Unit I Electrochemistry

Transcription

1 Uit I lectrochemistry Itroductio : Sigle lectrode Potetial: lectrode potetial is defied as the potetial developed at the iterface betwee metal ad solutio, whe it is i cotact with solutios of its ios. Or It is a measure of the tedecy of the metal electrode to lose or gai electros, whe it is i cotact with of its ow ioic solutio. It is represeted as. Stadard electrode potetial (º); it is the potetial measured uder stadard coditios i.e., Whe, 1. Cocetratio of the metal io solutio is 1M. 2. Temperature is 298 K. 3. Pressure is 1 atm. The stadard electrode potetial is represeted as º If Calculatio of cell MF: cell = 0 cell cathode 0 cathode aode 0 aode G is egative, reactio is spotaeous if cell = +ve, Similarly if cell = -ve, G = -ve, cell reactio is spotaeous. G = +ve, cell reactio is o-spotaeous. 1

2 Nerst quatio I 1889, Nerst derived a quatitative relatioship betwee electrode potetial ad cocetratio of electrolyte species. Accordig to thermodyamics, decrease i free eergy (- G ) represets maximum amout of work that ca be obtaied from a chemical reactio. G = W max (1) Work doe i electrochemical reactio depeds o (i) Number of coulombs that flow (ii) ergy available per coulomb. Number of coulombs = F Where = umber of electros ivolved i the cell reactio F = Faraday ergy available per coulomb = cell (V) 1V = 1 JC -1 Whe cell = maximum, work derived from cell is maximum. Thus, W F (2) max = moles of electros, F = Cmol -1, = JC -1 quatig equatios (1) & (2) - G = F G = - F (3) Uder stadard coditios, whe cocetratios of all species is uity, stadard free eergy chage is give by equatio G 0 F (4) Where is a costat called stadard electrode potetial. Cosider the reductio reactio M e M The equilibrium costat ( K ) is related to chage i free eergy by equatio C 2

3 G = G + RT l K (5) C K C = M M (6) Therefore by substitutig values of we get -F = -F + RT l M M -F = -F + RT l [M] RT l [M + ] Dividig whole equatio by F G, G ad K C from equatio (3), (4) ad (6) ito equatio (5), F F = - F F RT l[ M ] RT l[ F F M ] = - RT l[ M ] RT l[ M ] F F Assume [M] = 1, l1 = 0 = + RT M l[ ] F (7) Substitutig the values for R (8.314 JK -1 mol -1 ), F (96500Cmol -1 ), T (298K) ad covertig atural log to base 10 i eq (7) we get = log[M ] This is the mathematical represetatio of Nerst equatio. Nerst equatio for cell potetial = log Products Reactats Or, = Species at cathode log Species at aode 3

4 For example, for Daiel Cell Z + Cu 2+ Z 2+ + Cu 2 = log Z Cu 2 Cosider the cell reactio aa + bb cc + dd = c d log C D b A a Types of electrodes B lectrode is a device cosists of metal ad its ow ioic solutio. Followig are some importat types of sigle electrodes (a) Metal metal io electrode: It cosists of a metal dipped i a solutio cotaiig its ow ios. eg: Z/Z 2+, Cu/Cu 2+, Ag/Ag + etc. (b) Metal metal salt io electrode or Metal isoluble salt electrode: It cosists of metal i cotact with a sparigly soluble salt of the same metal dipped i a solutio cotaiig aio of the salt. x.. Calomel electrode (Hg/Hg 2 Cl 2 /Cl - ), silver silver chloride electrode (Ag/AgCl/ Cl - ). (c) Gas electrode: I this type of electrode, a gas is i cotact with a iert metal dipped i solutio cotaiig ios to which gas is reversible. x. H 2 electrode(pt/ H 2 /H + ), Cl 2 electrode(pt/ Cl 2 /H + ), (d) Oxidatio-Reductio electrode: I this electrode potetial arises from the presece of oxidized ad reduced forms of same substace i solutio. Potetial developed is picked up by a iert electrode like Platium (Pt). x. Pt/Fe 2+, Fe 3+ ad Pt/ S 2+, S 4+. (e) Io Selective lectrode: these are the electrodes, selects oly specific ios igorig other ios. I this electrode, a glass membrae is i cotact with a solutio, with which it ca exchage ios. x: Glass electrode (exchages H + ios with solutios). 4

