Unit 3. Electrochemistry

Transcription

1 Unit 3 Electrcheistry One ark questins 1. What is an electrlyte? An electrlyte is a cpund which cnducts electricity either in its aqueus slutin r in its lten state. e.g Acids HCl, CH 3 COOH, HNO 3 Bases NaOH, NH 4 OH Salts CuSO 4, NaCl etc 2. Define cnductivity f an electrlytic slutin. Cnductivity f a slutin f an electrlyte is the cnductance f a slutin placed between tw electrdes each f ne square eter area kept at a distance f 1 eter apart. 3. Write the S.I unit fr cnductivity. SI unit fr cnductivity is S Give the S.I unit fr lar cnductivity. S 2 l State Khlrausch Law. The liiting lar cnductivity f an electrlyte can be represented as the su f the individual cntributins f the anin and catin f the electrlyte. 6. Define electrde ptential. The ptential difference develped between the electrde (etal) and the electrlyte (slutin cntaining its wn ins) when bth the etal and the slutin are in equilibriu is called electrde ptential. 7. Define standard electrde ptential. Standard electrde ptential is the electrde ptential when the cncentratins f all the species invlved is unity (1M) and if a gas is invlved its pressure shuld be 1 bar. 8. Write Nernst Equatin E n+ = E n+ - lg ( M / M) ( M / M) 10 n+ n M 9. State Faradays secnd law f electrlysis. The aunts f different substances liberated by the sae quantity f electricity passing thrugh the electrlytic slutin are prprtinal t their cheical equivalent weights.

2 10. Define cell ptential. Cell ptential is the ptential difference between the tw electrdes f the galvanic cell. 11. Define EMF f the cell. It is the difference between the electrde ptential f the cathde and ande when n current is drawn thrugh the cell. 12. What is Fuel cell? Galvanic cells that are designed t cnvert the energy f cbustin f fuels like hydrgen, ethane etc directly int electrical energy are called fuel cells. 13. Give a ethd t prevent rusting. Rusting ay be prevented by barrier prtectin like painting, etal plating etc. 14. Write the relatinship between cell ptential and Gibb s energy Δ rg = nfe cell 15. Write the relatinship between equilibriu cnstant and E cell 0.059V E cell = lg Kc n 2 ark questins 1. What are redx reactins? Give an exaple. Reactins in which bth xidatin and reductin taken place siultaneusly are called redx reactins. e.g Zn+Cu Zn +Cu In this Zn is xidised t Zn 2+ Cu 2+ is reduced t Cu 2. Mentin any tw factrs n which the cnductivity f an electrnic cnductr depends. The electrnic cnductance depends n (i) The nature and structure f the etal (ii) The nuber f valence electrns per at. (iii) Teperature (it decreases with increase in the teperature) (any tw) 3. Mentin any tw factrs n which the cnductivity f an electrlytic cnductr depends. The cnductivity f electrlytic slutin depends upn (i) The nature f the electrlyte (ii) Size f the ins prduced and their slvatin. (iv) The nature f the slvent and its viscsity. (iv) Cncentratin f the electrlyte and (v) Teperature (increases with increase in teperature (any tw)

3 4. Give tw difference between the cnductivity f an electrnic cnductr and electrlytic cnductr. 1. On passing direct current cpsitin f electrnic cnductr des nt change but that f electrlytic cnductr changes. 2. On increasing the teperature in case f electrnic cnductr cnductivity decreases in case f electrlytic cnductr cnductivity increases. 5. What is a strng electrlyte? Give an exaple. A strng electrlyte is an electrlyte that dissciates cpletely int ins at derate cncentratins f its aqueus slutin Ex: acids HCl, H 2 SO 4, HNO 3 Base NaOH, KOH Salts NaCl, CuSO 4 (any salt) 6. What is a weak electrlyte? Give an exaple. A weak electrlyte is an electrlyte that dissciates partially int ins in its aqueus slutin. Ex: CH 3 COOH, NH 4 OH 7. Define lar cnductivity. Hw is it related t cnductivity? Mlar cnductivity f a slutin at a given cncentratin is the cnductance f the vlue V f a slutin cntaining ne le f electrlyte kept between tw electrdes with area f crss sectin A and distance f unit length. It is represented by λ λ = kv where k is cnductivity and v is vlue f the slutin cntaining 1 le f the electrlyte r If λ is in S 2 l - 1 and k in S - 1 k λ = 1000C r where C is cnc. in l L - 1 When λ is in S c 2 l - 1 and k is in Sc k λ = C 8. Hw des cnductivity f a slutin change with change in cncentratin f the slutin? Give reasn. Cnductivity f a slutin decreases with decrease in cncentratin f the slutin due t decrease in the nuber f ins per unit vlue f the slutin.

