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Examples of Galvanic Cells Analysis Examples of Galvanic Cells Analysis A quote of the week (or camel of the week): A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing. George Bernard Shaw Physical Chemistry EPM/08 1 Examples of galvanic cells analysis From the structure of the cell, the reactions may be deduced: Zn(s)|ZnCl 2(aq)|AgCl(s)|Ag(s) Cathode (right): AgCl(s) + e – = Ag 0(s) + Cl –(aq) Anode (left): Zn 0(s) = Zn +2 (aq) + 2e – Overall (cell): 2AgCl(s) + Zn 0(s) = 2Ag 0(s) + Zn +2 (aq) + 2Cl –(aq) = ⋅ 2 = 3 Assuming activity coefficients are equal to 1: Q c 2+ c − 4c Zn Cl ZnCl 2 0 = 0 − 0 0 RT E E − E 2+ E = E − ln Q cell Ag |AgCl |Cl Zn |Zn cell cell 2F Physical Chemistry EPM/08 2 Examples of galvanic cells analysis (2) From the overall reaction, the half-reactions and cell structure may be deduced: – + – +2 3I (aq) + MnO 2(s) + 4H (aq) = I 3 (aq) + Mn (aq) + 2H 2O(l) + – +2 Cathode (reduction): MnO 2(s) + 4H (aq) + 2e = Mn (aq) + 2H 2O(l) – – – Anode (oxidation): 3I (aq) = I 3 (aq) + 2e – – +2 + Cell diagram: Pt| I 3 (aq),I (aq)||Mn (aq),H (aq)|MnO 2(s)|C ⋅ cMn 2+ cI − Q = 3 Assuming activity coefficients are equal to 1: 3 ⋅ 4 cI − cH + 0 0 0 0 RT = + + − − − = − Ecell EMnO |Mn 2 ,H EI |I Ecell Ecell ln Q 2 3 2F Physical Chemistry EPM/08 3 Equilibrium constant and cell potential Once more moving back to the study of chemical equilibria: ∆ = ∆ 0 + Gr Gr RT ln Q we can find that at equilibrium hence ∆ = = ∆ 0 = − Gr 0 and Q K Gr RT ln K 0 0 At the same time: ∆ = − Gr zFE zFE 0 0 = = RT Therefore: zFE RT ln K and K e Physical Chemistry EPM/08 4 Faraday law The law says that mass of an electrolysis product at a given electrode is directly proportional to the charge passed across the electrode and molar mass of the product. M m = k ⋅q = k∫ (ti )dt = ∫ (ti )dt zF 1 Number of moles of product is n = ∫ (ti )dt equal to: zF In well defined half-cells the law is obeyed exactly. For many years before the SI system, Coulomb was the basic electrical unit and it was defined using Faraday law for a silver/silver nitrate cathode. Physical Chemistry EPM/08 5 Primary batteries Leclanché cell: carbon rod with a copper cap (cathode) E0=1,5-1,6V seal zinc housing (anode) MnO 2 paste electrolyte paste Physical Chemistry EPM/08 6 Primary batteries (2) Zn(s)|NH 4Cl(aq)|MnO 2(s)|C A: Zn(s) = Zn 2+ (aq) + 2e – – – C: 2MnO 2(s) + 2H 2O(l) + 2e = 2MnOOH(s) + 2OH (aq) 2+ – E: Zn (aq) + 2NH 4Cl(aq) + 2OH (aq) = Zn(NH 3)2Cl 2(aq) + 2H 2O(l) O: 2MnO 2(s) + Zn(s) + 2NH 4Cl(aq) = 2MnOOH(s) + Zn(NH 3)2Cl 2(aq) This is Leclanché cell chemistry for normal discharge rate. Batteries with ZnCl 2 electrolyte are also known. Alkaline batteries: Zn(s)|NaOH(aq)|MnO 2(s)|C 2MnO 2(s) + Zn(s) + 2H 2O(l) = 2MnOOH(s) +Zn(OH) 2(s) Physical Chemistry EPM/08 7 Secondary batteries R Lead battery mV E0 = 2,14 V Gaston Planté, 1859 Pb PbO 2 H2 SO 4 ; 36% Physical Chemistry EPM/08 8 Secondary batteries (2) discharging → + + − Pb + H 2SO 4← PbSO 4 + 2H e2 charging discharging → + - ← PbO 2 + H 2SO 4 + 2H + 2e charging PbSO 4 + 2H 2O discharging → PbO 2 + Pb + 2H 2SO 4← 2PbSO 4 + 2H 2O charging Physical Chemistry EPM/08 9 Secondary batteries (3) Other secondary batteries: Nickel-cadmium battery, E 0=1,26 V Thomas Alva Edison discharging → 2NiOOH + Cd + 2H 2O← 2Ni(OH) 2 + Cd(OH) 2 charging lithium battery (non-aqueous), E 0<4 V discharging → nLi + MO 2← Li nMO 2 charging Physical Chemistry EPM/08 10 Fuel cells Traditional (conventional) way of production of electric energy: chemical energy → heat → mechanical energy → electric energy fuel combustion steam rotation current furnace boiler turbine generator Overall efficiency is low, esp. the heat → work (mechanical energy) transition is strictly limited by the II law of thermodynamics. The new approach, environmentally friendly to some extent, though also afflicted by some weaknesses is using fuel cells (FCs). Physical Chemistry EPM/08 11 Fuel cells (2) William Robert Grove First fuel cell 1118111811811811--- 18181896 18 969696 TheTheTheLondon InstitutionInstitution, , 1818183918 393939 Physical Chemistry EPM/08 12 Fuel cells (3a) Almost any reaction may be carried out in a galvanic cell !!! Anode(–): Zn 0=Zn 2+ + 2e – Cathode(+): Cu 2+ + 2e – =Cu 0 Overall: Zn 0 + Cu 2+ = Cu 0 + Zn 2+ Physical Chemistry EPM/08 13 Fuel cells (3b) Almost any reaction may be carried out in a galvanic cell !!! + – Anode(–): 2H 2(g) = 4H + 4e – + Cathode(+): O 2(g) + 4e +4H = 2H 2O Overall: 2H 2(g) +O 2(g) = 2H 2O Physical Chemistry EPM/08 14 Fuel cells (3c) Almost any reaction may be carried out in a galvanic cell !!! + – Anode(–): CH 3OH + H 2O = CO 2 + 6H + 6e + – Cathode(+): 1½O2 + 6H + 6e = 3H 2O Overall: CH 3OH + 1½O2 = CO 2 + 2H 2O Physical Chemistry EPM/08 15 Fuel cells (4) R mV The simplest model of Liquidmostek elektrolitycznyjunction a hydrogen/oxygen FC H2 O2 p=1 Atm p=1 Atm E0 = 1,23 V Pt Pt H2 SO 4 H2 SO 4 0 + + 0 Pt(black)|H 2(g,P )|H (aq,1M)||H (aq,1M)|O 2(g,P )|Pt Physical Chemistry EPM/08 16 Fuel cells (5) A real hydrogen/oxygen PEMFC obwód elektryczny external circuit paliwofuel katalizatoranodic membrana membrane, katalizatorcathodic catalystanodowy (elektrolit polimerowy) katodowy (polymeric electrolyte) catalyst exhaustspaliny gases Physical Chemistry EPM/08 17 Fuel cells (6) Main types of FCs: • alkaline FCs • proton exchange membrane FCs • phosphoric acid FCs • molten carbonate FCs • solid oxides FCs • direct methanol FCs Physical Chemistry EPM/08 18 Battery discharging curve EMF short circuit Physical Chemistry EPM/08 19.
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