Question

how is reduction potential related to stability?

Answer 1

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 E0=0E0=0.For example, in your case, you may say that that Cu2+Cu2+ has a greater tendency to be reduced than Na+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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