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 O2 + 4H+ + 4e- 2H2O eo1/2 = +1.23 V This leads to an eocell that is positive for this process = spontaneous Mo + O2 MxOy eocell = + Most metals don’t completely decompose because MxOy protects the vulnerable Mo underneath from further corrosion Aluminum Example Al3+ + 3e- Alo eo = -1.70 V O2 + 4H+ + 4e- 2H2O eo = +1.23 V Al2O3 produced has eo1/2 = -0.60 V, resulting in a much less favorable corrosion process, once the aluminum underneath is covered. eocell = +2.93 V

The “Noble Metals” (Ag, Au, Cu, Pt) do not react with oxygen as easily Auo eo1/2 = +1.50 V no corrosion Ago eo1/2 = +0.80 V Ag2S tarnish formed instead of the oxide Cuo eo1/2 = +0.16 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 Fe2+ + 2e- Cathodic Region: O2 + 2H2O + 4e- 4OH- Fe2+ then acts as the salt bridge electrolyte if wet (added salt speeds up corrosion) Cathode: 4Fe2+ + O2 (4 + 2n)H2O 2Fe2O3 • nH2O + 8H+

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 Fe2+ + 2e- -eo1/2 = +0.44 V Zn Zn2+ + 2e- -eo1/2 = +0.76 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 = +2.37 V Bars of Ti attached to ship -eo1/2 = +1.60 V

Electrolysis = using electric energy to produce chemical change (opposite of cell) Example Consider the Cu/Zn Galvanic Cell Anode: Zn Zn2+ + 2e- Cathode: Cu2+ + 2e- Cu eocell = +1.10 V If we attach a power source of eo > +1.10 V, we can force e- to go the other way Anode: Cu Cu2+ + 2e- Cathode: Zn2+ + 2e- Zn Called an Electrolytic Cell

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. Cu2+ + 2e- 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 + O2 2H2O (Fuel Cell) Electrolytic Cell: Anode: 2H2O O2 + 4H+ + 4e- -eo = -1.23 V Cathode: 4H2O + 4e- 2H2 + 4OH- eo = -0.83 V Overall: 6H2O 2H2 + O2 + 4(H+ + OH-) 2H2O 2H2 + O2 eocell = -2.06 V We must add a salt to increase the conductance of pure water [H+] = [OH-] = 10-7

Commercial Electrolytic Processes Electrolysis of Mixtures Mixture of Cu2+, Ag+, Zn2+; What is the order of plating out? Ag+ + e- Ag eo1/2 = +0.80 V Cu2+ + 2e- Cu eo1/2 = +0.34 V Zn2+ + 2e- Zn eo1/2 = -0.76 V Reduction of Ag+ is easiest (eo = most positive) followed by Cu, then Zn Example: Ce4+ (eo1/2 = +1.70 V), VO2+ (eo1/2 = +1.00 V), Fe3+ (eo1/2 = +0.77 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 = 1.66 V) Al was more valuable than gold or silver

Hall—Heroult Process Al3+ + 3 e- Al eo1/2 = -1.66 V 2H2O + 2e- H2 + 2OH- eo1/2 = -0.83 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.

Electrorefining = purification of metals Impure Cuo anode: Cuo Cu2+ + 2e- Pure Cuo cathode: Cu2+ + 2e- 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- Cl2 + 2e- 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 = -0.83 V) before Na+ (eo1/2 = -2.71 V) Cathode: 2H2O + 2e- H2 + 2OH- Chlor-Alkali Process = second largest electricity user in U.S. (after Al)

Downs Cell

Mercury Cell for Chor-Alkali Process