Principles of Chemistry II © Vanden BoutTodayPotentials vs ConcentrationRelationship between !G and EElectrolytic CellsForcing the "non-sponteneous" Reaction Principles of Chemistry II © Vanden BoutFirst some nomenclatureGalvanic CellVoltaic CellBatterySpontaneous!G<0E > 0volt meterE>0Anode CathodeCathode at a higher potential than the anodeSo cathode get the PLUS sign+- Principles of Chemistry II © Vanden BoutFirst some nomenclatureElectrolytic CellNon-Spontaneous!G>0E < 0volt meterE<0Anode CathodeAnode at a higher potential than the cathodeSo anode get the PLUS sign-+ Principles of Chemistry II © Vanden BoutIn the following standard Ecell, what is the sign of the cathode?! A.!! +! B.!! -! C.!! neither E°cell = 0Zn(s) | Zn2+ || H+ | H2 (g)Zn2+ + 2e- ---> Zn(s) E° = -0.76 V2H+ + 2e- ---> H2 E° = 0.0 VPrinciples of Chemistry II © Vanden BoutIn the following standard Ecell, what is the sign of the cathode?! A.!! +! B.!! -! C.!! neither E°cell = 0Zn(s) | Zn2+ || H+ | H2 (g)Zn2+ + 2e- ---> Zn(s) E° = -0.76 V2H+ + 2e- ---> H2 E° = 0.0 VE°cell = E°cathode - E°anode = 0 - (-0.76) = +0.76 VVoltaic Cell therefore cathode + 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 BoutWhat about other concentrations?volt meterX?????10-3 M Zn2+ (aq) and 10-1 M Cu2+ (aq) ??? Principles of Chemistry II © Vanden BoutRelationship between E and !G !G is energyE is electrical potentialElectric work (energy) is -charge x potentialwork = -q x E!G = workmax!G = - q x EmaxFrom now on well now the Potential we calculate are the theoretical maximumReal world never actually that goodPrinciples of Chemistry II © Vanden BoutRelationship between E and !G !G = - q x EWhat is the charge q?q = n X Fn is number of moles of electronsF is the charge of one mole of electrons F = 96,485 C (Faraday's Constant)!G = - nFE Principles of Chemistry II © Vanden BoutHow do we measure q in the lab?Current = Charge/TimeAmpere (amp) = Coulomb (C)/ Second (s)If we have the total charge in Coulombwe can figure out how many moles of electrons using F Principles 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 C1 A x (10 min) x (60 s min-1) = 600 C Principles 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 FF is C mol-Therefore the number of moles of electrons is q/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 10-22H+ + 2e- -----> H2(g)For every mole of H2 you need two moles of electrons Principles of Chemistry II © Vanden BoutYou reduce H+ to H2 in an electrochemical cell.Your cell has a current of 1 Amp for 10 minutes. How many moles of H2 are formed?! A.!! 6.2 x 10-3! B.!! 3.1 x 10-3! C.!! 1.2 x 10-22H+ + 2e- -----> H2(g)For every mole of H2 you need two moles of electrons Principles of Chemistry II © Vanden BoutOther concentrations and equilibrium!G = !G° + RTlnQat equilibrium !G = 0so !G° = -RTlnK-nFE = -nFE° + RTlnQE = E° -RTnFlnQE = E° -0.0591nlogQlog!assume 25°C Principles 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)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) = 1.16V Principles of Chemistry II © Vanden BoutE = E° -0.0591nlogQCurrent will flow until E = 0EquilibriumE° = +0.0591nlogKlogK = nE°0.0591 Principles of Chemistry II © Vanden BoutConcentration Differences will lead to potential difference Principles of Chemistry II © Vanden BoutIf E < 0, then the reaction can be force in the non-spontaneous direction by applying a potential greater than E to the
View Full Document