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Suppose the half reactions:

$$\begin{split}\ce{Na^+ + e^- &-> Na} ~~~~~~~~~~~~&\text{ $E^o = -2.71$}\\ \ce{Cu^{2+} + 2 e^- &-> Cu} &\text{ $E^o = +0.34$}\end{split}$$

Can I say that $\ce{Na+}$ is more stable than $\ce{Na}$, so $\ce{Na+}$ has negative potential to reduce? If so then can I also say that $\ce{Cu^2+}$ is less stable than $\ce{Cu}$ so $\ce{Cu^2+}$ has a positive potential to reduce?

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Remember that reduction potentials do not have any meaning by themselves. They are measured against a hydrogen cell to which arbitrarily we assign the value $E^0=0$. You can compare between different potentials. For example, in your case, you could say that that $Cu^{2+}$ has a greater tendency to be reduced than $Na^+$ because its reduction potential is more positive. However, I do not think the potential itself tells you anything with regard to the stability (and, in any case, what do you mean by that) of the species involved in a particular half reaction.

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  • $\begingroup$ Yes that was what i wanted to assure. Thank you. $\endgroup$ – Pranoy De Mar 8 '17 at 17:06

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