Goal: to understand and explore the process of electrolysis
Electrolytic cells use an external source of direct current (DC) to drive reactions that would not otherwise be spontaneous. Electrolytic cells do not produce a cell potential but require a potential to operate. Electrolytic reactions are used purify metals and to plate metals on many types of substrates.
Using the electrolytic process to purify a metal (refining):
Because impurities can dramatically decrease the conductivity of copper wires, impure copper must be purified. One method of purifying copper is by electrolysis.
When a strip of impure metallic copper is used as the anode in the electrolysis of an aqueous preparation of copper(II) sulfate, copper is oxidized. The oxidation of copper is more facile than the oxidation of water (see the standard oxidation potentials below) so metallic copper dissolves into solution as copper(II) ions, leaving behind many of the impurities (less active metals):
Cu(s) Cu2+(aq) + 2 e (anode)
E°ox(Cu) = 0.34 V vs. E°ox(H2O) = 1.23 V
The copper(II) ions formed at the anode migrate to the cathode where they are more easily reduced than water and metallic copper plates on the cathode where it can be collected:
2 e + Cu2+(aq) Cu(s) (cathode)
E°red(Cu2+) = +0.34 V vs. E°red(H2O) = 0.80 V
We had to pass sufficient current between the electrodes to cause the otherwise non-spontaneous reaction to occur! By carefully regulating the electrical potential, the metallic impurities that are active enough to be oxidized with copper at the anode are not reduced at the cathode and copper is selectively deposited.
Ready for a recap?
Use the previous table of standard reduction potentials to answer the following questions. There may be more than one correct answer!
Which of these metal ions can be selectively reduced in an aqueous solution?
Is the zirconium ion easier to reduce than water? Try again.
Well done! Silver ion is much easier to reduce than water... A nice way to silverplate flatware!
Is the magnesium ion easier to reduce than water? Try again.
Which of these metals can be refined using the experiment described for copper in the preceeding section?
Though zirconium ion is easier to oxidize than water, it is more difficult to reduce. You would be unable to efficiently deposit zirconium metal at the cathode. Try again.
Great job! Silver ion is much easier to reduce that is water and is sufficiently easier to oxidize.
Right! Mercury ion is both easier to reduce and oxidize than is water.
Before moving to the next page, you should understand what is meant by electrolysis and how to determine if oxidation or reduction of a chosen species can selectively occur in the presence of water.