" ---wikipedia $\endgroup$ Commented Jul 23, 2020 at 6:35
  • $\begingroup$ Then there is electrode potential(1) and electrode potential(2). $\endgroup$
    – Poutnik
    Commented Jul 23, 2020 at 6:38
  • $\begingroup$ what does that mean? $\endgroup$ Commented Jul 23, 2020 at 6:40
  • $\begingroup$ It means 2 different potential refererence levels. 1 = SHE, 2=electrolyte. but the latter is quite hard to define, determine or reproduce. $\endgroup$
    – Poutnik
    Commented Jul 23, 2020 at 6:42

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I think your source of confusion is the name cathode and anode. Anode and cathodes have nothing to do with the electrostatic sign. They have to do with the processes.

Cathode, whether electrostatically positive OR negative, is the electrode where reduction is occurring.

Anode, whether electrostatically positive OR negative, is the electrode where oxidation is occurring.

Hope that clarifies your confusion about the signs.

Electrode potential is the potential difference between the electrode and the surrounding electrolyte.

This is not the electrode potential but is rather called interfacial potential difference.

Mathew wanted to elaborate this further on "Electrode potential is NOT the potential difference between the electrode and the surrounding electrolyte". I think this an electrochemist's unfulfilled dream to find the absolute potential difference, i.e., we dip Cu electrode in a copper solution and we measure the potential difference between the two. I think one can improvise what Poutnik is saying. The tabulated electrode potentials, found commonly, are not the potential differences of metals and their solutions. The reason is that it is impossible to measure such a hypothetical construct.

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  • $\begingroup$ Make sense. However, I'd include the definition of electrode potential as well. $\endgroup$ Commented Jul 23, 2020 at 19:37
  • $\begingroup$ Also, @Poutnik says "Electrode potential is NOT the potential difference between the electrode and the surrounding electrolyte." What is your take of this? $\endgroup$ Commented Jul 23, 2020 at 19:44
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    $\begingroup$ @MathewMahindaratne, I edited the post. $\endgroup$
    – ACR
    Commented Jul 24, 2020 at 0:56
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Electrode potential is NOT the potential difference between the electrode and the surrounding electrolyte.

The electrode potential is conventionally the relative potential to the reference standard hydrogen electrode(SHE) with conventionally assigned potential 0 V.

The "absolute" potential of the SHE wrt a free electron potential is then estimated as $\pu{+4.44 \pm 0.02 V}$.

Cathodes are more negative than anodes in electrolytic cells and more positive than anodes in galvanic cells. But more positive does not mean positive and more negative does not mean negative. The convention of marking positive and negative cell contacts is good enough for every day life, but it says nothing about the sign of electrode potentials ( either wrt a free electron in vacuum either wrt SHE ).

The electrode potentials can be positive + negative, or both positive or both negative wrt SHE. They are always positive wrt a free electron.

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  • $\begingroup$ so it is the potential difference between the electrode and the hydrogen electrode? $\endgroup$ Commented Jul 23, 2020 at 6:38
  • $\begingroup$ Yes, unless it is explicitly stated you consider the potential difference between the electrode and the electrolyte. $\endgroup$
    – Poutnik
    Commented Jul 23, 2020 at 6:40
  • $\begingroup$ my textbook says: $\endgroup$ Commented Jul 23, 2020 at 6:40
  • $\begingroup$ "a potential difference develops between the electrode and the electrolyte which is called electrode potential" $\endgroup$ Commented Jul 23, 2020 at 6:41
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    $\begingroup$ @Maurice There is need to distinguish formal potential of the electrode half reaction, bound to the change of Gibbs energy, that switch the sign with the reaction direction, and the physical potential of electrode wrt SHE, that does not switch the sign. $\endgroup$
    – Poutnik
    Commented Jul 23, 2020 at 11:57
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The question: Why is the electrode potential for a cathode positive?

Since question didn't directed to what kind of cell, the answer is it is not always positive:

Cathode Signs

As M. Farooq clarified, cathode is where reduction is happening in a cell. Thus, in Galvanic cell it is positive as the reduction reaction, $\ce{Y+ + e- -> Y}$, happens. The electrons needed for this purpose came from the anode. However, in an electrolytic cell, electrons from an outer source (e.g., a battery), flow to the negative end where the reduction reaction, $\ce{M+ + e- -> M}$, happens. Thus, the cathode is negative here.


Edit: I agree with Poutnik's comment below. Accordingly, it may be better to say cathodes are more negative than anodes in electrolytic cells and more positive than anodes in galvanic cells. As more positive does not mean positive and more negative does not mean negative. The convention of marking positive and negative cell contacts is good enough for every day life, but it says nothing about the sign of electrode potentials either with respect to free electron or SHE (e.g., relevant half-cell reduction reaction's potential may carry positive or negative sign, based on the corresponding reaction).

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    $\begingroup$ It may be better to say cathodes are more negative than anodes in electrolytic cells and more positive than anodes in galvanic cells. As more positive does not mean positive and more negative does not mean negative. The convention of marking positive and negative cell contacts is good enough for every day life, but it says nothing about the sign of electrode potentials ( either wrt free electron either wrt SHE ). $\endgroup$
    – Poutnik
    Commented Jul 24, 2020 at 6:28
  • $\begingroup$ @ Poutnik: I have edited the answer accordingly. It's a very good point. Thank you for checking. $\endgroup$ Commented Jul 24, 2020 at 15:55
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    $\begingroup$ See also my last comment to the Maurice below my answer. I had in mind rather physical measurable potentials that do not flip signs. There can be cells with both electrode potentials negative or both positive wrt SHE. And all are positive wrt the potential of a free electron. $\endgroup$
    – Poutnik
    Commented Jul 24, 2020 at 16:24
  • $\begingroup$ @Poutnik, very important point that ALL are positive wrt to free electron!! $\endgroup$
    – ACR
    Commented Jul 24, 2020 at 17:43
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I think AChem does a great job explaining the meaning of cathode and anode. (It has to do with the process occuring, not the electrostatic potential).

The anode always gives electrons (oxidation).

The cathode always accepts electrons (reduction).

The postive electrode is the one with the higher potential than the negative electrode. Since the potentials are all defined relative to a theoretical standard electrode (see Poutnik's answer), this designation may or may not match the actual sign of the potential.

When the reaction proceeds spontaneously (galvanic cell), the negative electrode is the anode. Electrons flow from the negative electrode to the positive electrode.

When the reaction must be driven by an external energy source (electrolytic cell), the negative electrode is the cathode. Electrons are forced into the negative electrode from the positive electrode.

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