Section18_Corrosion_and_Refinement

Section18_Corrosion_and_Refinement - Electrochemistry III...

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Page 1 of 1 Electrochemistry III Corrosion and Electrolysis By the end of this lecture, you will be able to: (1) Define corrosion, and understand its spontaneity in an O 2 -filled environment. (2) Calculate the equilibrium constant for an oxidation process. (3) Understand the rusting of iron. (4) Understand electrolysis both qualitatively and quantitatively. (5) Understand the Hall-Heroult process for the refining of aluminum.
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Page 2 of 2 Corrosion Corrosion is the return of elemental metals to their thermodynamically preferred compounds (ores). Consider the following table: Reduction ε ° (V) O 2 + 4 H + + 4 e 2 H 2 O 1.23 Cu 2+ + 2 e Cu 0.34 Pb 2+ + 2 e Pb 0.13 Fe 2+ + 2 e Fe 0.44 Al 3+ + 3 e Al 1.66 The common metals all have reduction potentials less than that of O 2 . So, oxidation will be spontaneous (produce a positive cell potential).
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What is the equilibrium constant for the O 2 -oxidation of Fe(0) to Fe(II)? 2 Fe + O 2 + 4 H + 2 Fe 2+ + 2 H 2 O ε ° cell = ε °(O 2 H 2 O) ε °(Fe 2+ Fe) = 1.23 0.44 = 1.67 V The Nernst equation (at 298 K): ε cell = ε ° cell (0.0591/ n )log( Q ) At equilibrium, Q = K and ε cell = 0. Here,
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Section18_Corrosion_and_Refinement - Electrochemistry III...

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