5 Referece lectrodes: These are the electrodes, used to determie the potetials of ukow electrodes. They are of two types; I. Primary Referece lectrodes: These are the electrodes whose potetials are kow from the method of costructio. x. Stadard hydroge electrode (SH). Limitatios of primary referece electrode (SH) (a) Costructio is difficult. (b) lectrode becomes iactive i presece of impurities i H 2 gas. (c) It caot be used i the presece of oxidizig agets. (d) It caot be used i solutios of redox systems. To overcome all these difficulties, secodary referece electrodes are used to measure the electrode potetial of ukow electrodes II. Secodary Referece lectrodes: These are the electrode whose potetial is kow or costat ad stable does ot vary with tempeterure. x; 1.Calomel electrode 2. Silver-silver chloride electrode Calomel electrode It is a metal -metal isoluble salt electrode is also called as secodary referece electrode. Costructio: lectrode cosists of glass tube. The bottom of the glass tube is filled with pure mercury (Hg) o which a calomel paste (Hg + Hg 2 Cl 2 ) is placed. The remaiig part of the glass tube is filled with sat. or std KCl Solutio. It also cotais side tube serves as salt bridge ad a platium wire is dipped ito the mercury so that it serves as electrical cotact. 5

6 Half cell represetatio: The calomel electrode is represeted as, KCl /HgCl 2, Hg Half cell reactios: (a). Whe it acts as aode(ox ): 2Hg + 2Cl - Hg 2 Cl 2 + 2e - (b). Whe it acts as cathode(red): Hg 2 Cl 2 + 2e - 2Hg + 2Cl - The electrode potetial of the calomel electrode is maily depeds o the cocetratio of KCl used, lectrode potetial is calculated usig Nerst equatio, = RT log[ Cl ] For, 0.344V for [KCl] = 0.1N 0.281V for [KCl] = 1.0N V for [KCl] = saturated KCl Advatages: 1. It is simple to costruct. F 2. The cell potetial is stable ad reproducible does ot vary with temperature. Applicatios: It is used as secodary referece electrode i all potetiometric determiatios Used i glass or combied electrode to determie the ph of the ukow solutio. Limitatios: 1. Calomel electrode caot be used beyod a temperature of 60 C. 2. Hg employed i costructio poses pollutio problem. 6

7 Ag AgCl electrode It is a metal -metal isoluble salt electrode is also called as secodary referece electrode. Costructio: It cosists of glass tube. The bottom of which glass tube is filled with agar, o which saturated or stadard solutio of KCl is placed. The silver wire coated with AgCl is used for electrical coectios is as show i fig. Cell represetatio: lectrode is represeted as Ag(s) / AgCl(s) / KCl(aq) Cell reactios: Whe it acts as aode the electrode reactio is Ag + Cl - Ag Cl + e- Whe it acts as cathode the electrode reactio is AgCl + e- Ag + Cl - The electrode potetial of the Ag-AgCl electrode is maily depeds o the cocetratio of KCl used, Ad it is determied by Nerst equatio, For, = V: for [KCl] = 0.1N 0.199V: for [KCl] = saturated KCl.303 RT log[ Cl ] F 7

8 Applicatios: It is used as secodary referece electrode i all potetiometric determiatios. Advatages: 1. Costructio is simple ad electrode is portable. 2. Coditios of ideal referece electrodes are obeyed to better extet. 3. lectrode ca be used beyod a temperature of 60 C. Measuremet of sigle electrode potetial usig a referece electrode (saturated Calomel electrode): The give electrode, say zic electrode, is coupled with saturated calomel electrode as i the figure. Sice the reductio potetial of zic electrode less tha that of calomel electrode, zic acts as aode ad calomel as cathode. The cell assembly is represetatio: The cell assembly is represeted as, Z/ ZSO4 (1 M) // KCl (satd )/ Hg2Cl2/Hg Cell reactios: At Aode (Ox): Z Z e At Cathode (Red): Hg 2 Cl 2 + 2e 2Hg + 2Cl - Net cell reactio: Z + Hg 2 Cl 2 Z Hg + 2Cl - 8

9 The MF of the cell assembly is, cell = right left = cal - Z/Z = Z/Z 2+ Z/Z 2+ = Z/Z2+ = volt. Cocetratio Cell It is a electrochemical cell i which two idetical electrodes immersed i same ioic solutio but of differet cocetratio. x: Copper cocetratio cell Costructio: It cosists of two copper electrodes immersed i copper sulphate solutios of differet cocetratios C 1 ad C 2 is as show i the fig. Cell represetatio: The cocetratio Cell is represeted as: Cu / Cu 2+ ( C 1 ) // Cu 2+ (C 2 ) / Cu Cell reactios: At aode (oxidatio): Cu Cu 2+ (C 1 ) + 2e - At cathode (Reductio): Cu 2+ (C 2 ) + 2e - Cu Net cell reactio: Cu 2+ (C 2 ) Cu 2+ (C 1 ) 9

