ELECTROCHEMISTRY ONE Mark Each 1. The difference between the electrde ptentials f tw electrdes when n current is drawn thrugh the cell is called.. Under what cnditin an electrchemical cell can behave like an electrlytic cell? 3. What is the quantity f charge in faraday is required t btain ne mle f aluminum frm Al O 3? 4. Hw the cell cnstant f a cnductivity cell changes with change f electrlyte, cncentratin and temperature? 5. What will happen at ande during the electrlysis f aqueus slutin f CuSO 4 in the presence f Cu electrdes? 6. Under what cnditin is E Cell = r Δ r G =? OR Give the cnditin fr Daniell Cell in which there is n flw f electrns r current. 7. Why is alternating current used fr measuring resistance f an electrlytic slutin? 8. Hw will the ph f brine (aq. NaCl slutin) be affected when it is electrlyzed? 9. Unlike dry cell, the mercury cell has a cnstant cell ptential thrughut its useful life. Why? 1. Mentin the purpse f salt-bridge placed between tw half-cells f a galvanic cell? 11. Tw metals A and B have electrde ptential values f.5v and.8v respectively. Which f these will liberate hydrgen gas frm dilute H SO 4? 1. What is the effect f temperature n mlar cnductivity? 13. What is the rle f ZnCl in the dry cell? 14. Why is the equilibrium cnstant K, related t nly E cell and nt E cell? 15.Rusting f irn is quicker in saline water than in rdinary water. Why is it s? 16. Why rusting f irn prevented in alkaline medium?
114 17.Which will have greater mlar cnductivity and why? 1 mle KCl disslved in cc f the slutin OR 1 mle KCl disslved in 5 cc f the slutin. 18.Why Lead strage battery as a secndary cell can be recharged? 19.Write the name f a chemical substance which is used t prevent crrsin..write the unit f Faraday cnstant. 1. Cell emf Answer f ne mark. When E ext > E cell 3. 3F 4. Remain unchanged fr a cell 5. Cpper will disslve at ande 6. When the cell reactin reaches equilibrium 7. Alternating current is used t prevent electrlysis s that cncentratin f ins in the slutin remains cnstant. Otherwise if DC is used the ins will get discharged and electrlysis will ccur 8. The ph f the slutin will rise as NaOH is frmed in the electrlytic cell.) 9. Ins are nt invlved in the verall cell reactin f mercury cells. 1. Neutralize the tw half cell. 11. Metal - A 1. Mlar cnductivity f an electrlyte increases with increase in temperature. 13. ZnCl absrbs ammnia prduced in the reactin by frming a cmplex [Zn(NH 3 ) 4 ] 14. This is because E cell is zer at equilibrium. 15.Due t presence f ins in saline water cnductivity is mre than the rdinary water. Hence in miniature electrchemical cell flw f electrns will increase, cnsequently rusting f irn is increased. 16. In alkaline medium, atmspheric xygen is unable t take electrn which is given by the xidatin f Fe. 17. 1 mle KCl disslved in 5 cc f the slutin, Due t mre mbility f ins and mre degree f dissciatin.
115 18. Recharging is pssible in this case because PbSO 4 frmed during discharging is a sticky slid which sticks t the electrde. Therefre it can either take up r give up electrns during recharge. 19. Bisphenl. Culmb/ ml ELECTROCHEMISTRY TWO Marks Each 1.Slutins f tw electrlytes A and B are diluted. The Λ m f B increases 1.5 times while that f A increases 5 times. Which f the tw is a strng electrlyte? Justify yur answer.. When acidulated water (dil.h SO 4 slutin) is electrlysed, will the p H f the slutin be affected? Justify yur answer. 3. What advantage d the fuel cells have ver primary and secndary batteries? 4. Hw des the density f the electrlyte change when the lead strage battery is discharged? 5. Why n dilutin the Λ m f CH 3 COOH increases drastically, while that f CH 3 COONa increases gradually? 6. What is the relatinship between Gibbs free energy f the cell reactin in a galvanic cell and the emf f the cell? When will the maximum wrk be btained frm a galvanic cell? 7.Define crrsin. Write chemical frmula f rust. 8.Can yu stre cpper sulphate slutins in a zinc pt? 9.Write the cell reactin which ccur in the lead strage battery (a) when the battery is in use (b) when the battery is n charging. 1.Write the prduct f electrlysis f aqueus cpper sulphate by using platinum electrde. Answer 1.Electrlyte B is strng as n dilutin the number f ins remains the same, nly interinic attractin decreases therefre increase in is small..ph f the slutin will nt be affected as[ H ] remains cnstant.
