Principles of Chemistry II © Vanden BoutTo d ayVoltage and EquilibriaPrinciples of Chemistry II © Vanden BoutHow do we know what the voltage is?Principles of Chemistry II © Vanden BoutThe voltage depends on the concentrations(we’ve all had dead batteries)Mix up “standard” concentrations1 M Zn2+ and 1 M Cu2+(note this is very concentrated)Principles of Chemistry II © Vanden BoutLet’s look at an actual cellPrinciples of Chemistry II © Vanden BoutOn which side of the cell are the electrons at a higher potential energy at these concentrations?! A.! ! the anode! B.! ! the cathode! C.! ! they are the samePrinciples of Chemistry II © Vanden BoutZn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)Zn(s) | Zn2+ || Cu2+ | Cu ReductionCathodeOxidationAnodeWe’ve made a1.1 V battery!Principles of Chemistry II © Vanden BoutPotential EnergyZn|Zn2+(1M)Cu2+(1M)|CuPrinciples of Chemistry II © Vanden BoutNow we can measure every possible combinationof electrochemical cells!What if I would like to predict the voltage from a cell for any reaction at standard conditions?First we need to think about potential energyPrinciples of Chemistry II © Vanden BoutWhat is my gravitational potential energy?zero if I am on the groundBut if a hole appears beneath me?then it is no longer zero Energy is relative!We pick where zero isPrinciples of Chemistry II © Vanden BoutWe need to pick a zero potential for electrochemistryWe chose this reaction2H+ + 2e- H2(g)note standard conditionswe pick this as E°= 0Vpotential energyNow compare everything to thisPrinciples of Chemistry II © Vanden BoutSo potential for Zn Zn2+ + 2e- is 0.76 VPrinciples of Chemistry II © Vanden Bout! A.! ! -0.76 V! B.! ! 0.76 V! C.! ! 0 V! D.! ! it can’t be measuredIf the potential for Zn Zn2+ + 2e- is 0.76Vwhat is the potential forZn2+ + 2e- ZnPrinciples of Chemistry II © Vanden BoutWrite everything as a reduction reactionPrinciples of Chemistry II © Vanden BoutPrinciples of Chemistry II © Vanden Bout! A.! ! Zn! B.! ! Fe! C.! ! Zn2+!D.!!Fe2+Given thatFe2+ + 2e- Fe -0.44 VZn2+ + 2e- Zn -0.76V which is easier to oxidize?Principles of Chemistry II © Vanden BoutHow to find E°cell?E°cell = E°cathode - E°anodeUse the reduction potential for both half reactionsThe number of electrons does not matteronly the half-reactionsPrinciples of Chemistry II © Vanden BoutHow to find E°cell?E°cell = E°cathode - E°anodeZn | Zn2+ || Cu2+ | CuZn2+ + 2e- Zn E°=Cu2+ + 2e- Cu E°=Principles of Chemistry II © Vanden BoutHow to find E°cell?E°cell = E°cathode - E°anodeCu | Cu2+ || Zn2+ | ZnZn2+ + 2e- Zn E°=Cu2+ + 2e- Cu E°=Principles of Chemistry II © Vanden BoutHow to find E°cell?E°cell = E°cathode - E°anodePt | H2(g) | 2H+ || O2 (g) | H2O | Pt2H+ + 2e- H2 E°=O2 + 4H+ 4e- 2H2O E°=Principles of Chemistry II © Vanden BoutWhat is cell notation for the following reaction?! A.! ! Ni2+ | Ni || Fe2+ | Fe3+ | Pt! B.! ! Ni | Ni2+ || Fe3+ | Fe2+ | Pt!C.!!Ni | Ni2+ || Fe2+ | Fe3+ | Pt!D.!!Ni | Ni2+ || 2Fe2+ | 2Fe3+ | Pt! E.! ! Ni | Ni2+ || 2Fe3+ | 2Fe2+ | PtNi(s) + 2Fe3+(aq) Ni2+(aq) + 2Fe2+(aq)Principles of Chemistry II © Vanden BoutWhat is E° for the following reaction?! A.! ! +0.54 V! B.! ! +0.77 V! C.! ! +1.0 V! D.! ! -1.0 V! E.! ! -0.54 VNi(s) + 2Fe3+(aq) Ni2+(aq) + 2Fe2+(aq)Principles of Chemistry II © Vanden BoutCould the following reaction make a battery?! A.! ! yes! B.! ! noNi(s) + 2Fe3+(aq) Ni2+(aq) + 2Fe2+(aq)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)Zn | Zn2+ || Cu2+ | CuPrinciples of Chemistry II © Vanden BoutWe'll look at standard concentrationsvolt meterX-1.1 V1 M Zn2+ (aq) and 1 M Cu2+ (aq) (note this is ridiculously concentrated)Cu | Cu2+ || Zn2+ | ZnPrinciples of Chemistry II © Vanden BoutWhat is voltage for the following reactionat equilibrium?! A.! ! 1.1 V! B.! ! zero! C.! ! -1.1 V! D.! ! something between 0 and 1.1 VZn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)Principles of Chemistry II © Vanden BoutRelationship between E and ∆G ∆G is energyE is electrical potentialElectric work (energy) is -charge x potentialwork = -charge x E∆G = workmax∆G = - charge x EmaxFrom now on we’ll now the Potential we calculate is the theoretical maximumReal world never actually that goodPrinciples of Chemistry II © Vanden BoutRelationship between E and ∆G ∆G = - charge x EWhat is the charge?charge = n X Fn is number of moles of electrons (per mole rxn)F is the charge of one mole of electrons F = 96,485 C (Faraday's Constant)∆G = - nFEPrinciples of Chemistry II © Vanden BoutOther concentrations and equilibriumLet’s remember
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