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Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010 This course is approximately at this level CHEMISTRY E CH9 Electrochemistry

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ELECTROCHEMISTRY CH9 Ohm's law where R is resistance [ ], I is current and U is voltage [V]. Basic concepts Faraday's law where n is the amount of reacted substance [mol], I is current [A], t is time [s], z is the number of electrons in the electrode reaction and F is the Faraday constant [96487 C/mol]. Remark: 1 Coulomb is electrical charge of × electrons. 1 Ampere is electrical current of 1C/second. 1mole of reactant releases 6.02x10 23 z electrons, therefore 6.02x10 23 z/ × =96487z Coulombs.

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Galvanic cell CH9 Zn Zn ++ Zinc electrode submerged into an electrolyte has the tendency to be dissolved (oxidised) into cations. Free electrons are produced. Cu Cu ++ Copper electrode submerged into electrolyte absorbs (reduces) cations and neutral Cu. There is a tendency to push out electrons from electrode, manifested by electromotive force (potential) There is a tendency to reduce cations from electrolyte. ANODE CATHODE Negative potential is necessary if K + or Zn ++ would be reduced Overpressure of electrons Underpressure of electrons

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GALVANIC Daniel CELL CH9 Anode (Zn is dissolved into electrolyte) oxidation Cathode (Cu++ is removed from electrolyte) reduction Anions Cl- compensate charge of dissolved Zn++ anions SO 4 -- flow through porous plug

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Lemmon cell CH9

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ELECTROCHEMISTRY CH9 The dependence of an electrode potential on the concentration of reduced and/or oxidised substances is given by the Nernst equation, which relates the potential in a non-standard state, the standard potential and the reaction quotient for the reaction. For the reduction half-reaction the Nernst equation has the form For the cell reaction the Nernst equation has the form

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Tutorial Daniel cell CH9 Calculate the potential at 25 C for the Daniell cell. The Cu 2+ concentration is 2 mol.dm -3, the Zn 2+ concentration is 0.5 mol.dm -3. cathodeE red 0 =0.337 V anode E ox 0 =0.763 V sumE cell 0 =1.10 V The standard cell potential is 1.1 V. The cell does not operate under standard conditions, so we have to use the Nernst equation to determine the dependence of the cell potential on the concentrations of reactants and products. Cell potential will decrease with decreasing concentration of copper ions and increasing concentration of zinc ions.

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