3
$\begingroup$

I am wondering if the direction of electron flow in a voltaic cell be reversed such that the cathode becomes the anode and vice versa. I understand this is not possible for two standard half-cells, but what about non-standard half-cells at STP ?

$\endgroup$
  • $\begingroup$ Electrolysis perhaps $\endgroup$ – Liam Feb 23 at 18:18
  • 1
    $\begingroup$ A voltaic cell ( or a galvanic cell) is made up of two half cells between which a current will flow when they are connected. In other words, a battery. The Nernst equation gives the half cell voltage under non-standard conditions. However the Nernst equation assumes a infinitesimally small current flow. A simple model of real battery uses an internal resistance to account for the fact that the battery's voltage drops as more current passes. // Some batteries are rechargeable, and some are not. $\endgroup$ – MaxW Feb 23 at 18:43
  • $\begingroup$ @MaxW is the x intercept for a graph of the Nernst equation, where x represents the concentration of the products, equal to concentration after which the reaction become non-spontaneous and the reverse reaction becomes spontaneous ? $\endgroup$ – Physics Feb 23 at 19:02
  • $\begingroup$ @Physics - Sorry, I'm not a mind reader. I have no idea what graph you mean. Please explain in detail... $\endgroup$ – MaxW Feb 23 at 19:05
  • $\begingroup$ @MaxW Sure. Say you have the Daniell cell at non-standard conditions. E = 1.1 - (R(298)/2F) x ln(x/1.0) where E is the emf of the cell at non-standard conditions and x is the concentration of Zn(2+) ions. So, is the x-intercept, the concentration beyond which the reaction becomes non-spontaneous and hence the reverse reaction becomes spontaneous ? In the case of this cell, the x intercept is physically impossible (due to the solubility limit), but may be it is possible for other cells. In case : The reaction is : Zn(s) + Cu(2+) --> Zn(2+) + Cu(s) $\endgroup$ – Physics Feb 23 at 19:32
6
$\begingroup$

Can going to non-standard conditions reverse the cell potential of a voltaic cell?

I have already answered that question, in the affirmative, for a simple tin and lead galvanic cell, here: https://chemistry.stackexchange.com/a/116734/79678.

What follows is an elaboration with some specific values and illustrative figures. In fig. 1, the tin and lead voltaic cell is shown:

Voltaic cell 1

In fig. 1, the tin and lead ion concentrations are each exactly $\pu{1 M}$ and the cell potential is $\pu{+0.014 V}$, with tin as the anode. All necessary equations are shown in the figure and the usual simplifying assumptions are made: unity activity coefficients, zero cell resistance, etc. The electron flow is from $\ce{Sn}$ to $\ce{Pb}$.

In the next figure, the lead ion concentration is $\pu{0.3363 M}$ while the tin ion concentration remains at exactly $\pu{1 M}$. This results in zero cell potential, as shown in fig. 2:

Voltaic cell 2

Since the cell potential is zero, it is a moot point to call tin the anode: the cell is not doing (net) electrochemistry. There is no net electron flow.

In the final figure, the lead ion concentration is exactly $\pu{0.1 M}$ while the tin ion concentration remains at exactly $\pu{1 M}$. This results in a cell potential of $\pu{+0.016 V}$, as shown in the figure:

Voltaic cell 3

But now lead is the anode and tin is the cathode. The electron flow is from $\ce{Pb}$ to $\ce{Sn}$.

So changing the ion concentration to non-standard values can reverse the electron flow direction. But this was an especially easy case because the two metal electrodes had standard potentials that were relatively close. For the Daniell cell, with $\ce{Zn}$ and $\ce{Cu}$, it would not be feasible.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Tangible and plausible answer $\endgroup$ – Adnan AL-Amleh Feb 24 at 3:41
  • $\begingroup$ Wonderful answer! But please don't make it a habit to write EDIT whenever you add something to your post, we have the edit history to keep a record of that, further you could use a bit of MathJax whenever you feel so, though its not always compulsory. For a comprehensive read refer to this meta post $\endgroup$ – Sir Arthur7 Feb 24 at 11:12

Not the answer you're looking for? Browse other questions tagged or ask your own question.