Chem 6c chap 12 week 1

Chem 6c chap 12 week 1 - Chemistry 6C Spring Week 1 Bill...

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Chemistry 6C Spring Week 1 Bill Trogler
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Course Web Page, Check Weekly http://chem- courses.ucsd.edu/CoursePages/LowerDiv/6C_Trogler/
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Review Redox Rxns F77-83 Oxidant + Reductant Ox(+n e) + Red(-n e) Generally acid-base reactions have small energy changes, as compared to redox reactions. Therefore, energetic reactions involved in fuels, explosions, rockets, batteries, etc. are redox. 1783 Luigi Galvani(frogs legs) 1796 Alessandro Volta (battery) e Reductant Oxidant
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Galvanic Cells A clever trick to separate reduction and oxidation into separate compartments to avoid direct reaction between the oxidant and reductant. This uses an anode as the conducting electrode where oxidation occurs and a cathode as the electrode where reduction occurs. The anode and cathode are conducted by a wire through which electrons flow and are used to do electrical work as they flow from reductant to oxidant.
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Key Features of Galvanic Cells 1) Physical barrier (e.g. porous separator or salt bridge) to prevent direct mixing of oxidant and reductant. 2) Conducting electrolyte (ionic solution, ionic conducting polymer) to allow charged ions produced at anode and cathode to move away from electrodes and prevent charge buildup. 3) Conducting electrodes, which may or may not be directly involved in redox reactions (e.g. Pt is an inert electrode but Zn often is oxidized in Galvanic cells) 4) Electrical connection between the anode and cathode which allows readout of the cell voltage E.
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Zn/Cu Galvanic Cell
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Zn/Cu Galvanic Cell Observations Net Reaction: Zn (s) + Cu 2+ (aq) Cu (s) + Zn 2+ (aq) Left compartment: anode Zn (s) Zn 2+ (aq) + 2e Right compartment: cathode Cu 2+ (aq) + 2e Cu (s) Cell Voltage is 1.10 Volts at standard conditions, 25 C, 1 M concentrations, 1 atm partial pressures of gases.
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Zn/Cu Galvanic Cell Quantitative Aspects Due to Michael Faraday 1797-1867 How to relate physics units to chemical units? In electrolysis experiments Faraday showed that 96485 Coulombs = 1 mole of charge, which is called Faraday’s constant 96485 C/mole chg. Also recall from physics: current I in amps and 1 amp = 1C/sec. Suppose the Zn/Cu cell runs at 0.01 amps for 30 min. How much Cu metal is deposited on the cathode? Use dimensional analysis: [0.01 C/s]/[96485 C/mole chg] = 1.04 x 10 -7 moles chg/s In 30 min (1800 s) the moles of chg flowing = (1800 s) 1.04 x 10 -7 moles chg/s = 1.87 x 10 -4 moles chg Since 2 moles of charge are needed to reduce 1 mole of Cu 2+ to Cu, only ½ mole of Cu forms per mole of charge or 9.36 x 10 -5 moles of Cu or on multiplying by the MW 0.0059 g Cu.
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Energetic Considerations G = maximum work a system can perform ( G o at stand. cond.)
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This note was uploaded on 06/12/2008 for the course CHEM 6C taught by Professor Hoeger during the Spring '08 term at UCSD.

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Chem 6c chap 12 week 1 - Chemistry 6C Spring Week 1 Bill...

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