116 At ande: H O O 4H 4 e- At cathde: 4 H 4 e- H 3.Primary batteries cntain a limited amunt f reactants and are discharged when the reactants have been cnsumed. Secndary batteries can be recharged but take a lng time t recharge. Fuel cell runs cntinuusly as lng as the reactants are supplied t it and prducts are remved cntinuusly. 4.Density f electrlyte decreases as water is frmed and sulphuric acid is cnsumed as the prduct during discharge f the battery. Pb PbO H SO 4 PbSO 4 H O 5.In the case f CH 3 COOH, which is a weak electrlyte, the number f ins increase n dilutin due t an increase in degree f dissciatin. In the case f strng electrlyte the number f ins remains the same but the inter inic attractin decreases. 6. rg = nfe (cell) If the cncentratin f all the reacting species is unit. 7.Crrsin is a prcess f frmatin sulphides, xides, carbnates, hydrxides, etc. f metal n its surface as a result f its reactin with air and water, surrunding it. Frmula f rust- Fe O 3.XH O 8.N, We cannt stre CuSO 4 slutin in zinc pt, because electrde ptential f zinc is less than cpper,s Cu ins get replaced by Zn ins in slutin. Zn is mre reactive metals than Cu. (Displacement reactin) 9. (a) When battery is in use Oxd n React n Red n React n Cell React n Pb SO - 4 PbSO 4 PbO e- SO - 4 H 4 e- PbSO 4 Pb PbO SO 4 - H O 4 H PbSO 4 H O (b) When the battery is n charging Red n React n Oxd n React n PbSO 4 e- Pb SO 4 - PbSO 4 H O PbO SO 4-4 H e- Cell React n PbSO 4 H O Pb PbO SO - 4 4 H 1.
117 Electrlysis f aqueus CuSO 4 using Pt electrde CuSO 4 Cu Ande [ SO 4 - ] SO 4 - H O H O HO - 4 HO - H O O 4 e - Cathde [ Cu H ] Cu e - Cu ELECTROCHEMISTRY THREE Marks Each 1.Calculate the EMF f the cell in which the fllwing reactin take place:.if a current f 5 ampere flws thrugh a metallic wire fr hurs, then hw many electrns flw thrugh the wire 3.Calculate the ptential f hydrgen electrde in cntact with a slutin whse P H is 1. 4.The mlar cnductivity f 5 ml L 1 methanic acid is 461 S cm ml 1. Calculate its degree f dissciatin and dissciatin cnstant. Given H = 3466 S cm ml 1 and HCOO = 546 S cm ml 1 5.If a current f 5 ampere flws thrugh a metallic wire fr hurs, then hw many electrns flw thrugh the wire? 6. Calculate m fr CaCl and MgSO 4 frm the data given in the table f Bk. 7.The Cnductivity f 18 ml L 1 acetic acid is 495 1 5 S cm 1. Calculate its dissciatin cnstant if fr acetic acid is 395 S cm ml 1. 8. A slutin f CuSO 4 is electrlysed fr 1 minutes with a current f 15 amperes. What is the mass f cpper depsited at the cathde? 9.The cnductivity f M slutin f KCI at 98 K is 48 S cm 1. Calculate its mlar cnductivity. 1. Write the Nernst equatin and find emf f the fllwing cells at 98 K: Mg(s) Mg (1 M) Cu (1 M) Cu(s) H - Answer 1.Ni Ag (M) Ni (16 M) Ag
