Electrochemical Cells

Transcription

1 Electrchemistry invlves redx rectins. Terminlgy: Electrchemicl Cells Red-x rectin chemicl rectin where ne species underges lss f electrns nther species gins electrns. e - Cu(II)+ Cu(II) + (II) xidnt reductnt Further, the rectin cn be crried ut s tht xidtin nd reductin ccur t physiclly different lctins electrdes - in electrchemicl set-ups. The electrde rectins re, (II) + 2e 2e + Cu(II) Cu Tw electrdes, cupled, cnstitute n electrchemicl. The electrn mvement; q = quntity f chrge, i = rte f mvement f chrge. The electricl ptentils f the electrdes, E el nd the ptentil difference f the electrdes E tht re invlved nd cn be mesured.

2 Tw types f s re studied:. Glvnic (Vltic) : Glvnic uses spntneus red-x chemicl rectins t prduce electricl energy; tht wuld result in flw f electrns. G <. b. Electrlytic : An electrlytic decmpses chemicl cmpunds by red-x prcesses using electricl energy frm n utside surce - electrlysis. This is n energy demnding prcess nn-spntneus. G >. In ny type f : Ande xidtin ccurs Cthde reductin ccurs Hlf Cell M(s) immersed in n queus slutin f M +2 (q) estblishes n equilibrium between n the tw phses. Depending n the psitin f equilibrium there wuld be n ccumultin r depletin f electrns frm the metl(s), e.g. / 2 (q) system (hlf-), electrns ccumulte (stndrd stte) n the electrde. A negtive chrge builds up n the electrde nd psitive chrge builds up in the surrunding slutin. This chrging seprtin leds t difference in the electricl ptentil difference between the metl (electrde) nd the slutin (~1V); hlf- ptentil, electrde ptentil. Cupling tw hlf-s give n electrchemicl. Hlf Cell 2 ()() s 2 q e Psitin f equilibrium determines the vlue f. 2 ()() s 2 q e 2 Cu()() s 2Cu q e 2 Difference in electrsttic ptentil = The electricl ptentil,, develped ffects the chemicl ptentil (energy) f the chrged species. The ptentil energy f the chrged species wuld hve n dditinl term in dditin t the nrml chemicl ptentil, ; electrchemicl (energy) ptentil. () z F fr = nd/r z =.

3 Electrchemicl ptentil is thermdynmic mesure f chemicl ptentil tht tht tkes int ccunt the energy cntributin frm the electricl ptentil. Fr n electrchemicl rectin; G = R i i i Fr n electrchemicl equilibrium; G = R i i i In this exmple the (II) in d nt exist in isltin, but with cunter in. Tht will be true fr ll cmpunds in slutin. Therefre fr electrchemicl rectin ; G = [()] z F R i i i i i i i i () z F F z i i i i i i i i i i i i i i i Therefre cnsidering ll species present in the slutin; we cn set =, nd s result. Fr ny inic cmpund in slutin The ffects ll ins, in the slutin, regrdless f their prticiptin in the redx equilibrium hlf rectin. Nte: The verll rectin include the nn-prticipting cunter ins s well. Hlf Cell 2 F fr electrns in metl. e 2 () q 2() e s At equilibrium the chemicl/electrchemicl ptentils f the rectnt side nd prduct side re equl. 2 z e e F per electrn in metl / 2 cnsidering the cunter ins z e 2 fr (); ( q2 ) 2e s z ln 2 ln 2/ / ln 2 E E 2/ 2/ 2 el el el ln E E ln Q Electrde Ptentil

4 2 2 cnsidering the cunter ins z e fr () q 2e (); s z 2 ln / ln 2 2 2/ 2/ 2/ ln ln 2 1 E E ln E E ln Q 2/ 2/ el el el 2 Electrde Ptentil, Nernst Equtin Primry Reference Electrde: Stndrd Hydrgen Electrde Hwever the bslute electricl ptentil f n individul hlf cnnt be mesured. Therefre, it is cnvenient t chse ne hlf- s reference nd rbitrrily ssign n electricl ptentil f zer t this hlf-. Cupling with the reference electrde the electricl ptentil sscited with ny ther hlf, the new ptentil difference cn be determined. The stndrd hydrgen electrde fulfills the rle f reference hlf- f zer ptentil. The rectin in the stndrd hydrgen electrde is, Stndrd Hydrgen Electrde: SHE Setting unit ctivities fr ll species; E SHE 25 C by definitin.

5 Ptentimeter Circuit Stndrd electrde: All ctivities f rectin cmpnents is unity. Reference Electrde Test Electrde ph meter is very clse t n idel vltmeter. Impednce Drws negligible current. Negtive terminl - reference electrde slt. - BNC uter cnnectr Fr Cell: +2 ( 1 ) Cu +2 ( 2 ) Cu Q rxn 2 Cu 2 Cu2 Cu2 E E ln Qrxn Cell Ptentil, Nernst Equtin

