Electrochemistry-Dr. Surendran Parambadath


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Electrochemistry-Dr. Surendran Parambadath

  1. 1. Dr. SURENDRAN PARAMBADATH (M.Sc, M.Phil, M.Tech) Formerly: Post Doctoral Research Associate,Nano-Information Materials Research Laboratory, Pusan National University, Busan-South Korea Currently: Assistant Professor Govt. Polytechnic College, Perinthalmanna
  2. 2. Electrochemistry is the study of Inter-convention of electrical energy and chemical energyElectrical Energy Chemical Energy
  3. 3. Electrolytic CellThis device can convert electrical energy in to chemical energy Electrochemical CellThis device can convert chemical energy in to electrical energy
  4. 4. Conductors and InsulatorsThis classification is based on their ability to allow electric currentto pass through them.Conductors are those substances which allowelectric current to pass through them.Examples: Metals, Alloys, Graphite, Ionic compounds in fusedor dissolved state.Insulators are those substances which do notallow electric current to pass through them.Examples: Glass, Wood, Paper, Organic Compounds etc.
  5. 5. Types of Conductors1. Metallic Conductors Eg: Metals and Alloys2. Electrolytic Conductors Eg: NaCl, CuSO4, etc3. Semi Conductors Eg: Ge doped with Ar or P.4. Super Conductors Eg: Mercury at 4K
  6. 6. Metallic Conductors Electrolytic Conductors1. Due to movement of electrons 1. Due to movement of ions2. No Chemical Change takes place 2. Electrolysis takes place3. No transfer of matters 3. Transfer of matter in the forms of ions4. Conductance of metals 4. Conductance of metals increasesdecreases with increase of with increase of temperaturetemperature
  7. 7. Electrolytes are substances which conductelectric current through them either in themolten state or in the dissolved state.Eg: NaCl, H2SO4, KOH, HNO3 etcNon electrolytes are substances which do notconduct electricity in the fused state or indissolved state.Eg: Sugar. Urea, alcohol etc.
  8. 8. Strong electrolytes: Electrolytes that dissociate almostcompletely into ions even at moderate concentrationare called strong electrolytes.They have high conductivity.Eg: HCl, HNO3, H2SO4 etc. NaOH, KOHetc, NaCl, CuSO4 etc.Weak electrolytes: Electrolytes which dissociate intoions partially at moderate concentrations.They have low conductivity.Eg: Acetic acid, Oxalic acid, NH4OH etc.
  9. 9. Electrolysis is the process of decomposition of an electrolyteby passage of electric current. - battery + Cl2 (g) escapes Na (l) NaCl (l) Na+ Cl- Na+ Cl- (-) (+) electrode electrode half-cell Cl- Na+ half-cell Na+ + e-  Na 2Cl-  Cl2 + 2e-
  10. 10. Molten NaCl Observe the reactions at the electrodes - + battery Cl2 (g) escapes Na (l) Na+ NaCl (l) Cl- Na+ Cl (-) - (+) electrode electrode half-cell Cl Na+ half-cell - Na+ + e-  Na All rights reserved. 2Cl-  Cl2 + 2e- http://academic.pgcc.edu/~ssinex/E_cells.ppt. 11
  11. 11. 1st Law………….The mass of substance discharged at an electrode duringelectrolysis is directly proportional to the quantity of electricitypassed through the electrolyte.m Q ………………………m = Zitm = mass in grams of substance dischargedQ= quantity of electricity in coulombst= time in secondsZ= Electrochemical equivalentECE may be defined as the mass of the substance discharged by passing onecoulomb of electricity.
  12. 12. 2nd Law…….The law states that when the same quantity of electricity ispassed through different electrolytes connected in series, theamount of substance discharged at the electrodes are directlyproportional to their chemical equivalent.E = Equivalent weight m1/m2 = E1/E2 e- - battery + - + - + - + e- e- e-
  13. 13. The process of depositing a superior metal on am inferior metalby passing electric current is called electroplating.The base metal object, which is to be plated ismade the cathode in the electrolytic cell.The rod of pure metal to be deposited on theobject is made the anode.The electrolyte is a solution of a soluble salt of thesuperior metal.
