Chap11_S09-2 - ELECTROCHEMISTRY Electrochemistry deals with...

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ELECTROCHEMISTRY Electrochemistry deals with chemical changes produced by electric current and with the production of electricity by chemical reactions. Electrochemical reactions are oxidation-reduction reactions. The reacting system is contained in a cell. Electrochemical cells are devices in which an electric current is produced as a result of a spontaneous chemical reaction, or is used to cause a non-spontaneous chemical reaction to occur. Therefore we have two types of electrochemical cells: galvanic cells and electrolytic cells. GALVANIC CELLS (VOLTAIC CELLS) These are cells in which spontaneous oxidation-reduction reactions produce electrical energy. The two halves of the redox reactions are separated and the electron transfer occurs through an external circuit. WHAT DOES A GALVANIC CELL CONSIST OF? Each half-cell consists of the oxidized and reduced forms of a species in contact with each other. A common kind of a half-cell consists of a piece of metal immersed in a solution of its ions. Electrical contact between the two electrodes is frequently made by a wire and a salt bridge. A common kind of salt bridge consists of a piece of glass tubing bent into a U-shape filled with hot saturated salt (5 % agar solution) and allowed to cool. The cool mixture “sets” out to the consistency of gelatin, which keep the solution from running out when the tube is inverted; all while enabling the ions to move. A salt bridge serves three functions: 1. It makes electrical contact between the two solutions, which completes the circuit. 2. It maintains electrical neutrality in each half-cell. 3. It prevents the mixing of the two half-cell solutions. NOTE: A cell in which all reactants and products in their thermodynamic standard states (1 M for dissolved species and 1 atm partial pressure for gases) is called a standard cell. 1
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THE ZINC-COPPER CELL ( SEE FIG . 11.6) This cell consists of a strip of copper immersed in a 1.0 M copper (II) sulfate, CuSO 4 , solution and a strip of a zinc immersed in 1.0 M zinc sulfate, ZnSO 4 , solution. A wire and a salt bridge (or a porous disk in a tube) complete the cell. We observe the following: 1. The initial voltage is 1.10 volts. 2. The mass of the zinc electrode decreases. The concentration of Zn 2+ increases in the solution around the zinc electrode as the cell operates. 3. The mass of the copper electrode increases. The concentration of Cu 2+ decreases in the solution around this electrode as the cell operates. Anode: (oxidation) Zn Zn 2+ + 2e - Cathode: (reduction) Cu 2+ + 2e - Cu Overall reaction Zn + Cu 2+ Zn 2+ +Cu Electrons are released at the anode and consumed at the cathode. In all voltaic cells, electrons flow spontaneously from the negative electrode (anode) to the positive electrode (cathode). In contrast with electrolytic cells, in a voltaic cell, the anode is negative and the cathode is positive. To maintain electrical neutrality and complete the circuit, two Cl
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Chap11_S09-2 - ELECTROCHEMISTRY Electrochemistry deals with...

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