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Unformatted text preview: 4/24/2009 1 Electrochemistry Electrochemistry Electrochemistry Electrochemistry Cells, and the Nernst Equation Half-cells, 1 Last Update: 4/24/2009 6:48 PM Objective: Examine electrical consequences of some chemical reactions and derive some chemically useful information from electrical measurements Concepts: Oxidation / Reduction Electric potential Cell/Half Cell Nernst Equation Concentration Cell Reduction Potential Solubility Product Salt Bridge Basic Electrical Units Anode, Cathode Techniques: Constructing Cells Measuring Voltage Apparatus: pH Meter Salt Bridge 2 Basic Electrical Units &amp; Concepts: Basic Electrical Units &amp; Concepts: Units Charge, q: coulomb Potential, V: volt Current, i: ampere = coulomb/sec Energy, E: joule = watt-second = volt-coulomb Power, P: watt = volt-ampere Resistance, R: ohm = volt/ampere Ohms Law i = V / R 1 volt The resistor will give off energy at 1 watt + 1 ohm 1 ampere 1 volt of potential applied to a resistance of 1 ohm will cause 1 ampere to flow. and dissipate 1 joule of energy per second. 3 Michael Faraday defined the cathode as the electrode to which cations flow (positively charged ions, like silver ions Ag + ), to be reduced by reacting with (negatively- charged) electrons on the cathode. Likewise he defined the anode as the electrode to which anions flow (negatively charged ions, like chloride ions Cl- ), to be oxidized by depositing electrons on the anode anode. Thus, (positive) electric current flows from the cathode to the anode. To an external wire connected to the electrodes of a battery, the cathode is positive and the anode is negative and: electrons flow from the anode to the cathode. 4 One more unit One more unit - The Faraday The Faraday - A Chemical unit A Chemical unit If we agree to include electrons in chemical equations, we need to be able to account for them stoichiometrically, just as we do with moles of other chemical substances. To that end, we define the Faraday, F , to be the (absolute value) of the charge on Avogadros number (1 mole) of electrons: How many Faradays of charge must flow to keep a 100 watt bulb (@ 110 v) lit for 1 day? 1 F = Ne = 6.022141 X 10 = Ne = 6.022141 X 10 23 23 mol mol 1 X 1.6021765 X 10 1.6021765 X 10 19 19 C 100 watt X 1 day X 86,400 sec/day 110 volts = 78.5 X 10 3 coulombs = 0.814 F = 96,485.3383 C/mol = 96,485.3383 C/mol (volt-coulomb/sec) 5 Voltages, Equilibrium Constants and Voltages, Equilibrium Constants and...
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