Principles of Chemistry II © Vanden BoutTo d ayVoltage and Equilibria (again)Batteries and Fuel CellsElectrolytic CellsPrinciples 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 BoutWhat about other concentrations?volt meterX?????[Zn2+] ≠1M and [Cu2+] ≠ 1M ???Principles 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 BoutWhat is voltage for the following reactionif [Cu2+] = 10-4 M and [Zn2+] =1.9 M! 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 = - charge x E∆G = - nFEPrinciples of Chemistry II © Vanden BoutOther concentrations and equilibriumLet’s remember equilbrium!∆G = ∆G° + RTlnQPrinciples of Chemistry II © Vanden BoutOther concentrations and equilibriumLet’s remember equilbrium!∆G = ∆G° + RTlnQat equilibrium ∆G = 0so ∆G° = -RTlnK-nFE = -nFE° + RTlnQE = E° -RTnFlnQE = E° -0.0591nlogQlog!assume 25°CPrinciples of Chemistry II © Vanden BoutWhat about other concentrations?volt meterX?????10-3 M Zn2+ (aq) and 10-1 M Cu2+ (aq) ???Principles of Chemistry II © Vanden BoutZn(s) + Cu2+(aq) Zn2+ (aq) + Cu(s)1 M Zn2+ (aq) and 1 M Cu2+ (aq) standardPrinciples of Chemistry II © Vanden BoutZn(s) + Cu2+(aq) Zn2+ (aq) + Cu(s)1 M Zn2+ (aq) and 1 M Cu2+ (aq) standardQ = [Zn2+][Cu2+]=11= 1E = E° -0.0591nlogQE = 1.10 V -0.05912log(1) = 1.10VPrinciples of Chemistry II © Vanden BoutZn(s) + Cu2+(aq) Zn2+ (aq) + Cu(s)10-3 M Zn2+ (aq) and 10-1 M Cu2+ (aq) ???Principles of Chemistry II © Vanden BoutZn(s) + Cu2+(aq) Zn2+ (aq) + Cu(s)10-3 M Zn2+ (aq) and 10-1 M Cu2+ (aq) ???Q = [Zn2+][Cu2+]=(10-3)(10-1)= 10-2E = E° -0.0591nlogQE = 1.10 V -0.05912log(10-2) = 1.16VPrinciples of Chemistry II © Vanden BoutE = E° -0.0591nlogQCurrent will flow until E = 0EquilibriumE° = +0.0591nlogKlogK = nE°0.0591Principles of Chemistry II © Vanden BoutWhat will happen to the voltage if I lower the Zn2+ concentration?! A.! ! the voltage will increase! B.! ! the voltage will decrease! C.! ! the voltage will stay the sameZn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)Principles of Chemistry II © Vanden BoutWhat about this cell?volt meterX?????10-4 M Zn2+ (aq) and 1 M Zn2+ (aq) ???Zn | Zn2+ || Zn2+ | ZnPrinciples of Chemistry II © Vanden BoutDoes this cell have a non-zero voltage?! A.! ! yes! B.! ! noZn | Zn2+ (10-4 M) || Zn2+ (1 M) | ZnPrinciples of Chemistry II © Vanden BoutWhich side has the lower free energy?! A.! ! 1 M solution! B.! ! 10-4 M solution! C.! ! they are the same (its at equilibrium)Zn | Zn2+ (10-4 M) || Zn2+ (1 M) | ZnPrinciples of Chemistry II © Vanden BoutWill electrons flow spontaneously to the cathode? ! A.! ! yes! B.! ! noZn | Zn2+ (10-4 M) || Zn2+ (1 M) | ZnPrinciples of Chemistry II © Vanden BoutWill the potential for this cell be positive? ! A.! ! yes, E > 0! B.! ! no, E < 0 C. it is the same reaction so E = 0Zn | Zn2+ (10-4 M) || Zn2+ (1 M) | ZnPrinciples of Chemistry II © Vanden BoutZn | Zn2+ (10-4 M) || Zn2+ (1 M) | ZnPrinciples of Chemistry II © Vanden BoutQ = [Zn2+]anode[Zn2+]cathode=10-41= 10-4E = E° -0.0591nlogQSame reaction! E° = 0 VE = 0 V -0.05912log(10-4) = 0.118Veach factor of ten will be another 0.0591 VZn | Zn2+ (10-4 M) || Zn2+ (1 M) | ZnPrinciples of Chemistry II © Vanden BoutTake home message!Voltage is a direct measure of the free energyTherefore it is a direct measure of Q!If you set up a system where one half of the cell is known,the the other half can be used as a sensor!Principles of Chemistry II © Vanden BoutLet’s look at this reaction2H2O(l) 2H2(g) + O2(g)E°cell = -2.06 V Not spontaneous, but if we apply a voltage > 2.06 we can force the reaction to go!Principles of Chemistry II © Vanden BoutPrinciples of Chemistry II © Vanden BoutYou reduce H+ to H2 in an electrochemical cell.Your cell has a current of 1 Amp for 10 minutesWhat is the total charge that is passed through the cell?! A.! ! 1 C! B.! ! 10 C! C.! ! 600 C! D.! ! 6000 CPrinciples of Chemistry II © Vanden BoutYou reduce H+ to H2 in an electrochemical cell.Your cell has a current of 1 Amp for 10 minutesHow many moles of electrons pass through the cell?! A.! ! 600 C / F! B.! ! 600 C x F! C.! ! 1 A x FPrinciples of Chemistry II © Vanden BoutYou reduce H+ to H2 in an electrochemical cell.The number of moles of electrons that pass through the cell is 6.2 x 10-3 . How many moles of H2 are formed?! A.! ! 6.2 x 10-3! B.! ! 3.1 x 10-3! C.! ! 1.2 x
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