118 Given that E Cell 1 5V 59 Ni (Accrding t Nernst equatin : ECell ECell lg n Ag 59 16 15 lg 9143V.Quantity f electricity (Q) = Current (ampere) time (secnd) = 5 6 6 = 36 C (Culmbs) A flw f 69487 C f electricity 6 1 3 electrns 36 C f electricity = 3 6 1 96487 36 461 electrns 3.P H = 1 means [H ] = 1-1 M Nw fr the electrde; H e 1 H (Here n = 1) E E H H H H 59 1 - lg n H 1 = 59 lg = 59 lg 1 1 = 59 1 = 59 v 1 1 4. C = 461 S cm ml 1 C = 5 ml L 1 HCOO H = 349.6 546 = 44 S cm ml 1 HCOO 461 44 Degree f dissciatin 114 C 4 c 5 114 Dissciatin cnstant Ka 3667 3671 1 1 114 5.Quantity f electricity (Q) = Current (ampere) time (secnd) = 5 6 6 = 36 C (Culmbs) A flw f 69487 C f electricity 6 1 3 electrns 36 C f electricity = 3 6 1 96487 36 461 electrns 6. m Cacl - 119 76 3 119 15 6 71 Ca cl m MgSO - 16 16 4 Mg SO 4 66Scm -5 k 1 4951 1 7. m = 4815 cm ml 1 C 18 ml -1 6Scm ml -1
119 m m 4815 133 39 Dissciatin cnstant (K a ) = C 1-18 1 133 133 5 1781 8.Quantity f electricity (Q) = Current time = 15 1 6 = 9 C Accrding t the reactin : Cu e Cu We required F r 96487 C f electricity t depsit 1 ml r 63 g f Cu 9 C electricity will depsit = 63 9 = 938 g f Cu at the cathde 96487 9. M mles KCI present in 1 litre i.e. 1 cm 3 f slutin 1 1 mle KCI present in cm 3 f slutin K = 48 S cm 1 Cnductance f 1 cm 3 slutin = 48 S 1 Cnductance f cm 3 1 slutin = 48 = 14 S cm ml 1 S Mlar cnductivity () = 14 S cm ml 1 1. Oxidatin Half Mg Mg e Reductin Half Cu e Cu ---------------------------------------------------------------------- Cell Reactin Mg Cu Mg Cu Here number f mles f electrns (n) = E E = 34 ( 37) = 71 V E cell Cu Cu Mg Mg The Nernst equatin fr the cell : 59 [Mg Ecell Ecell lg [Cu ] ] E cell 59 1 71 lg = 71 95 lg 1 = 71 95 = 685 V 1 FIVE Marks Each
1 1. (a) A Leclanche cell is als called dry cell. Why? (b)why is the vltage f a mercury cell cnstant during its wrking? (c)name tw metals that can be used fr cathdic prtectin f irn? (d)what d yu mean by primary and secndary battery? [111]. (a)what d yu understand by strng and weak electrlytes? [1 ] (b)state Faraday s Laws f electrlysis? (c)silver is depsited n a metallic vessel by passing a current f. amps. fr 3 hrs. Calculate the weight f silver depsited. (At mass f silver = 18 amu, F = 965 C? 3. (a)define the term resistivity and give its SI unit. [1] (b) What are the factrs n which cnductivity f an electrlyte depend? (c) The mlar cnductivity f.1m CH 3 COOH slutin is 4.6 cm resistivity f the slutin? ml -1. What is the cnductivity and 4.(a) State the factrs that affect the value f electrde ptential? [1] (b) Write Nernst equatin fr a Al-ZnSO 4 cell? (c) write the chemistry f rusting f irn 5.(a) Can an electrchemical cell act as electrlytic cell? Hw? [1] (b) Explain cnstructin and wrking f standard Hydrgen electrde? (c) What is an electrchemical series? Hw des it predict the feasibility f a certain redx reactin? Answer 1. (a) Leclanche cell cnsists f zinc ande (cntainer) and carbn cathde. The electrlyte isa mist paste f MnO, ZnCl, NH 4 Cl and carbn black. Because there is n free liquid inthe cell, it is called dry cell. (b)as all the prducts and reactants are either in slid r liquid state, their cncentratin des nt change with the use f the cell.