  14. 14. Examples for Superior Metals Cr, Ni, Ag & AuExamples for Inferior Metals Fe & Cu
  15. 15. 1. To protect the inferior metal object from corrosion.2. To increase the resistance to chemical attack3. To improve its physical appearance so as to make it more attractive.4. To modify hardness5. To repair damaged part of the machinery.6. To strengthen light weight non metallic like wood, glass, leather, cloth etc.7. To obtain conducting surfaces, eg copper plating on wooden or plastic radar antenna masts.
  16. 16. Step: 1 Wash the object with an organic solventto remove any grease or oil on it. Then washwith dilute sulphuric acid to remove oxide filmfrom the surface.Finally wash with chromic acid or detergent toclean the surface thoroughly.
  17. 17. Step: 2 The metal surface should be rough sothat the deposit sticks firmly and permanently.
  18. 18. Step 3: The electrolyte is nickel sulphatesolution containing nickel chloride or nickelammonium sulphate solution.
  19. 19. Step 4: pH of the electrolytic solution ismaintained between 4 to 5.
  20. 20. Step 5: The cleaned object to be plated is madethe cathode of the electrolytic cell, and purenickel plate or block, the anode.
  21. 21. Reaction:At Anode,Ni Ni2+ + 2e-At Cathode,Ni2+ + 2e- Ni
  22. 22. The Electrolytes used for electroplating should be1. Highly soluble2. Stable towards oxidation, reduction or hydrolysis3. A good conductor
  23. 23. Anodizing is a process of coating a base metal like Aluminium, orMagnesium with a thin uniform and protective oxide film.Anode: Base metalElectrolyte: Chromic acid, dil. H2SO4 or Phosphoric acidCathode: Graphite rod or lead sheet The anode coating being thicker than the natural oxide film, it has greater resistance to corrosion and mechanical injury. By addition of suitable dyes and pigment to the electrolyte, brightly colored, lustrous surface coating are obtained.
  24. 24.  “Cells” are containers of liquid with electrodes: Source or use of electricity Cell Electrode – + Molten or – + – + aqueous chemicals• In “electrolytic cells”, electricity is used to force chemicals to undergo a redox reaction• In “galavanic cells”, electricity is produced spontaneously from a redox reaction
  25. 25.  An apparatus that allows a redox reaction to occur by transferring electrons through an external connector. Product favored reaction > voltaic or galvanic cell --> electric current Batteries are voltaic cells Reactant favored reaction > electrolytic cell ---> electric current used to cause chemical change.
  26. 26. The device in which chemicalenergy is converted intoelectrical energy is calledgalvanic cell.Working w.r.t: Oxidation-Reduction Reaction.
  27. 27. 1. Two half cells, namely zinc half cell and copper half cell. In the former is a zinc rod dipped in a ZnSO4 solution and the latter is a copper rod dipped in a CuSO4 solution. The two metallic rods are called electrodes.2. The two half cells are connected externally by a metallic wire to a galvanometer through a key and internally by a salt bridge.3. The salt bridge is an inverted U-tube containing an arouse solution of an inert salt like KCl, KNO3, NH4NO3 to which some agar-agar or gelatin has been added to convert it into a semi solid, ie gel. The ends of the U-tube are plugged with glass wool.
  28. 28. 1.Permits the passage of electric current internally,2.Maintains the electrical neutrality of the solution,3.Prevents intermixing of the solutions,4.It does not take part in cell reaction. Zn(s) / Zn2+(aq) // Cu2+(aq)/Cu(s)
  29. 29. Anode CathodeZinc plate is eaten away and Copper deposits on thecopper plate. Electrons produced at the zinc anode flowthrough the outer circuit to the copper cathode.Electric Current is assumed to flow from copper to zinc,ie, from positive terminal to negative terminal.