4 9. Define liiting lar cnductivity. Write the relatinship between lar cnductivity and liiting lar cnductivity. Liiting lar cnductivity is the lar cnductivity f a slutin when cncentratin appraches zer r lar cnductivity at infinite dilutin. 1 2 λ = λ - AC where λ is lar cnductivity and λ is liiting lar cnductivity, C is cncentratin in le/l and A is cnstant which depends n nature f the electrlyte, slvent and teperature. 10. Draw a graph f lar cnductivity verses square rt f the lar cncentratin fr KCl and CH 3 COOH entining clearly each. 11. Hw is liiting lar cnductivity fr a strng electrlyte fund ut by extraplatin ethd? Prepare fur slutins f given strng electrlyte f different cncentratins. Measure the cnductivities f each slutins using cnductivity cell and calculate the lar cnductivities f each slutin. Plt a graph f lar cnductivity verses square rt f the lar cncentratin fr these slutins. A straight line is btained which is t be extraplated back s as t tuch the vertical axes.this pint f intersectin n the vertical axes gives the liiting lar cnductivity. 12. State and illustrate Faradays first law f electrlysis. The aunt f cheical reactin which ccurs at any electrde during electrlysis by a current is prprtinal t the quantity f electricity passed thrugh the electrlyte either thrugh its aqueus slutin r lten state. If w is the ass f the substance depsited and Q is the current passed in culbs w Q But Q = I t where I is the current strength in apere and t is tie in secnds. 13. Cnductivity f 0.01 M NaCl slutin is 0.12 S - 1. Calculate its lar cnductivity. k C λ = = = S / l

5 14. The lar cnductivity f 0.1M nitric acid is 630 S c 2 /l. Calculate its cnductivity. 1000k λ = C 1000k 630 = k= =0.063 Sc A slutin f Ni(NO 3 ) 2 is electrlysed between platinu electrdes using a current f 5 aperes fr 20 inutes. What ass f nickel is depsited at the cathde? (Ml ass f Ni = 58.7) Q = I t = = 6000C 2+ - Ni + 2e Ni C C 58.7g Fr C f electricity ass f nickel btained = 58.7g Fr 6000C f electricity =1.812g Hw lng it will take fr the depsitin f 0.2g f silver when silver nitrate slutin is electrlysed using 0.5 apere f current (Ml ass f Ag = 108) + - Ag + e Ag 96500C 108g Fr 108g f silver t be depsited current required is 96500C. Fr 0.2g f Ag But Q = I t =178.7C =Q 108 Q t= = =357.4 se I The cell in which the fllwing reactin ccurs 2Fe + 2I 2Fe + I ( aq) ( aq) ( aq) 2( s) Has E cell = 0.236V at 298K. Calculate the standard Gibb s energy and the equilibriu cnstant fr the cell reactin. n = 2 Δ.G = - nfe Ecell = = J = lg K n

6 = lg K lg K = = Taking the antilg K = Write the reactin taking place at cathde and ande when aqueus slutin f cpper sulphate is electrlysed using cpper electrdes. At ande Cu Cu + 2e xdn 2+ ( s) ( aq) A Cu e Cu 2+ redn t cathde ( aq) + 2 ( s) Thus cpper fr ande disslves and an equivalent aunt f pure cpper is depsited n cathde. This technique is used in electrlytic refining f crude cpper. 19. Write the reactin taking place at ande and cathde when lten NaCl is electrlysed. When lten sdiu chlride is electrlysed using inert electrdes At ande 2Cl xdn Cl + 2e + redn At cathde Na + e Na Thus chlrine gas is liberated at ande and Sdiu etal is fred at cathde Write the reactin taking place when aqueus slutin f NaCl is electrlysed. When aqueus slutin f NaCl is electrlysed, NaCl Na + Cl + + HO 2 áàà ààü H + OH The reactin taking place at cathde is + 1 H aq + e H 2 The reactin taking place at ande is Cl aq Cl 2 aq + e 2 2( g ) 21. What is a priary battery/cell?give an exaple. Priary battery is ne in which reactin ccurs nly nce and cannt be recharged. Eg Dry cell r Leclanche cell and Mercury cell 22. What is a secndary battery/cell? Give an exaple. Secndary battery is ne which can be recharged by passing current thrugh it in ppsite directin,s that it can be Reused. Eg: Lead strage battery and Nickel cadiu cell.

