1. Electrochemistry and Society
Corrosion = oxidation of pure metals to their oxides
Corrosion Basics
Metals (Mo) are easily oxidized to cationic forms (Mn+) [Table 18.1]
eo1/2 of O2 gas reduction > oxidation of most metals
O H e H2O eo1/2 = V
This leads to an eocell that is positive for this process = spontaneous
Mo + O MxOy eocell = +
Most metals don’t completely decompose because MxOy protects the vulnerable Mo underneath from further corrosion
Aluminum Example
Al e Alo eo = V
O H e H2O eo = V
Al2O3 produced has eo1/2 = V, resulting in a much less favorable corrosion process, once the aluminum underneath is covered.
eocell = V

2. The “Noble Metals” (Ag, Au, Cu, Pt) do not react with oxygen as easily
Auo eo1/2 = V no corrosion
Ago eo1/2 = V Ag2S tarnish formed instead of the oxide
Cuo eo1/2 = V Cu2CO3 forms green “patina”
The Corrosion of Iron
This is the most economically important corrosion process due to structural steel
Steel has a non-uniform surface due to physical stress
Anodic Region: Fe Fe e-
Cathodic Region: O H2O + 4e OH-
Fe2+ then acts as the salt bridge electrolyte if wet (added salt speeds up corrosion)
Cathode: 4Fe O2 (4 + 2n)H2O Fe2O3 • nH2O + 8H+

3. Preventing Corrosion
Paint covers the surface to prevent the contact of oxygen and the metal
Plating steel with Cr or Sn to produce very stable oxides
Galvanizing = coating with Zinc
Fe Fe e eo1/2 = V
Zn Zn e eo1/2 = V
Corrosion occurs on Zn rather than Fe (sacrificial coating)
Stainless Steel = Fe + Cr + Ni eo1/2 ~ Noble metal
Cathodic Protection = protects buried steel or ships with a sacrificial reactant
Active metal (Mg) connected to pipe by a wire -eo1/2 = V
Bars of Ti attached to ship -eo1/2 = V

4. Electrolysis = using electric energy to produce chemical change (opposite of cell)
Example
Consider the Cu/Zn Galvanic Cell
Anode: Zn Zn e-
Cathode: Cu e Cu eocell = V
If we attach a power source of eo > V, we can force e- to go the other way
Anode: Cu Cu e-
Cathode: Zn e Zn
Called an Electrolytic Cell

5. B. Calculations with Electrolytic Cells
How much Chemical Change? Is usually the question.
Find mass of Cuo plated out by passing 10 amps (10 C/s) through Cu2+ solution.
Cu e Cuo(s)
Steps: current/time, charge (C), moles e-, moles Cu, grams Cu
Example: How long must a current of 5.00 amps be applied to a Ag+ solution to produce 10.5 g of silver metal?
Electrolysis of Water
Galvanic: 2H2 + O H2O (Fuel Cell)
Electrolytic Cell:
Anode: 2H2O O H e eo = V
Cathode: 4H2O + 4e H OH eo = V
Overall: 6H2O H2 + O (H+ + OH-)
2H2O H2 + O eocell = V
We must add a salt to increase the conductance of pure water [H+] = [OH-] = 10-7

7. Commercial Electrolytic Processes
Electrolysis of Mixtures
Mixture of Cu2+, Ag+, Zn2+; What is the order of plating out?
Ag+ + e Ag eo1/2 = V
Cu e Cu eo1/2 = V
Zn e Zn eo1/2 = V
Reduction of Ag+ is easiest (eo = most positive) followed by Cu, then Zn
Example: Ce4+ (eo1/2 = V), VO2+ (eo1/2 = V), Fe3+ (eo1/2 = V)
Commercial Electrolytic Processes
Production of Aluminum
Most metals are found naturally as their oxides, MxOy
Only the noble metals are typically found as the pure metal
Bauxite = aluminum ore; Al2O3
Aluminum is the third most abundant element on crust (oxygen and silicon)
No commercial process for pure Al until 1854 (eo1/2 = V)
Al was more valuable than gold or silver

8. Hall—Heroult Process
Al e Al eo1/2 = V
2H2O + 2e H OH- eo1/2 = V
Can’t make Al in water because water gets reduced before Al3+
Use molten Al2O3/Na3AlF6 mixture at 1000 oC
Aluminum alloys with Zn, Mn are most useful because they are stronger
Aluminum production uses 5% of the electricity consumed in the U.S.

9. Electrorefining = purification of metals
Impure Cuo anode: Cuo Cu e-
Pure Cuo cathode: Cu e Cuo (99.95% pure)
Also useful for purification of Zn, Fe
Gold, Silver, Platinum fall to the bottom of the tank as sludge (won’t plate out)
Metal Plating
Coat easily corrodable metal object with a noble metal
Ag+ + e Ago on a spoon
Electrolysis of NaCl
Production of Na metal from NaCl in a “Downs Cell”
Anode: 2Cl Cl e-
Cathode: Na e Nao
Cell is designed to to keep products apart so they can’t reform NaCl
Production of Cl2, OH- in a Mercury Cell
Water is reduced to OH- (eo1/2 = V) before Na+ (eo1/2 = V)
Cathode: 2H2O + 2e H OH-
Chlor-Alkali Process = second largest electricity user in U.S. (after Al)

10. Downs Cell

11. Mercury Cell for Chor-Alkali Process