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Well here's a question from my book:

A solution containing one mole per liter of each $\ce{Cu(NO3)2}$, $\ce{AgNO3}$, $\ce{Hg2(NO3)2}$ and $\ce{Mg(NO3)2}$ is being electrolysed by using inert electrodes. The values of standard electrode potentials in volts (reduction potentials) are:

  • $\ce{Ag+/Ag} = +0.80$,
  • $\ce{Hg2^2+/Hg} = +0.79$,
  • $\ce{Cu^2+/Cu} = +0.34$,
  • $\ce{Mg^2+/Mg} = -2.37$

The sequence of deposition of the metals on the cathode will be?

The answer to the above question is: Ag, Hg, Cu.
Why isn't Mg deposited on the cathode? Is it because the reduction potential is negative?
If yes, what exactly is the reason?

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  • $\begingroup$ I have improved the formatting with $\LaTeX$ (edit pending approval). For more information on how to do so yourself, see here and here. $\endgroup$ – Jan Jun 12 '15 at 18:30
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    $\begingroup$ Your solution is aquaeous. There is another cation present. It very much has something to do with magnesium's potential being positive. Do you maybe see it? ;) $\endgroup$ – Jan Jun 12 '15 at 18:32
  • $\begingroup$ @Jan Water! Right. Didn't think about that. Thanks. $\endgroup$ – Tabish Mir Jun 13 '15 at 15:11
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You are in aquaeous solution. Everywhere where there is an aquaeous solution, you have a certain concentration of a further cation: $\ce{H+}$. It has a potential of $\pm 0\,\mathrm{V}$ by definition, thereby being nobler than $\ce{Mg^2+}$.

Thus, hydrogen gas is ‘deposited’ on the cathode first and bubbles away while the solution gets removed.

It is not possible to deposite considerably less noble metals on the cathode in normal, aquaeous solutions.

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Reduction takes place on cathode. Thus the element having most reduction potential will get reduced and get deposited. In your question the reduction potential of $\ce{Mg}$ is the least of all the ones that you have mentioned hence it will not get deposited.

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  • $\begingroup$ It doesn't answer this. $\endgroup$ – Mithoron Sep 17 '15 at 14:45
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Since the reduction potential is negative, the process becomes non spontaneous. Moreover, there are hydrogen ions already present having zero assumed reduction potential. Therefore, Mg won't get deposited

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