= Solution
The desired net process transfers <copper> from an impure <anode> to a purer <cathode>. In the idealized common-electrolyte approximation, both interfaces use the same $\mathrm{Cu}^{2+}$/Cu equilibrium potential. Setting solid <copper> activities to one and interfacial <ion> activities equal gives \b[$E_{\rm rev}=0$ V]: subtracting two equal 0.34 V <reduction> potentials yields zero, rather than 0.34 V.
A strictly thermodynamic refining <voltage> need not be exactly zero. At equal dissolved-ion activity, the required reversible applied <voltage> is
$$
V_{\rm rev}=\frac{RT}{2F}\ln\frac{a_{\rm Cu,cath}}{a_{\rm Cu,an}}.
$$
Dilution of <copper> in the impure phase can therefore require positive reversible <work>. Weight percentage alone does not specify that phase's <thermodynamic activity> or impurity molar composition. The zero result is the intended pure-solid approximation, not a precise chemical-potential measurement from the stated weight purities.
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