Electrochem

Electrochem - Electrochemistry I. Terms and Definitions. A....

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1 Electrochemistry I . Terms and Definitions. A. Electrochemistry. 1. Study of the interconversion of electrical and chemical energy. Two processes, use electrical energy to bring about a chemical transformation, or can use a chemical reaction to do electrical work. 2. Cel l = device in which the interconversions take place. a. In an electrolysis cell one uses electrical energy, from a battery or generator, to bring about a nonspontaneous, usually endothermic, redox reaction. b. In a voltaic or galvanic cell one uses a spontaneous redox reaction to generate a voltage. B. Electrical Terms. 1. Charge (q) = coulomb (C) a. The magnitude of the electron charge = 1.6022x10 –19 C. b. The charge associated with 1 mole of electrons = Faraday (F) 1 F = (6.02x10 23 )(1.60x10 -19 ) = 96,500 coulombs. 2. Voltage (V, ε ) = the difference in electrical potential energy between two points. a. Basic unit is the volt (= joule coulomb ). b. If one moves a charge of 1 coulomb across a potential energy difference of 1 volt, will gain or expend 1 joule of energy. 3. Current (I) = measure of the rate of flow of charge through a body. a. 1 Ampere (Amp) = 1 coulomb of charge flowing past a point per second. b. Dimension = coulomb second c. Number of coulombs = (current in amps)x(time in seconds). Number of Faradays of charge = Number of coulombs 96500C/F 4. Resistanc e (R) = measure of the resistance to charge flow. a. A voltage difference will generate a current. If a substance has a high resistance, the current will be small for a given voltage. Ohm’s Law : V = IR b. Conductance = 1 R
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2 c. A good conductor will have a low resistance, while a good insulator will have a high resistance C. Types of conductors to be considered. 1. Metallic conductor - metals a. Charge carriers = electrons. b. Very low resistances. Resistance increases as temperature increases . 2 . Electrolytic conductors - molten salts and solutions of electrolytes . a. Charge carriers = cations and anions. b. Resistances higher than metals. Resistance decreases as temperature increases. II. Voltaic Cells. A. Introduction. 1. Consider the reaction Cu 2+ + Zn(s) -----> Zn 2+ + Cu(s) a. This is a spontaneous redox reaction whose G° = -212.3 kJ/mol b. Half reactions: Oxidation : Zn(s) -----> Zn 2+ + 2e- Reduction : Cu 2+ + 2e- -----> Cu(s) c. If the two half reactions could be physically separated so that the transfer of the two electrons from Zn to Cu 2+ takes place through a wire, then a current will flow and electrical work can be done. This is the principle behind all voltaic cells. A working voltaic cell is a battery. 2. The maximum electrical work that can be done by such a cell is – G. a. – G can be expressed in volts by the equation G = n F ε G is in J/mol of the balanced equation; n = the number of electrons transferred per balanced equation; F = Faraday constant (96500 Coulombs/mol electrons); = Voltage (J/Coulomb) b. Note that (nF ε ) is just a unit conversion factor—from joules/mol to volts.
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This note was uploaded on 04/07/2008 for the course CHEM 1304 taught by Professor Prof.maguire during the Spring '08 term at SMU.

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Electrochem - Electrochemistry I. Terms and Definitions. A....

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