Presentation on theme: "Electrochemistry and Society"— Presentation transcript:
1 Electrochemistry and Society Corrosion = oxidation of pure metals to their oxidesCorrosion BasicsMetals (Mo) are easily oxidized to cationic forms (Mn+) [Table 18.1]eo1/2 of O2 gas reduction > oxidation of most metalsO H e H2O eo1/2 = VThis leads to an eocell that is positive for this process = spontaneousMo + O MxOy eocell = +Most metals don’t completely decompose because MxOy protects the vulnerable Mo underneath from further corrosionAluminum ExampleAl e Alo eo = VO H e H2O eo = VAl2O3 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 corrosionAgo eo1/2 = V Ag2S tarnish formed instead of the oxideCuo eo1/2 = V Cu2CO3 forms green “patina”The Corrosion of IronThis is the most economically important corrosion process due to structural steelSteel has a non-uniform surface due to physical stressAnodic 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 CorrosionPaint covers the surface to prevent the contact of oxygen and the metalPlating steel with Cr or Sn to produce very stable oxidesGalvanizing = coating with ZincFe Fe e eo1/2 = VZn Zn e eo1/2 = VCorrosion occurs on Zn rather than Fe (sacrificial coating)Stainless Steel = Fe + Cr + Ni eo1/2 ~ Noble metalCathodic Protection = protects buried steel or ships with a sacrificial reactantActive metal (Mg) connected to pipe by a wire -eo1/2 = VBars of Ti attached to ship -eo1/2 = V
4 Electrolysis = using electric energy to produce chemical change (opposite of cell) ExampleConsider the Cu/Zn Galvanic CellAnode: Zn Zn e-Cathode: Cu e Cu eocell = VIf we attach a power source of eo > V, we can force e- to go the other wayAnode: Cu Cu e-Cathode: Zn e ZnCalled 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 CuExample: 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 WaterGalvanic: 2H2 + O H2O (Fuel Cell)Electrolytic Cell:Anode: 2H2O O H e eo = VCathode: 4H2O + 4e H OH eo = VOverall: 6H2O H2 + O (H+ + OH-)2H2O H2 + O eocell = VWe must add a salt to increase the conductance of pure water [H+] = [OH-] = 10-7
7 Commercial Electrolytic Processes Electrolysis of MixturesMixture of Cu2+, Ag+, Zn2+; What is the order of plating out?Ag+ + e Ag eo1/2 = VCu e Cu eo1/2 = VZn e Zn eo1/2 = VReduction of Ag+ is easiest (eo = most positive) followed by Cu, then ZnExample: Ce4+ (eo1/2 = V), VO2+ (eo1/2 = V), Fe3+ (eo1/2 = V)Commercial Electrolytic ProcessesProduction of AluminumMost metals are found naturally as their oxides, MxOyOnly the noble metals are typically found as the pure metalBauxite = aluminum ore; Al2O3Aluminum 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 ProcessAl e Al eo1/2 = V2H2O + 2e H OH- eo1/2 = VCan’t make Al in water because water gets reduced before Al3+Use molten Al2O3/Na3AlF6 mixture at 1000 oCAluminum alloys with Zn, Mn are most useful because they are strongerAluminum 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, FeGold, Silver, Platinum fall to the bottom of the tank as sludge (won’t plate out)Metal PlatingCoat easily corrodable metal object with a noble metalAg+ + e Ago on a spoonElectrolysis of NaClProduction of Na metal from NaCl in a “Downs Cell”Anode: 2Cl Cl e-Cathode: Na e NaoCell is designed to to keep products apart so they can’t reform NaClProduction of Cl2, OH- in a Mercury CellWater 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)
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