Question

This question is from a Chemistry lab on constructing a half-potential reduction series. In part A of the lab we must determine if a metal medallion is appropriate or not (could be iron, galvanized, or other appropriate metal). We want to convert Cu^2+ to Cu. How do we determine if the metal is appropriate?

Answer 1

The standard electrode potential (E⁰) for Cu²⁺, Cu electrode is +0.34 volts and the half-cell reaction in

Copper electrode : Cu²⁺ + 2e^-     ---------->     Cu    E¹ = +0.34 volts (1)

The metal electrode chosen for the conversion of Cu²⁺ to Cu should have negative electrode potential (or atleast less than +0.34 volts), i.e., the electrode should have oxidation potential.

Metal electrode : M²⁺   +   2e^-   --------->    M       E² volts (2)

substracting (2) from (1)

Cell reaction : Cu²⁺ + M    --------->    M²⁺ + Cu

The total cell potential is E = 0.34 - (E^₂)

Since ΔG = -nFE (F = faradays), the reaction is feasible if the cell potential E is positive and ΔG < 0.

Using standard hydrogen electrode (SHE) Pt; H₂(g) or saturated calomel electrode (SCE) Hg, Hg₂Cl₂; KCl (solution), the electrode potential of the chosen electrode can be detemined.

For example, standard electrode potential of -0.76 volts is determined for Zn, Zn²⁺ electrode and it can be coupled with the copper electrode for the conversion of Cu²⁺ to Cu.

Cell reaction : Cu²⁺ + Zn    --------->    Zn²⁺ + Cu          E = 0.34 - (-0.76) volts = 1.10 volts    Feasible

The iron electrode of the type Fe³⁺, Fe²⁺; Pt cannot be useful for this reaction since the standard electrode potential of this half cell reaction is 0.77 volts which gives negative cell potential for the cell reaction

Cell reaction : Cu²⁺ + 2 Fe²⁺    --------->    2 Fe³⁺ + Cu          E = 0.34 - 0.77 volts = - 0.43 volts   Not Feasible