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Experiment 2 - Electrochemistry

Experiment 2 - Electrochemistry - CHEM 257 Experiment 2...

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CHEM 257 Experiment 2 Electrochemistry and Oxidation-Reduction An electrochemical cell is composed of a conducting solution (the term electrolyte is used to refer both to this solution and to the ionic solute which it contains) and two electrodes which dip into the electrolyte. Oxidation occurs at one electrode (the anode ) and reduction at the other (the cathode ). When a metal electrode is inserted into a solution of its ions, an equilibrium is set up between the metal atoms in the electrode and the metal ions in solution. Me ++ + 2 - + + - or The metal ions formed at the electrode surface diffuse away into the solution; the electrons liberated in the process remain in the electrode and render it more negative. Conversely, if ions are reduced to the metal at the electrode-solution interface, electrons are consumed and the electrode becomes more positive. Because the equilibrium constants are different for different metal-metal ion half-cells, there will be a potential difference between the electrodes of a cell whose electrodes involve different half-cell reactions. This produces the voltage which is measured in this experiment. Cell potentials are related to the corresponding free energy changes by the relationship: G = -n FE where: G is the free energy change in Joules/mol n is the number of electrons involved in the cell reaction F is the Faraday, 96,500 coulombs per equivalent, joule/volt/mol 1 coulomb = 1 joule/volt E is the cell potential in volts. A positive cell potential corresponds to a negative free energy change and a spontaneous cell reaction. The equilibrium position of a metal-metal ion is best characterized by its standard reduction potential , E 0 . This is the potential observed for the cell. Me|Me + (a=1.0), H + (a=1.0)|H 2 (1 atm), Pt or Me|Me ++ (a=1.0), H + (a=1.0)|H 2 (1 atm), Pt with the sign of the half-cell being that of the metal vs . the platinum of the hydrogen electrode. E 0 for the hydrogen reduction is defined to be zero. The standard cell potential for an electrochemical cell is the sum of the two standard half-cell potentials of the half-cells that make up the cell. For example, the cell: Zn|Zn ++ (a=1.0)||Cu ++ (a=1.0)|Cu
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CHEM 257 Experiment 2 :Page 2 is composed of Zn|Zn ++ and Cu ++ |Cu half-cells. Zn → Zn ++ + 2e - Oxidation occurs at the E 0 Zn,Zn ++ = -(-0.76 volts) negative electrode minus sign due to oxidation Cu ++ + 2e - → Cu Reduction occurs at the E 0 Cu ++ ,Cu = +0.34 volts ___________ positive electrode number of electrons must be equal _________ Zn + Cu ++ → Zn ++ + Cu Overall reaction E 0 cell = +1.10 volts When all reactants activities are not at equilibrium, the cell potential is given by the following equations: G = G 0 + RT ln Q note if Q = 1 then G = G 0 -n FE = -n FE 0 + RT ln Q since G ≡ - n FE and G 0 ≡ - n FE 0 (definition) Q is the reaction quotient and is the ratio if the activities of product ion to the reactant ion in this case.
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Experiment 2 - Electrochemistry - CHEM 257 Experiment 2...

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