Ch12 II - L34 BATTERIES and FUEL CELLS Batteries Self...

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BATTERIES and FUEL CELLS Batteries: Self reading; ask questions at review session L34
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DEMO: Aluminum properties Al 3+ + 3e -> Al E 0 =-1.66 V Al: a very strong reducing agent. However, it is not oxidized by air, and does not react with water or even vinegar. It is protected by a thin Al 2 O 3 layer. If we remove the layer by treating the foil by HCl and HgCl 2 (forming aluminum amalgam which prevents Al 2 O 3 from forming), Al oxidizes very quickly and the foil disintegrates.
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The history of Aluminum 1809 - first characterized as a metal by Sir Humphry Davy as an alloy of Fe 1825 – obtained in pure form by Oersted Price of Al Date Price, $/lb 1855 100,000 1885 100 1886 Hall-Heroult process discovered 1890 2 1990 0.74
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Production of aluminum: Hall-Heroult process Al 3+ + 3e -> Al E 0 =-1.66 V Compare: 2H + + 2e -> H 2 E 0 =0.0 V O 2 + 2H 2 O + 4e -> 4OH - E 0 =0.4 V Cannot reduce Al from water solution Bauxite: Al 2 O 3 nH 2 O + oxides of Fe, Si, Ti + silicates Melting point of Al 2 O 3 : 2050 o C Mixture of Al 2 O 3 /Na 3 AlF 6 (cryolite) : 1000 o C Electrochemical cell with carbon anode: 2 Al 2 O 3 + 3C -> 4Al + 3CO 2 Al production: 5% of electricity in the US!
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Galvanic cell vs electrolytic cell <-> Exothermic vs endothermic rxns Example (gas phase): 2H 2 (g) + O 2 (g) -> 2 H 2 O (g) Δ H f 0 (H 2 O)=-286 kJ/mol Spontaneous at 25 o C; releases heat How do we reverse it? Heat H 2 O until it dissociates (or until -T Δ S term exceeds Δ H). Or electrolysis of water In aqueous acidic solution: O 2 + 4 H + + 4e -> 2 H 2 O E 0 = 1.24 V H 2 – 2e -> 2H + E 0 = 0.0 V Overall: 2H 2 (aq) + O 2 (aq) -> 2 H 2 O (aq) E=1.24 V left to right: spontaneous, used in fuel cells right to left: non-spontaneous, needs electrical energy (electrolysis of water)
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Electrochemistry: "cold reactions" No heat and high efficiency No burning - > environmentally friendly Batteries and fuel cells Chemical energy is directly transformed into electrical energy in a galvanic-cell type device Batteries: non-rechargeable and rechargeable Fuel cells: galvanic cell with fuel supply
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Ch12 II - L34 BATTERIES and FUEL CELLS Batteries Self...

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