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Unformatted text preview: Chem 260/ Lecture 35 F11 1 Chemistry 260 Lecture 35 November 30, 2011 Today in Chemistry 260 • electrochemical cells • reactions at electrodes • standard cell potentials • the electromotive force (E) • concentration dependencies (Nernst Equation) • batteries/fuel cells Later in Chemistry 260 • Next week Chs. 18.1, 18.2 • Chemical kinetics Electrochemistry: Electron transfer reactions Introduction. Cu Zn SO 4 2- Zn 2+---- SO 4 2- Zn 2+ + – Negative ions Postitive ions V – + – + Electrochemistry: Metal junction electron transfer Cu Zn Two different metals in contact: ions stay put, but electrons can move. Can we predict the direction? e – ionized Energy ! Zn ! Cu e – e – e – Cu Zn e – e – e – Cu Zn +++ --- ! Cu = 4.65 eV ! Zn = 4.33 eV Yes, We will see that this is related to The electrical potential to do work e – ionized ! Zn ! Cu Electrochemistry 2+ 2+ Zn( ) + Cu ( ) Zn ( ) + Cu( ) s a q a q s ⎯⎯→ ←⎯⎯ Immersing metal in metal ions solution Consider the reaction occurring on the Zn slab.. Electrochemistry 2+ 2+ Zn( ) + Cu ( ) Zn ( ) + Cu( ) s aq aq s ⎯⎯→ ←⎯⎯ Electrical ß à chemical, energy By oxidation – reduction reactions! Chem 260/ Lecture 35 F11 2 Oxidation-Reduction Reactions 2+ 2+ Zn( ) + Cu ( ) Zn ( ) + Cu( ) s a q a q s ⎯⎯→ ←⎯⎯ Oxidation process: A reduced species (Zn): • Can serve as a reducing agent by donating electrons. • It is oxidized. Reduction process: An oxidized species (Cu 2+ ) : • Can serve as an oxidizing agent by accepting electrons. • It is reduced. Electrochemistry: Electrical ß à chemical, energy By oxidation – reduction reactions! Electrochemical cells and Half cells reactions Electrochemistry: Electrical ß à chemical, energy By oxidation – reduction reactions! Electrochemical Cells Anode = Oxidation Cathode = Reduction Zn → Zn 2+ Cu 2+ → Cu Zn → Zn 2+ + 2 e – 2 e – + Cu 2+ → Cu Half reactions Cu Zn SO 4 2- Zn 2+---- -- e- SO 4 2- Zn 2+ Negative ions Postitive ions Anode Cathode – + OIL RIG : O xidation I s L oss, R eduction I s G ain LEO the lion says GER : L ose E lectrons: O xidation G ain E lectrons: R eduction 2+ 2+ Zn( ) + Cu ( ) Zn ( ) + Cu( ) s a q a q s ⎯⎯→ ←⎯⎯ Electrodes in a Galvanic/Voltaic Cell Cathode: Is where reduction occurs Is where electrons are l consumed z Is what cations migrate towards (Has a positive sign) Anode: Is where oxidation occurs Is where electrons are l produced z Is what anions migrate towards (Has a negative sign) Anode Cathode Cu Zn ---- -- e- Negative ions Postitive ions – + 1.10 V Half Reactions and Electrodes...
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