7 23. E Cu = +0.34V and E Zn =- 0.76V. Daniel cell is btained by cupling these tw electrdes. (i) represent the cell syblically (ii) calculate the EMF f the cell (i) Daniel cell can be represented as Zn/ Zn 2+ (aq) Cu 2+ (aq) /Cu (ii) EMF f Daniel cell E cell = E R - E L = E Cu - E Zn = (- 0.76) = 1.10V 24. Calculate the lar cnductivity f a slutin f MgCl 2 at infinite dilutin given that the lar inic λ =106.1 Sc l and λ = 76.3 Sc l ( Mg ) ( Cl ) cnductivities f 2+ - λ = λ + 2λ MgCl Mg Cl = ( 76.3) =258.7 Sc l The resistance f a cnductivity cell cntaining M KCl slutin at 298K is 1500Ω. What is the cell cnstant if the cnductivity f 0.001M KCl slutin at 298K is Sc - 1? Cell cnstant G*= Rk =resistance cnductivity = Sc S - 1 = c - 1 Questin carrying 3 r 4 arks 1. Explain the cnstructin f Daniel cell. Write the reactin taking place at ande and cathde and the net cell reactin. (3 ark) T prepare Daniel cell get a zinc electrde by dipping zinc rd in 1M ZnSO 4 slutin. Get a cpper electrde by dipping a cpper plate in 1 M CuSO 4 slutin. Cuple these tw electrdes using a salt bridge t get Daniel cell. Reactins taking place + xdn 2 At ande Zn Zn + 2e 2+ redn At cathde Cu + 2e Cu Net cell reactin Zn + Cu Zn + Cu ( s) ( aq) ( aq) ( s)

8 2. With a labeled digra explain standar hydrgen electrde. Represent it syblically. Write the reductin reactin at the ande. What is its electrde ptential? (4 arks) It cnsists f a platinu electrde cated with platinu black. The electrde is dipped in 1M HCl. Pure hydrgen gas is bubbled thrugh it under a pressure f 1 bar. S.H.E is represented as Pt(s) H 2 (g)(1bar) H + (aq)(1m) The reductin reactin taking place is + 1 H ( aq) + e H2( g) 2 S.H.E is assigned an electrde ptential f 0.0 V at all teperatures. 3. Explain the use f standard hydrgen electrde in easuring the standard electrde ptentials f cpper and zinc electrde (4 ark) Cnstruct a standard electrde f the given etal by dipping the pure etal in 1M slutin f its wn in at 25 C Cuple this standard electrde with SHE using a salt bridge t get galvanic cell. Measure the ef f the cell using suitable instruent like ptentieter. E = E R E L One f the electrdes f the cell is SHE and its electrde ptential is 0.0V. S the electrde ptential f the given electrde will be the ef f the cell in agnitude. If reductin takes place at the given electrde its E will be +ve but if xidatin takes place at the given electrde is E will be ve. e.g if SHE is cupled with standard cpper electrde reductin takes place at cpper electrde cell can be represented as Pt (s) H 2(g. 1bar) H + (aq1m) Cu 2+ (aq.1m) Cu E = E -E cell 2+ + Cu / Cu H / H = E - 0 E = 0.34V Cu / Cu Cu / Cu If SHE cupled with standard zinc electrde xidatin takes place at zinc electrde. Cell can be represented as

9 Zn / Zn Pt H H 2+ + ( aq.1 M ) ( s) ( s) 2( g.1 bar) ( aq.1 M ) Ecell= E E + 2+ H / H2 Zn / Zn 0.76 = 0 E E = 0.76V Zn / Zn Zn / Zn 4. Hw is Khlrausch law helpful in finding ut the liiting lar cnductivity f a weak electrlyte? (3 ) Let us try t calculate λ fr a weak electrlyte CH 3 COOH. Select three strng electrlytes whse λ can be fund by extraplatin ethd in such a way that if we subtract λ fr ne electrlyte fr the su f λ s f the reaining tw electrlyte λ fr CH 3 COOH can be btained. The three electrlytes t be selected are CH 3 COONa, HCl & NaCl λ = λ +λ - λ CH COOH CH COONa HCl NaCl The values f liiting lar cnductivities (λ ) fr NH 4 Cl, NaOH and NaCl are respectively ; and Sc 2 l - 1. Calculate the liiting lar cnductivity f NH 4 OH (3M) λ = λ +λ - λ NH OH NH Cl NaOH NaCl 4 4 = = Sc 2 l Calculate the equilibriu cnstant fr the reactin at 298K Cu + 2 Ag ( aq) Cu + 2Ag Given that E Ag + /Ag = 0.80V and E (Cu 2+ /Cu) = 0.34V E cell= lg Kc n ne cell lg Kc = E = E E cell + 2+ ( Ag / Ag) ( Cu / Cu ) = =0.46V lg K c = = Taking the antilg K c = ( s) ( aq) ( s) 7. In Leclanche cell (dry cell) what are ande and cathde? What is the electrlyte used? Write the reactins at each electrde. What is the rle f zinc chlride? It cnsists f a zinc cntainer as an ande. A graphite rd surrunded by a ixture f anganese dixide and carbn pwder is cathde. The space between the electrdes is filled with electrlyte a ist paste f aniu chlride and zinc chlride

