Skills to develop
- Apply electrochemical principle to discuss corrosion phenomena
Electrochemistry and Corrosion
Electrolysis of Mixtures of Ions
Let us consider an electrolytic cell containing a mixture of ions,
Ag+, Cu2+, Zn2+, and Mg2+,
all with a concentration of 1.0 M.
Pt | H2 | H+ || Ag+, Cu2+, Zn2+, Mg2+ | M
For such a mixture, we construct a table below using standard reduction
The first two couples has a positive voltage, indicating that the reactions
for the first two cells are spontaneous and exothermic.
The Ag+ ions have the strongest oxidizing power, and they
create the highest voltage for the cell.
The third and fourth reactions are not spontaneous, and require a
voltage to force the reaction.
|Anode ||DE° || Cathod
|Half-reaction || E° /V ||
|| E° /V ||Half-reaction
|H2 = 2 H+ + 2e ||0.00 ||0.80 V
|| 0.80 ||Ag+ + e = Ag
|H2 = 2 H+ + 2e ||0.00 ||0.34 V
|| 0.34 ||Cu2+ + 2 e = Cu
|H2 = 2 H+ + 2e ||0.00 ||-0.76 V
|| -0.76 ||Zn2+ + 2 e = Zn
|H2 = 2 H+ + 2e ||0.00 ||-2.36 V
|| -2.36 ||Mg2+ + 2 e = Mg
Let us assume that a Mg2+ ion zaps by and is electicuted,
(reduced). But Mg is more reactive than Ag, and in the fight for
electrons, the stongest oxidizing ion Ag+ wins.
So goes the theory, but the reality is often different.
Thus, apply electrolysis for the separation of metals still
require engineering research and development.
The argument provided earlier appears to be sound, but the reality
can be different. Consider the the case when a salt solution is
electrolyzed. The oxidation potentials are:
Since water has less negative (similar to more positive) oxidation potential,
the above argument suggests that oxygen gas will form when a voltage is
applied to the electrode.
The reality is that we get chlorine gas under this circumstance.
the mechanism of the process is complicated. An explanation
suggests that its easier for the chloride to transfer electrons
to the electrode than water.
|Half-reaction || E°
|| ||E° ||Half-reaction
|Cl2 + 2 e = 2 Cl-|| 1.36 V
|| ||0.00 ||2 H+ + 2e = H2
|O2 + 2 e + 4 H+ = 2 H2O || 1.23 V