10 The et cell reactio i cocetratio cell shows simply chage i cocetratio as a result currat flows. i.e, MF of cell is give by cell = cathode aode Accordig to Nerst equatio, 2.303RT = logc 2 F cell 2.303RT log F C C Coclusio: 1. If C 2 > C 1, the cell is Positive 2. If C 2 < C 1, the cell is Negative 3. If C 2 = C 1, the cell is Zero RT log F C 1 Io Selective electrodes: These are the electrodes, which respods to specific ios oly ad develops a potetial agaist that ios while igorig the other ios preset i the solutio. The types (classificatio) of io-sesitive electrode: 1) Glass membrae electrodes 2) Solid state electrode 3) Precipitate electrodes 4) Liquid liquid electrode Priciple; IS cosist of a io selective membrae i cotact with a aalyte solutio o oe side ad a iteral referece electrode o the other side. A iteral referece electrode is costituted i cotact with referece solutio. lectrode ca be represeted as; Sol to be aalyzed iteral stadard sol Iteral Referece electrode C C M 1 Membrae M 2 10

11 Glass lectrode Costructio: The glass electrode cosist of glass tube, the bottom of the glass tube is glass bulb covered with glass membrae made up of special glass of low meltig poit ad high electrical coductivity ( SiO 2,Na 2 O,Al 2 O 3 etc., ad it allows oly H + ios). Glass bulb cosists of 0.1 N HCl (Assume cocetratio is C 2 ) ad Ag/AgCl electrode it serves as iteral referece electrode as show i fig. t The glass electrode is dipped ito ay solutio cotaiig H + ios the glass electrode develops potetial called as glass electrode potetial. It is represeted as G. The, G = b + Ag/AgCl + assy.. (1) Where, b = Boudary potetial Ag/AgCl = Potetial due to Ag/AgCl assy = asymmetric potetial whe C 1 =C 2 Boudary potetial ( b ); It is a potetial developed across the glass membrae whe cocetratio of the solutio iside ad outside the glass membrae are differet. Mathematically it is represeted as, b = 1-2 Where, 1 = Potetial due to H + preset i out side solutio (Ukow solutio) 2 = Potetial due to H + preset i out side solutio (Ukow solutio) Accordig to Nerst equatio = º log C 1 (º log C 2 ) = log C log C 2 Where, C 2 = 0.1 M ad = 1 (H + ) Combied Glass lectrode 11

12 = log C 2 K or = K log C 1 Glass selects oly H + ios igorig other ios. Hece C 1 = H + b = K log [H + ] Where, log [H + ] = -ph b = K pH (2) Substitutig eq (2) i (1) G = K pH + Ag/AgCl + assy G = º G pH Where, º G = Stadard glass potetial = K + Ag/AgCl + assy Advatages of Glass electrode; 1. It ca be employed i presece of strog oxidizig /reducig substaces ad metal ios. 2. It does ot get poisoed easily. 3. It ca be used i ubuffered solutio ad ca be adopted for measuremets with very small quatities of solutio. Determiatio of ph usig glass electrode or calomel electrode To determie the ph of give solutio, glass electrode is dipped i a solutio whose ph has to be determied. It is combied with a saturated calomel electrode through a salt bridge. 12

13 Cell assembly represetatio: Hg / Hg 2 Cl 2 / Cl - // Ukow solutio / glass membrae / 0.1N HCl / AgCl / Ag From the emf, the ph of the solutio is calculated as below: cell = right left cell = cal glass cell = cal ( G ph) cell = cal - G ph ph = cal o G cell ph = K cell Applicatios of Io-sesitive electrode: i) To determie ios like H+, K+, Li+, etc. ii) To determie hardess of water (Ca+ 2 ad Mg +2 ios) iii) To determie cocetratio of F-, NO 3-, CN- etc. iv) To determie cocetratio of a gas usig gas-sesig electrodes. v) To determie ph of a solutio usig H + io sesitive electrode. Importat Questios: 13

14 1. Defie the followig: a. Sigle electrode potetial b. Stadard electrode potetial. 2. Derive the Nerst equatio for sigle electrode potetials. 3. What are referece electrodes? What are the limitatios of stadard hydroge electrode? 4. Defie cocetratio cell? xplai the costructio ad workig of a copper cocetratio cell. 5. xplai the costructio ad workig of the followig: a. Calomel electrode b. Ag-AgCl electrode 7. What are io selective electrodes? xplai the costructio ad workig of glass electrode. 8. xplai determiatio of ph usig glass electrode. 14

15 15