Chapter 19 Electrochemistry Week 1 2009

Chapter 19 Electrochemistry Week 1 2009 - CHAPTER 19....

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CHAPTER 19. ELECTRON TRANSFER AND ELECTROCHEMISTRY ** Electrochemistry represents the interconversion of chemical energy and electrical energy. ** Electrochemistry involves redox reactions because electrical energy (flow of electrons) results from loss of electrons (oxidation) at one place and gain of electrons (reduction) at another place. Important questions: 1. Can we use the energy released ( G) when a spontaneous reaction occurs to do work? YES (common e.g. car, batteries, barbecue, etc) 2. Can we cause a non-spontaneous reaction to occur by supplying energy? YES We have already seen one way to accomplish 2, by supplying thermal energy (raising the temperature) if S > 0. Remember: G = H - T S if S > 0, then -T S < 0, and we can make a non-spontaneous reaction spontaneous by raising the temperature so that -T S term dominates. But, is there a convenient and general way capable of driving reactions in their non- spontaneous directions? How about supplying electrical energy? The answer is YES . ** Electrochemistry brings together, as a technique, several topics we have already studied: redox reactions, equilibrium, and thermodynamics. ** Electrochemistry influences many areas of our lives from batteries (car, flashlights, calculators, watches, and electric cars) to electroplating (e.g., surgical instruments), and many, many others SECTION 19.1. REDOX REACTIONS (REDUCTION/OXIDATION) Oxidation = removal of one or more electrons Reduction = addition of one or more electrons 1
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Oxidizing agent = substance that is doing the oxidizing (taking the electrons) Reducing agent = substance that is doing the reducing (giving the electrons) If something is oxidized, its oxidation state (ox. state) is increased. If something is reduced, its oxidation state decreases. Example . Consider adding copper metal to a solution of silver nitrate (AgNO 3 ) - the colorless solution turns blue and silver metal appears. The equation is Cu(s) + 2 Ag + (aq) Cu 2+ (aq) + 2 Ag(s) For the reaction left-to-right: Cu(s) is the reducing agent, giving electrons (e - ) to Ag + (aq) Ag + (aq) is the oxidizing agent, taking e - from Cu(s) Cu(s) is being oxidized (it is losing e - ) - ox. state Cu 0 increases to Cu 2+ Ag + (aq) is being reduced (it is gaining e - )-ox. state Ag + decreases to Ag 0 For the reaction right-to-left Ag(s) is the reducing agent, Cu 2+ (aq) is the oxidizing agent Ag(s) is getting oxidized, Cu 2+ (aq) is getting reduced. 2
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Note : The oxidizing agent is the substance being reduced. The reducing agent is the substance being oxidized. Balancing Redox Reactions We must keep track of the exchange of electrons among species in a redox reaction. The best way is to consider the two “half-reactions” occurring, one describing the oxidation,
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This note was uploaded on 05/30/2010 for the course CHM 2046 taught by Professor Veige/martin during the Spring '07 term at University of Florida.

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Chapter 19 Electrochemistry Week 1 2009 - CHAPTER 19....

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