20_Electrochem2011

20_Electrochem2011 - ELECTROCHEMISTRY Balancing redox...

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141 Balancing redox reactions in acid or base solutions Electrochemical cells oxidation-reduction transfer of e s Half cells - cell reaction, cell potential Standard reduction potentials direction of redox reactions calculation of cell potentials G and cell potential criterion for reaction Nernst eqn - connection to equilibrium constant concentration cells Applications batteries lead acid corrosion
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142 Redox rxns involve a transfer of electrons eg. Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) 2 e s transfered Can we harness the transferred e s? Yes , but we must physically separate the oxidation and reduction half-reactions - An electrochemical cell uses the natural direction of a chemical reaction to generate useful electricity
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143 Why does the current flow? There is an electric potential difference a voltage - measured in volts Potential difference = work done on unit charge upon completing one passage around circuit 1 J = 1 V × 1 C What gives rise to the potential difference of an electrochemical cell & determines its value? A chemical driving force - Zn atoms get oxidized while Cu 2+ gets reduced a favorable chemical reaction the resulting cell potential, E cell , depends on T & the concentration of aqueous species if there are gases involved in the cell reaction, the gas partial pressures affect the cell potential 1 Joule work done 1 Volt potential difference 1 Coulomb of charge passed through circuit
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144 Under standard conditions ... T = 25 ° C, all concentrations = 1 M & all gas partial pressures = 1 atm We measure the standard cell potential , E ° cell eg. E ° cell (Zn Cu) = 1.10 V another eg.
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20_Electrochem2011 - ELECTROCHEMISTRY Balancing redox...

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