Chem208_Lecture_25_Color - Today: Electrochemistry (Ch. 20)...

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Unformatted text preview: Today: Electrochemistry (Ch. 20) - Galvanic Cells - Read section 20-5 (Batteries) PS10 is due Friday, 4/25, at 2:00 1 Introducing Aurora Davis, born 4/19/08 2 Review At anode: oxidation half reaction occurs here; electrons are lost At cathode: reduction half reaction occurs here; electrons are gained 3 Relationship between ∆G and E welec = nFE (work done by the system) n: # of moles of electrons transferred F: charge on 1 mole of electrons (the Faraday) (F = 96,485 coulombs) E: cell potential 4 From thermodynamics, ∆G = -w So, ∆G = –nFE Under standard conditions: ∆G° = –nFE° 5 How much of the 0.46 volt is a measure of the tendency for Ag+ to be reduced to Ag, and how much is a measure of the tendency for Cu to be oxidized to Cu2+? Problem: E° can not be measured for a half reaction 6 Standard Hydrogen Electrode (SHE) 2 H3O+(aq) + 2 e– → H2(g) E° = 0.00 volt Both the selection of the SHE and the assignment of E°=0.00 volt are completely arbitrary! E° is a state function (∆G° = –nFE°). Consequently, E°(R → P) = –E°(P → R) 7 8 Should Cu2+ oxidize Al under standard conditions? From Table of Standard Reduction Potentials: Cu2+(aq) + 2 e– → Cu(s) Al3+(aq) + 3 e– → Al(s) E° = +0.340 volt E° = -1.676 volt 9 Terry’s First, and so far only, Law Just because something should occur doesn’t mean it will occur; and even if it does occur, it will not necessarily occur as rapidly as we might wish. 10 Cell Potential under any conditions: The Nernst Equation ∆G = -nFE and ∆G = ∆G° + RTlnQ 11 Start with the standard Cu/Ag cell. Allow a current of 1.0 amp to flow for 2.0 hours. (1 amp = 1 coulomb/sec) Calculate: (a) the change in mass for each electrode and (b) the cell potential, Ecell after the two hours have passed. 12 ...
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This note was uploaded on 03/29/2009 for the course CHEM 2080 taught by Professor Davis,f during the Spring '07 term at Cornell University (Engineering School).

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Chem208_Lecture_25_Color - Today: Electrochemistry (Ch. 20)...

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