Week 12- Electrochemistry
Week 12- Electrochemistry 112
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This 3 page Class Notes was uploaded by BettyAnn Mead on Friday April 24, 2015. The Class Notes belongs to 112 at University of Massachusetts taught by Voigtman in Spring2015. Since its upload, it has received 91 views.
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Date Created: 04/24/15
Chemistry 112 042 52015 Electrochemistry Electron Transfer Reactions Consider what happens when calcium metal is added to water 0 A vigorous reaction spontaneously takes pkace 0 Gas H2 is evolved o A white precipitate occurs 0 Heat is evolved The reactions are 0 Ca 2H20 I CaOH2 H2 Product favored o CaOH2 I Ca 20H Reactant favored The first reaction is spontaneous exothermic AH lt 0 and AS gt 0 but relatively small So AG lt 0 but where did it go 0 Answer AG was wasted as heat Heat Thermal Energy Lowest Quality Energy Can NEVER be converted with 100 efficiency to higher quality forms of energy Electrical energy light mechanical energy are all higher quality than heat energy Electrical energy is BEST in terms ability to control and convert to light heat mechanical motion sound energy etc Some Electrical Notions An electrical current is a ow of charged particles In metal wires the charged particles that move in response to an applied potential difference are electrons The charge of one electron 16021e19 Coulomb One mole of electrons One equivalent of electrons The charge of one equivalent of electrons is 96485 Coulombs F Faraday Constant Charge of one equivalent electron 1 Joule 1 Volt 1 Coulomb 1 1V 1C 1Watt 1Volt 1Amp 1W 1V 1A 1Amp 1Coulomb second 1Amp 62418e18 electrons s 0 One equivalent of electrons reduces one mole of H ions to atoms In this chapter we want to see how chemical energy can be used to DIRECTLY produce electrical energy via controlled REDOX reactions and we want to see how electrical energy can DIRECTLY cause REDOX chemical reactions to occur even nonspontaneous 0 Chemical Electrical Voltaic Cells 0 Electrical Chemical Electrolytic Cells Review from Chemistry 111 Redox is short for quotreductionoxidation or quotoxidationreduction o Oxidation Loss of electrons 0 Reduction Gain of electrons o LEO the lion goes GER Loss of Electrons is Oxidation Gain of Electrons is Reduction 0 All redox reactions involve transfer of one or more electrons o Oxidizing agents TAKE one or more electrons from what they oxidized thereby themselves becoming reduced gaining one or more electrons Reducing agents have one or more electrons taken from them thereby becoming oxidized losing one or more electrons Oxidation Number Rules 0 Atoms in pure elements have an oxidation number of zero 0 For monatomic ions the oxidation number is equal to the ions charge Fluorine has an oxidation number of 1 except F2 0 Oxygen has an oxidation number of 2 in most compounds 0 Exceptions 1 When combined with F 2 Peroxides Oxidation number of 1 3 Superoxides Oxidation number of 12 0 Cl Br and l have oxidation numbers 1 except when combined with F or O or in interhalogen compounds ie lCl H has an oxidation number 1 except in metal hydrides where it is 1ieCaH2 The algebraic sum of oxidation numbers for the atoms in a neutral compound is zero CH3CH20H O l 2 H 1 C l 2 In a polyatomic ion the oxidation numbers must sum to the ions charge Balancing Redox Equations Half reactions Adjust stoichiometric coefficients to get mass balance of all elements other than hydrogen and oxygen Balance oxygen by adding H20 as needed In acidic solution ass H or H3O to balance hydrogen Add electrons e to achieve charge balance Electrons ost must be equal to electrons gained So multiply by factors to get equal numbers of electrons Add and cancel spectators If any H are present add H20 to both sides to make H30 Aternativey in basic solution add H20 or OH to balance hydrogen A balanced redox equation is the sum of two balanced half reactions the oxidation and reduction half reactions There are E leftover or missing electrons The oxidation and reduction half reactions are called half ce reactions when they are used in voltaic and electrolytic cells The overall redox reaction is then called the cell reaction The purpose of voltaic and electrolytic cells is to physically separate the half ce reactions so that AG can be electrical energy obtained or provided Measuring Cell Potentials Ecell Ec E2 0 Ec Cathode o Ea Anode Ce Standard Notation Oxidation at anode Reduction at cathode Electrochemistry Equations AG cell nFE ce o AG Free energy change under standard state conditions kJ mole o F Faraday constant 96485 CoulombsEquivalent o n Equivalentsmole reaction 0 E Standard cell potential in volts for full voltaic cell 1 equivalent of e number of electrons need to reduce 1 mole of H ions to H atoms n moles of electronsmole of redox reaction E ce E cathode E anode