Lecture 8 6pp - Corrosion and Electrochemistry Conventional...

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1 Corrosion and Electrochemistry • Silverware cleaning magic electrolyte silverware Al foil 2 Conventional Example Zn + 2HCl ! ZnCl 2 + H 2 " Zn Zn 2+ 2e # 2H + H 2 Zn ! Zn 2+ + 2e 2e + 2H + ! H 2 " Oxidation - Anode Reduction - Cathode Oxidation Is Loss, Reduction Is Gain (OILRIG) 3 Measuring Electrode Potential • A measure of the tendency to give up electrons 1 atm H 2 1 M HCl 1 M ZnCl 2 Zn Pt 0.76 V e e + KCl salt bridge 4 Standard Electrode Potentials Fe 2+ + 2e ! Fe E o = +0.44 volt 5 Standard Conditions • Metal in contact with 1M metal ions • Hydrogen at one atmosphere in contact with 1M H + ions • Temperature = 25 C (298K) 6 Nernst Equation V = V o + RT nF ln C • Note that V becomes more negative with smaller concentration C --> concentration cells • Example: Zn in contact with HCl. Zn in contact with solution of lower Zn 2+ concentration dissolves preferentially.
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7 Energy and Voltage " G = neV o N A = nFV o • In principle, we can take a “downhill” chemical reaction and extract electrical power from it (negative Gibbs free energy) • F = Faraday = 96500 Coul. • V 0 = standard electrode potential 8 Cu-Zn • Zn ! Zn 2+ + 2e # ( V o = # 0.76V) • Cu 2+ + 2e # ! Cu ( V o = + 0.34V) Net: Zn + Cu 2+ # ! Cu + Zn 2+ ( V o = # 1.10V) 1 M CuSO 4 1 M ZnCl 2 Zn KCl salt bridge Cu 1.10 V e e + 9 Ag-Al Al ! Al 3+ + 3e ( V o = # 1.66 volt) Ag ! Ag + + e ( V o = +0.80 volt) – Al is oxidized (forward reaction) – Ag is reduced (reverse reaction • Electrolyte: NaOH – To conduct ionic currents – To dissolve aluminum oxide What is the standard electrode potential? 10
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This note was uploaded on 11/21/2011 for the course MAT SCI 201 taught by Professor Matsci during the Spring '10 term at Northwestern.

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Lecture 8 6pp - Corrosion and Electrochemistry Conventional...

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