Presentation is loading. Please wait.

Presentation is loading. Please wait.

Electrochemistry and Society

Similar presentations

Presentation on theme: "Electrochemistry and Society"— Presentation transcript:

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

Download ppt "Electrochemistry and Society"

Similar presentations

Ads by Google