Principles of Chemistry II © Vanden BoutTodayStandard PotentialsFiguring out which way the electrons"want to go" Principles of Chemistry II © Vanden Bout Principles of Chemistry II © Vanden BoutVoltmeter measures the voltage (or potential) across the anode and the cathodeIf the voltage is positive then electrons will flow from anode to cathode Principles of Chemistry II © Vanden BoutOxidationZn -> Zn2+ + 2 e-ReductionCu2+ + 2e- -> CuPrinciples of Chemistry II © Vanden BoutShorthand notation for electrochemical cell (Ecell)AnodeZn --> Zn2+ + 2e-Zn | Zn2+Saltbridge|| CathodeCu2+ + 2e- --> CuCu2+ | Cu Principles of Chemistry II © Vanden BoutIn the following Ecell, what is being reduced?! A.! ! Zn (s)! B.! ! Zn2+ (aq)! C.! ! H+ (aq)! D.! ! H2 (g)!Zn(s) | Zn2+ || H+ | H2 (g)Reduction reaction at the cathode H+ is reduced to H2 (g) Principles of Chemistry II © Vanden BoutHow can we figure out the voltage acrossand electrochemical cell?volt meterX??????The voltage (potential) depends on the concentrations of the solutions Principles of Chemistry II © Vanden BoutWe'll look at standard concentrationsvolt meterX1.1 V1 M Zn2+ (aq) and 1 M Cu2+ (aq) (note this is ridiculously concentrated)Principles of Chemistry II © Vanden BoutPotential is the result of potential forboth the oxidation and reduction reactionWe can't go measuring every possible combination ofreactions in the worldWe need another standardWe'll choose this reaction2H+ + 2e- ----> H2 (g) Principles of Chemistry II © Vanden BoutThis will be our "zero" Principles of Chemistry II © Vanden BoutElectrical potential is potential energyWe need to measure it relative to somethingPotential energy of the chicken is relativeWe say it is zero because we are used tomeasuring it relative to the groundBut clearly it is not zero relative to the center of the earth Principles of Chemistry II © Vanden BoutWe define a standard half cell potential asthe potential of that reaction in an electrochemicalcell with a cathode of 2H+ + 2e- ---> H2(g)So for Zn(s) --> Zn2+ + 2e- E° is 0.76 VPrinciples of Chemistry II © Vanden BoutStandard Reduction PotentialsWe write all standard potentials as reduction reactions2H+ (aq) + 2e- ---> H2(g)Zn2+ (aq) + 2e- ---> Zn(s)E° = 0.0 Vby definitionE° = -0.76VZn (s) ---> Zn2+(aq) + 2e- E° = +0.76VThis is the standard reduction potential for Zn2+ | Zn(s) Principles of Chemistry II © Vanden Bout Principles of Chemistry II © Vanden BoutEasy to reduce(Strongest oxidizing agents)Easy to oxidize(strongest reducing agents) Principles of Chemistry II © Vanden BoutGiven the following standard reduction potentialswhich is easier to oxidize?! A.! ! Zn(s)! B.! ! Fe(s)! C.! ! they are the same!Zn2+ + 2e- ---> Zn(s) E° = -0.76Fe2+ + 2e- ---> Fe(s) E° = -0.44the more negative the reduction potentialthe easier it is to oxidizePrinciples of Chemistry II © Vanden BoutHow do we the potential for the standard cell?Need half reactions for anode and cathodeZn | Zn2+ || Cu2+ | CuAnodeZn2+ + 2e- ---> Zn(s) E°anode = -0.76CathodeCu2+ + 2e- ---> Cu(s) E°cathode = +0.34E°cell = E°cathode - E°anode = (0.34) - (-0.76) = +1.10 V POSITIVE POTENTIAL IS SPONTANEOUS REACTION AS WRITTEN (you could make a battery) Principles of Chemistry II © Vanden BoutA moment to think again aboutFree Energyand Standard Free EnergyE° and "G° are related!Measuring the cell potential is measuring the"chemical potential" or Gibb's free energyWe need to "convert" fromvolts to J Principles of Chemistry II © Vanden
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