CHAPTER 7 ELECTROCHEMISTRY THE UNION OF GIBBS FREE ENERGY ELECTRON FLOW AND CHEMICAL TRANSFORMATION Free Energy Electron Flow and Electrochemistry Any reaction can be expressed as Reactants Products Free Energy Electrochemistry draws together the concepts of oxidation reduction electron transfer free energy release and electrical potentials o Separates the flow of electrons physically from the reactants and products Oxidation Reduction Reactions Movement of electrons away from an atom or element oxidation process o Caused by an atom w greater electronegativity extracting electron density from a less electronegative atom Reduction transfer of electron density to or toward an atom or element Electrons involved are explicitly and completely transferred from the site of oxidation to the site of reduction o Electrons flow through external circuit and can be individually counted Cations ions Anions ions Oxidizing agent takes electrons from the reducing agent o Oxidation denotes a loss of electrons o Reduction denotes a gain of electrons o Oxidation number of oxidizing agent decreases o Reducing agent s oxidation number increases Clean copper in silver nitrate 2Ag aq Cu s 2Ag s Cu2 aq o Reduction of Ag o Oxidation of Cu o Original metal copper turns into blob of silver and copper o Exothermic but no usable energy other than heat released can be harnessed only a process of transfer of electrons Goal of electrochemistry is to extract electrical free energy o Must isolate two oxidation reduction half reactions in separate cells The Galvanic or Voltaic Cell Consists of two electrodes o First electrode dipped in solution has ability to release positive ions from surface into solution releases cations positive ions Electrode that releases cations ANODE oxidation occurs o Second electrode dipped in generally different solution that has the ability to acquire cations from the solution Electrode that acquire cations CATHODE reduction occurs When anode and cathode connected by a good conductor excess electrons leave the negatively charged anode and travel to positively charged cathode to restore charge balance o Will register a voltage when this happens Typical Galvanic Cell o Anode Zinc Zn in solution of Zn NO3 2 o Cathode Copper Cu in solution of Cu NO3 2 o To sustain electrical circuit must be a closed loop Accomplished by having a salt bridge or a tube containing a solution of KNO3 that dissociates into ions Connected by semi permeable membrane o Anode and cathode connected with conducting wire and voltmeter o Zn2 cations released from anode into solution excess electrons move from zinc Process of Galvanic Cell anode to external wire o K cations enter the anode solution to balance charge o NO3 anions enter cathode solution to balance charge o Electrons traveling on wire reach cathode Cu2 cations in solution combine with electrons on the interface of metal form Cu s on surface of cathode o Current flow of electrons from anode to cathode It is the ability of the cation of one metal to more strongly attract electrons than the cation of another metal that ultimately determine whether an electrochemical cell will produce electricity o Battle of cations to gain electrons No net charge build up in cell because of salt bridge ions of K and NO3 keep it balanced The Half Cell Reactions Electrons seek whatever pathway is available to them to find the lowest free energy state that they can o Generates a potential to move from anode to cathode E flow because there is an electrical potential difference generated between the two half cells of electrochemical cell Cell Voltage J Coulomb Reduction Potential ability to extract electrons by one element in competition with another Standard Reduction Potential E o E cell E cathode E anode Standard Hydrogen Electrode Need a standard way to compare combinations of anodes and cathodes Establish a reference value of zero for the reduction potential of that standard electrode Standard electrode is Hydrogen gas H2 o 2H aq 2e H2 g Impossible to have an electrode entirely of H2 use Pt catalyst Provides means to set a quantitative scale for anode cathode combinations Patterns in Standard Electrode Potentials reflect patterns in electronegativities in periodic table Calculation of the Cell Potential Determine anode or cathode via voltmeter positive side or consult table E cell Standard Reduction Potential of the electrode that gains electrons Standard Reduction Potential of the electrode that loses electrons E cell E cathode E anode W establishment of open path for electron flow made by connecting half cells tug of war is set up between the two half cells for electrons Calculated cell potential for a spontaneous reaction is ALWAYS POSITIVE Discussion of Electrodes Cathode material solution surrounding it doesn t have to be identical to the metal of the cathode Active vs Inactive Electrodes Inert electrode example is H2 and Pt doesn t appear in half reactions Only acts as a stable surface for reaction Notation for and Electrochemical Cell A Shorthand Technique Double vertical lines separates anode and cathode means salt bridge Single vertical line inert electrode put on side that its used Maximum Work from a Cell Gibbs Free Energy Embodied in the oxidation reduction reaction that constitutes the motive force in the electrochemical cell is an electromotive force Maximum amount of work G nFEcell o n moles of electrons o F 9 65 x 104 Coulombs mole o Ecell volts j coulomb Keq e nFE cell RT Death of an Electrochemical Cell The Nernst Equation Battery goes dead Ecell RT nF ln Keq goes to Keq o Just as G goes to zero when Q goes to Keq so too does Ecell go to zero as Q The Master Diagram Non Spontaneous Reactions Driving the Electrochemical Cell Uphill Electrolysis o Battery must have higher voltage than spontaneous chemical reaction o Drive current in opposite direction o Breakdown of water into oxygen and hydrogen Corrosion Rust Oxidation of metals is spontaneous when paired with the reduction of O2 in water that is in contact with air because the CO2 in the atmosphere reacts in water to form carbonic acid