Question

What if you want to "plate out" copper metal from an aqueous Cu²⁺ solution? Use Table 18.1 to determine several metals you can place in the solution to plate copper metal from the solution. Defend your choices. Why can Zn not be plated out from an aqueous solution of ${\mathbf{Zn}}^{\mathbf{2}+}$using the choices in Table 18.1?

Short answer

The standard reduction potential of zinc is highly negative, it undergoes oxidation rather than reduction at cathode.

Step-by-step solution

Plating of copper metal and chemical equation

The copper metal can be plated using a Cu²⁺ aqueous solution. There is an applied electricity when two electrodes are placed in an aqueous solution. The copper ions can migrate towards the cathode and reduction occurs. The plating of copper is done at the cathode. The chemical equation can be written as,

$C{u}^{2+}\left(aq\right)+2e^{-}\to Cu\left(s\right)E_{red}=0.34V$

Standard reduction potential of metals

Various metals acts as an electrode for copper plating from aqueous solution of copper ions like lead, iron, cadmium, chromium, and nickel etc., The standard reduction potential can be denoted as,

$\begin{array}{c}F{e}^{3+}\left(aq\right)+3e^{-}\to Fe\left(s\right)E_{red}=-0.036V\\ F{e}^{2+}\left(aq\right)+2e^{-}\to Fe\left(s\right)E_{red}=-0.44V\\ P{b}^{2+}\left(aq\right)+2e^{-}\to Pb\left(s\right)E_{red}=-0.13V\\ S{n}^{2+}\left(aq\right)+2e^{-}\to Sn\left(s\right)E_{red}=-0.23V\\ N{i}^{2+}\left(aq\right)+2e^{-}\to Ni\left(s\right)E_{red}=-0.23V\\ C{d}^{2+}\left(aq\right)+2e^{-}\to Cd\left(s\right)E_{red}=-0.4V\\ C{r}^{3+}\left(aq\right)+3e^{-}\to Cr\left(s\right)E_{red}=-0.73V\end{array}$

Copper is positive whereas standard reduction potential for all elements is negative. Copper ions are deposited at the cathode.

Standard reduction potential of zinc is more negative than all other metals

Zinc cannot plate using an aqueous zinc solution. The equation can be denoted as,

$Z{n}^{2+}\left(aq\right)+2e^{-}\to Zn\left(s\right)E_{red}=-0.76V$

The standard reduction potential of zinc is highly negative, it undergoes oxidation rather than reduction at the cathode. Thus, zinc cannot plate from an aqueous solution using metals.