11 (c)names f the metals are Zinc and Magnesium. (d)in the primary batteries, the reactin ccurs nly nce and after the use ver a perid f time battery becmes dead and cannt be reused again. A secndary battery, after used,can be recharged by passing current thrugh it in the ppsite directin s that it can beused again.. (a)an electrlyte that inizes cmpletely in slutin is a strng electrlyte eg. NaCl, CaCl etc and an electrlyte that inizes partially in slutin is weak electrlyte eg CH 3 COOH, NH 4 OH etc. (b) Faraday s Laws f electrlysis First Law: The amunt f chemical reactin which ccurs at any electrde during electrlysis by a current is prprtinal t the quantity f electricity passed thrugh the electrlyte. Secnd Law: The amunt f different substances liberated by the same quantity f electricity passing thrugh the electrlytic slutin is prprtinal t their chemical equivalent weights. (c).417 g f silver. 3. (a) The resistivity f a substance is its resistance when it is ne meter lng and its area f crss Sectin is ne m. Unit: hm.meter (c) (b) The cnductivity f an electrlyte depends upn i) The nature f electrlyte ii) Size f the ins prduced iii) Nature f slvent and its viscsity. iv) Cncentratin f electrlyte. v) Temperature 4.(a) Factrs affecting electrde ptential values are a) Cncentratin f electrlyte b) Temperature. (b) TheNernst equatin fr a Al-ZnSO 4 cell:
1 The Cell is : Al Al 3 Zn Zn Ande reactin : Al Al 3 3 e - ] x Cathde reactin : Zn e - Zn ] x 3 Cell reactin : Al 3 Zn Al 3 3 Zn E = E -.59 6 lg [Al3 ] [Zn ] 3 (c) The chemistry f rusting f irn [1]Creatin f Acidic medium: Atmspheric carbn dixide and water vapur cmbine t frm. carbnic acid. CO H O H CO 3 [] Irn will xidise [ Ande- Oxidatin half ] Fe Fe e- X [3] In anther spt, xygen f air will take the tw electrns with help f H in and will be. reduced t H O [ Cathde - Reductin half ] Cell React n O 4 H e- 4 H O Fe O 4 H Fe H O In alkaline medium, atmspheric xygen is unable t take electrn which is given by the xidatin f Fe. [4] Atmspheric xygen further xidises ferrus in int ferric xide. Fe [5] Ferric xide will hydrlyse with water t frm rust. 1 H O O Fe O 3 4 H Fe O 3 x H O Fe O 3. x H O [ Rust ] --------------------------------------------------------------------------------------------------------------------------- 5.(a) Yes, An electrchemical cell can be cnverted int electrlytic cell by applying anexternal ppsite ptential greater than its wn electrical ptential.} (b) Standard Hydrgen electrde:
13 The Standard Hydrgen Electrde cnsists f a platinum electrde cated with platinum black. The electrde is dipped in an acidic slutin and pure hydrgen gas is bubbled thrugh it. The cncentratin f bth the reduced and xidised frms f hydrgen is maintained at unity. This implies that the pressure f hydrgen gas is ne bar and the cncentratin f hydrgen in in the slutin is ne mlar. Ande - H H e- Cathde - H e- H E H H = H gas at 1 bar Finely divided platinum cated n platinum fil If it act as cathde The maximum bubbling f hydrgen gas frm the slutin will evlve. If it act as ande The minimum bubbling f hydrgen gas frm the slutin will evlve (c) The arrangement f metals and ins in increasing rder f their electrde ptential values is knwn as electrchemical series. The reductin half reactin fr which the reductin ptential is lwer than the ther will act as ande and ne with greater value will act as cathde.reverse reactin willnt ccur.