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Electrochemistry and Society I.Corrosion = oxidation of pure metals to their oxides A.Corrosion Basics 1)Metals (M o ) are easily oxidized to cationic.

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Presentation on theme: "Electrochemistry and Society I.Corrosion = oxidation of pure metals to their oxides A.Corrosion Basics 1)Metals (M o ) are easily oxidized to cationic."— Presentation transcript:

1 Electrochemistry and Society I.Corrosion = oxidation of pure metals to their oxides A.Corrosion Basics 1)Metals (M o ) are easily oxidized to cationic forms (M n+ ) [Table 18.1] 2)  o 1/2 of O 2 gas reduction > oxidation of most metals a)O 2 + 4H + + 4e - 2H 2 O  o 1/2 = V b)This leads to an  o cell that is positive for this process = spontaneous c)M o + O 2 M x O y  o cell = + 3)Most metals don’t completely decompose because M x O y protects the vulnerable M o underneath from further corrosion 4)Aluminum Example a)Al e - Al o  o = V b)O 2 + 4H + + 4e - 2H 2 O  o = V c)Al 2 O 3 produced has  o 1/2 = V, resulting in a much less favorable corrosion process, once the aluminum underneath is covered.  o cell = V

2 5)The “Noble Metals” (Ag, Au, Cu, Pt) do not react with oxygen as easily a)Au o  o 1/2 = V no corrosion b)Ag o  o 1/2 = V Ag 2 S tarnish formed instead of the oxide c)Cu o  o 1/2 = V Cu 2 CO 3 forms green “patina” B.The Corrosion of Iron 1)This is the most economically important corrosion process due to structural steel 2)Steel has a non-uniform surface due to physical stress a)Anodic Region: Fe Fe e - b)Cathodic Region: O 2 + 2H 2 O + 4e - 4OH - 3)Fe 2+ then acts as the salt bridge electrolyte if wet (added salt speeds up corrosion) Cathode: 4Fe 2+ + O 2 (4 + 2n)H 2 O 2Fe 2 O 3 nH 2 O + 8H +

3 C.Preventing Corrosion 1)Paint covers the surface to prevent the contact of oxygen and the metal 2)Plating steel with Cr or Sn to produce very stable oxides 3)Galvanizing = coating with Zinc a)Fe Fe e - -  o 1/2 = V b)Zn Zn e - -  o 1/2 = V c)Corrosion occurs on Zn rather than Fe (sacrificial coating) 4)Stainless Steel = Fe + Cr + Ni -  o 1/2 ~ Noble metal 5)Cathodic Protection = protects buried steel or ships with a sacrificial reactant a)Active metal (Mg) connected to pipe by a wire -  o 1/2 = V b)Bars of Ti attached to ship -  o 1/2 = V

4 II.Electrolysis = using electric energy to produce chemical change (opposite of cell) A.Example 1)Consider the Cu/Zn Galvanic Cell a)Anode: Zn Zn e - b)Cathode: Cu e - Cu  o cell = V 2)If we attach a power source of  o > V, we can force e- to go the other way a)Anode: Cu Cu e - b)Cathode: Zn e - Zn c)Called an Electrolytic Cell

5 B.Calculations with Electrolytic Cells 1)How much Chemical Change? Is usually the question. 2)Find mass of Cu o plated out by passing 10 amps (10 C/s) through Cu 2+ solution. a) Cu e - Cu o (s) b)Steps: current/time, charge (C), moles e-, moles Cu, grams Cu 3)Example: How long must a current of 5.00 amps be applied to a Ag + solution to produce 10.5 g of silver metal? C.Electrolysis of Water 1)Galvanic: 2H 2 + O 2 2H 2 O (Fuel Cell) 2)Electrolytic Cell: a)Anode: 2H 2 O O 2 + 4H + + 4e - -  o = V b)Cathode: 4H 2 O + 4e - 2H 2 + 4OH -  o = V c)Overall: 6H 2 O 2H 2 + O 2 + 4(H + + OH - ) 2H 2 O 2H 2 + O 2  o cell = V 3)We must add a salt to increase the conductance of pure water [H + ] = [OH - ] = 10 -7

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7 D.Electrolysis of Mixtures 1)Mixture of Cu 2+, Ag +, Zn 2+ ; What is the order of plating out? a)Ag + + e - Ag  o 1/2 = V b)Cu e - Cu  o 1/2 = V c)Zn e - Zn  o 1/2 = V 2)Reduction of Ag + is easiest (  o = most positive) followed by Cu, then Zn 3)Example: Ce 4+ (  o 1/2 = V), VO 2+ (  o 1/2 = V), Fe 3+ (  o 1/2 = V) III.Commercial Electrolytic Processes A.Production of Aluminum 1)Most metals are found naturally as their oxides, M x O y 2)Only the noble metals are typically found as the pure metal 3)Bauxite = aluminum ore; Al 2 O 3 4)Aluminum is the third most abundant element on crust (oxygen and silicon) 5)No commercial process for pure Al until 1854 (  o 1/2 = 1.66 V) 6)Al was more valuable than gold or silver

8 8)Hall—Heroult Process a)Al e - Al  o 1/2 = V b)2H 2 O + 2e - H 2 + 2OH -  o 1/2 = V c)Can’t make Al in water because water gets reduced before Al 3+ d)Use molten Al 2 O 3 /Na 3 AlF 6 mixture at 1000 o C 9)Aluminum alloys with Zn, Mn are most useful because they are stronger 10)Aluminum production uses 5% of the electricity consumed in the U.S.

9 B.Electrorefining = purification of metals 1)Impure Cu o anode: Cu o Cu e - 2)Pure Cu o cathode: Cu e - Cu o (99.95% pure) 3)Also useful for purification of Zn, Fe 4)Gold, Silver, Platinum fall to the bottom of the tank as sludge (won’t plate out) C.Metal Plating 1)Coat easily corrodable metal object with a noble metal 2)Ag + + e - Ag o on a spoon D.Electrolysis of NaCl 1)Production of Na metal from NaCl in a “Downs Cell” a)Anode: 2Cl - Cl 2 + 2e - b)Cathode: Na + + e - Na o c)Cell is designed to to keep products apart so they can’t reform NaCl 2)Production of Cl 2, OH - in a Mercury Cell a)Water is reduced to OH- (  o 1/2 = V) before Na+ (  o 1/2 = V) b)Anode: 2Cl - Cl 2 + 2e - c)Cathode: 2H 2 O + 2e - H 2 + 2OH - d)Chlor-Alkali Process = second largest electricity user in U.S. (after Al)

10 Downs Cell

11 Mercury Cell for Chor-Alkali Process


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