6 Electrde ptentil E el : Clcultin f (hlf ) electrde ptentil E el s reductin ptentil (cnventin).. Write chrged nd mss blnced hlf rectin s reductin rectin, nte the # electrns n b. Write the rectin qutient (Q el ), sscited with n c. Obtin the Stndrd Electrde Ptentil fr the hlf rectin E el i.e. E M+n/M. E n E n ln Q M / M M / M el If Q =, i.e. ll ctivities t stndrd sttes = 1 E E n n M / M M / M Using Nernst equtin - electrdes. Fe + e = Fe MnO + 8H + 5e = Mn + 4H O E = E ln +2 Fe Fe / Fe Fe / Fe F +3 Fe E = E ln +2 Mn MnO 4 / Mn MnO 8 4 / Mn 5F -1 + MnO4 H reduced xidized By cnventin hlf rectins re written s reductin rectins. Lrger the electrde ptentil clculted (determined) fr the reductin prcess higher the prpensity fr tht t ccur. Then the reverse f the rectin hs less r n prpensity t ccur. Tht is the xidtin f the is unlikely if reductin Is the nturlly preferred prcess. And vice vers. Stndrd (reductin) ptentil vlues cmpre the inherent prpensity (spntneity) f reductin t ccur. Lrge nd psitive fvrs (spntneus) the reductin prcess; lrge nd negtive vlues indictive f nn-spntneus prcess, but reverse prcess, xidtin is spntneus. Cell ptentil (EMF) E : Clcultin f (hlf ) electrde ptentil E s reductin ptentil (cnventin).. Write chrged nd mss blnced full rectin s reductin rectin, nte the # electrns n, fr the full rectin. b. Write the rectin qutient (Q rxn ), sscited with n electrns. c. Obtin the Stndrd Cell Ptentil fr the rectin E i.e. E reductin rectin E xidtin rectin E E E ln Q ; E E ln Q If Q =, i.e. ll ctivities t stndrd sttes = 1 cthde nde rxn rxn E E

7 Using Nernst equtin Electrchemicl. MnO() 8H() 5Fe() Mn() 5Fe() 4H O E = E - ln 5 Mn+2 Fe F MnO4H+ Fe+2 E = E - ln F 1 23 E = E - ln 2 1 Mn+2 CO F MnO4H+ H2C2O4 E =( ) - ln 2 1 Mn+2 CO F MnO4H+ H2C2O4 E E E el, reductin _ prcess el, xidtin _ prcess E E ln Qrxn.591 E E lg Qrxn Vlts n Slpe =1/3 =1/2 =1 E E ln Q rxn E E 1 ln Q / F n rxn Experimentl determintin f n.

8 Lrger nd psitive the ptentil indictive f higher the prpensity (spntneus) fr the rectin t ccur. Then the reverse f the rectin hs less r n prpensity (nn-spntneus) t ccur. Stndrd ptentil vlues cmpre the inherent prpensity (spntneity) f rectin t ccur. Lrge nd psitive fvrs (spntneus) the rectin; lrge nd negtive vlues indictive f nn-spntneus rectin, but reverse rectin is spntneus. Energetics: G R G E ln Q R rxn At equilibrium G nd Q = K ln Ec ell ln K spntneity predictin, rectin E R E K G E G E el el Stndrd Gibbs Energy nd Entrpy Chnge f rectins G H TS G E G H () TS T T T () E T P P P S T P P E S Determintin f E el nd Activity Cefficients f Electrlytes: Stndrd electrde by definitin hs the ctivities f ll rectin cmpnents s unity. Unlike mst ther slutins electrlyte ctivities re devites cncentrtin even t lw cncentrtins. Cnstructin f stndrd electrde is nt s direct s expected. Experimentl determintin f stndrd electrde ptentils much difficult nd nn direct experimentl pprch is required.

9 Exmple Fr Cell: Pt H 2 (p=1tm) HCl(=1) AgNO 3 (m) Ag(s) Cell Rectin: Ag ( q,) m 1/ 2(1)()( H 2 tm, 1) Ag s H q Q rxn H Ag ; n 1 E E ln Qrxn E ln F E E ln E ln H F F m Ag Ag Ag H Ag 1 Men ctivity is define s; 2 Ag NO3 m nd m = γ γ m m = γ m m γ m ± ± + - ± m nt pssible t determine by expt. 1 E E ln E ln F F m H Ag E E.5916lg m.5916lg EMF f : Pt H 2 (p=1tm) HCl(=1) AgNO 3 (m) Ag(s) E m E m.5916lg.311 lg.592 z z I.592 z z m :DHE E E.5916lg m.5916(.592 z z ) m E.5916lg m E.311 m E E.5916lg m.5916lg

10 Applictins:. Slubility prduct f springly sluble slts, e.g. AgBr Use hlf-s invlving the slubility equilibrium. Bth f knwn stndrd electrde ptentil. Clculte stndrd ptentil. G E G E el Ptentils re nt dditive except in specil situtins, intensive prperty. Energy vlues re dditive extensive prperty. PbSO ()()() s Pb q SO q K sp 2 2 Pb SO4 PbSO () s 2()() e Pb s SO q 2 E G FE Pb()() s 2 Pb q e ( 2) E G FE Deceptive becuse n=1 is the sme fr hlf rectins. PbSO ()()() s Pb q SO q G 2FE ln K K sp 2FE sp G G G G 2() F E E Alternte pprch. Ag(s) AgNO 3 () Ag(NO 3 ) () Ag(s) Add Cl - s tht precipitte Ag +. At end pint EMF, E ; AgCl()()() s Ag q Cl q ; K K ; K 2 Ag sp Ag, LHS sp Ag LHS ClLHS Ag Cl sp Cnsidering bth electrdes s Ag(s) Ag + (q) electrdes. 1 1 E ERHS ln ELHS ln F Ag, RHS F Ag, LHS Ag, RHS Ag, RHS E ln ln F F K Ag, LHS sp Reference Electrdes secndry stndrd electrdes The SHE is cumbersme t cnstruct. Other hlf-s re being used s secndry stndrds. Silver/silver chlride: AgCl(s) +e - = Ag(s) + Cl - (q) 25 vs. SHE vs. SCE Ag/AgCl, KCl (.1M) Ag/AgCl, KCl (3M) Ag/AgCl, KCl (3.5M) Ag/AgCl, KCl (st'd) Ag/AgCl, NCl (3M) Ag/AgCl, NCl (st'd)