10 Reactin taking place At ande Zn 2+ - ( s) Zn + 2e At cathde MnO + NH + + e - MnO( OH) + NH NH 3 prduced in the reactin frs a cplex with Zn 2+ t fr [Zn(NH 3 ) 4 ] What are the ande and cathde f lead acid battery? What is the electrlyte? Write the reactins taking place at ande and cathde and the verall reactin during discharging f the battery. (3 M) It cnsists f lead ande and a grid f lead packed with lead dixide (PbO 2 ) as cathde. Electrlyte is 38% slutin f sulphuric acid. The reactins taking place when the battery is in use are Ande Pb + SO PbSO + 2e 2 ( s) 4 ( aq) 4( s) Cathde PbO ( s) + SO + 4H + 2e PbSO + 2 H O ( aq) ( aq) 4 ( s) 2 ( l) The verall reactin is Pb( s) + PbO2 ( s) + 2 H2SO4 ( aq) 2 PbSO4 ( s) + 2 H2 O ( l) 9. In Hydrgen xygen fuel cell (i) Draw the scheatic diagra entining the ande and cathde. What is the electrlyte? Write the reactin taking place at each electrdes and the net cell reactin. (4M) In this hydrgen and xygen gases are bubbled thrugh prus carbn electrdes int cncentrated aqueus sdiu hydrxide slutin. Catalyst like finely divided platinu r palladiu is incrprated int the electrdes fr increasing the rate f electrde reactin Reactin taking place are Cathde O + 2H O + 4e 4OH - - 2( g) 2 ( l) ( aq) - - Ande 2H + 4OH 4H O + 4e Overall reactin is 2H + O 2( g ) ( a q ) 2 ( l ) 2H O 2 ( g) 2 ( g) 2 ( l)

11 10. What is crrsin? During rusting f irn write the andic and cathdic reactins. Give the cpsitin f rust. (3M) When a etal is expsed t the atsphere it is slwly attacked by the cnstituents f the envirnent as a result f which the etal is slwly lst in the fr f its cpund. This is called crrsin. Reactin taking place are 2+ - At Ande 2Fe 2Fe + 4e ( s) - At Cathde O + 4H +( aq)+ 4e 2H O 2( g) 2 ( l) H + are prduced fr H 2 CO 3 fred due t disslutin f carbn dixide fr air int water The Fe 2+ ins are further xidised by atspheric xygen t ferric in which are ultiately cnverted t hydrated ferric xide called rust. Cpsitin f rust is (Fe 2 O 3.xH 2 O). 11. A cnductivity cell when filled with 0.01M KCl has a resistance f h at 25 C. When the sae cell was filled with an aqueus slutin f 0.05M CaCl 2 slutin the resistance was 876 h. Calculate (i) Cnductivity f the slutin (ii )Mlar cnductivity f the slutin (given cnductivity f 0.01M KCl = s - 1 ) (3M) Cell cnstant G* = Rk = = cell cnstant Cnductivity k = = = S R 876 h k Mlar cnductivity λ = = = s l 1000C The electrical resistance f a clun f 0.05M NaOH slutin f diaeter 1c and length 50c is h. Calculate its (i) resistivity (ii) cnductivity (iii) lar cnductivity (3M) l Cell cnstant l = 50 c G = a Diaeter = 1 c radius = 0.5 c Area f crss sectin A = πr 2 = 3.14 (0.5) 2 = c 3 50 G = = c * -1

12 1 1 Resistivity ρ= = = Ω -2 k k Mlarcnductivityλ = C = 0.05 = S c l Calculate the ef f the cell in which the fllwing reactin takes place. Ni + 2 Ag (0.002 M ) Ni (0.160 M ) + 2Ag + 2+ ( s) ( s) Given that E cell = 1.05V E cell [ Ni ][ Ag ] = E cell lg 2 [ ][ ] 2+ 2 ( s) Ni( s) Ag But [M] fr any eleent is taken as unity Ni E = E - lg 2 + Ag =1.05- lg cell cell 10 2 = 0.914V ( ) 2