421-821-chapter-22

421-821-chapter-22 - Introduction to Electrochemistry A...

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Introduction to Electrochemistry A.) Introduction : 1.) Electroanalytical Chemistry: group of analytical methods based upon electrical properties of analytes when part of an electrochemical cell 2.) General Advantages of Electrochemistry: a) selective for particular redox state of a species e.g. Ce III vs. Ce IV b) cost - $4,000 - $25,000 for a good instrument compared to $10,000 - $50,000 - $250,000 for a good spectrophotometer c) measures activity (not concentration) activity usually of more physiological importance d) fast e) in situ f) information about: oxidation states stoichiometry rates charge transfer equilibrium constants
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B.) Types of Electroanalytical Methods
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C.) Electrochemical Cell : 1.) Basic Set-up: a) Two electrodes b) electrolytes solution c) external connection between electrodes (wire) d) internal connection via contact with a common solution or by different solutions connected by a salt bridge . salt bridge – acts to isolate two halves of electrochemical cell while allowing migration of ions and current flow. - usually consists of a tube filled with potassium chloride - separate species to prevent direct chemical reactions
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2.) Flow of current (charge) in cell: a) electrons (e - ) within wires between two electrodes bridge c) electrochemical reactions at electrode At Cu electrode: Cu 2+ + 2e - n Cu(s) reduction gain of e - net decrease in charge of species At Zn electrode: Zn(s) n Zn 2+ + 2e - oxidation – loss of e - net increase in charge of species electrons Zn Cu K Cl SO SO
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3.) Net Reaction in Cell – sum of reactions occurring in the two ½ cells Zn(s) n Zn + 2e Potential of overall cell = measure of the tendency of this reaction to proceed to equilibrium at equilibrium , potential (E cell ) = 0 ˆ Larger the potential, the further the reaction is from equilibrium and the greater the driving force that exists Similar in concept to balls sitting at different heights along a hill
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4.) Types of Cells: Galvanic Cells – reaction occurs naturally - positive potential (E cell = +) - exothermic produces energy Electrolytic Cells – reaction does not occur naturally, requires external stimulus (energy) to occur - negative potential (E cell = -) - endothermic requires energy Chemically Reversible Cell – a cell in which reversing the direction of the current simply reverses the chemical reaction Galvanic Cell Electrolytic Cell External battery at higher power than cell potential
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Electrodes: a.) Cathode – electrode where reduction occurs Anode – electrode where oxidation occurs b.) Examples of cathode ½ reactions: Cu 2+ + 2e - n nCu(s) Fe 3+ + e - n Fe 2+ AgCl(s) + e - n Ag(s) + Cl - - e - supplied by electrical current via electrode - species (products/reactants) can both be in solution (Fe 3+ /Fe 2+ ) solids or coated on electrodes (AgCl(s)/Ag(s) or combination (Cu 2+ /Cu(s) c.) Examples of anode ½ reactions: Cu(s)
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421-821-chapter-22 - Introduction to Electrochemistry A...

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