  30. 30. Cu(s) / Cu2+(aq) // Ag+(aq)/Ag(s)Mg(s) / Mg2+(aq) // Ni2+(aq)/Ni(s)Fe(s) / Fe2+(aq) // Au3+(aq)/Au(s)Al(s) / Al3+(aq) // Sn2+(aq)/Sn(s)
  31. 31. e- - battery + Na+ Cl- Na+ Cl e- (-) - (+) (+) Cu Zn (-) Cl- Na+Cathode Anode Cathode Anode
  32. 32. galvanic electrolytic produces need electrical power current two source electrodesanode (-) conductive anode (+)cathode (+) medium cathode (-) salt bridge vessel No salt bridge
  33. 33. Electrolytic Cell Galvanic Cell1 Electrical Energy is converted Chemical Energy is converted into into chemical energy electrical energy2 Electrical energy brings about a Electrical energy is generated by a redox redox reaction reaction3 Anode is positive while cathode Anode is negative while cathode is is negative positive4 Redox reaction takes place in the Oxidation and reduction reactions are same container carried out separately5 No salt bridge is required Salt bridge is generally required6 Ions are discharged at both the Ions are discharged at the cathode while electrodes anode is consumed.
  34. 34. One of the main uses ofelectrochemical cells is the generationof portable electrical energy.Two or more cells are connected inseries to form a battery which acts asa source of electrical energy.
  35. 35. A com er ci al C l m m el ust f ul f i l l t hef ol l ow ng r equi r em s. i entI t s houl d bec o mp a c t a nd l i ghta n d e a s y t ot r a ns por t .I t s v ol t a ge mu s tn o t v a r y mu c hd u r i ng us e .
  36. 36. There are two category ofenergy producing galvanic cells
  37. 37. 1.Primary Cells (Disposable) Zinc carbon (flashlights, toys) Heavy duty zinc chloride (radios, recorders) Alkaline (all of the above) Lithium (photoflash) Silver, mercury oxide (hearing aid, watches)
  38. 38. Battery (Ancient) History1800 Voltaic pile: silver zinc1836 Daniell cell: copper zinc1859 Planté: rechargeable lead-acid cell1868 Leclanché: carbon zinc wet cell1888 Gassner: carbon zinc dry cell1898 Commercial flashlight, D cell1899 Junger: nickel cadmium cell1946 Neumann: sealed NiCd1960s Alkaline, rechargeable NiCd1970s Lithium, sealed lead acid1990 Nickel metal hydride (NiMH)1991 Lithium ion1992 Rechargeable alkaline1999 Lithium ion polymer
  39. 39. 1.Primary Cells (Disposable)In primary cells the redox reaction occurs only once and the cellbecomes dead since the chemical reactions in these are notreversible.Daniel cell, mercury cell, Dry cell etc….Daniel Cell in the commercialform consists of a zinc electrodedipping in zinc sulphate solutioncontained in a porous pot. The potis placed ia a cylindrical coppervessel containing copper sulphatesolution.
  40. 40. Zinc Rod(Anode) - .... .... + .... .... Copper Vessel (Cathode) .... .... .... .... .... .... ZnSO4 Solution .... .... .... .... .... .... .... .... .... .... Porous Pot .... .... .... .... .... .... .... .... CuSO4 Solution .... .... .... …. .... CuSO4 Crystals Daniel Cell (Commercial Form)
  41. 41. The porous pot allows the passage of onlyions from one solution to another and servesthe purpose of salt bridge in the conventionalgalvanic cell. When connections are made asshown electrons flow from zinc to copperand current is assumed to flow from copperto zinc.Zn(s) + CuSO4 (aq) ZnSO4 (aq) + Cu(s) Zn(s) / Zn2+(aq) // Cu2+(aq)/Cu(s) The e.m.f of the cell is 1.1 V
  42. 42. Dry Cell Battery NS Anode (-) Zn ---> Zn2+ + 2e- Cathode (+) 2 NH4+ + 2e-  2 NH3 + H2 Give the net E0 of the complete recation
  43. 43. Alkaline Battery NSNearly same reactions asin common dry cell, butunder basic conditions.Anode (-): Zn + 2 OH-  ZnO + H2O + 2e-Cathode (+): 2 MnO2 + H2O + 2e-  Mn2O3 + 2 OH-
  44. 44. Mercury Battery NSAnode: Zn is reducing agent under basic conditionsCathode: HgO + H2O + 2e- ---> Hg + 2 OH-
  45. 45. 2. Secondary cells (Rechargeable) Nickel cadmium Nickel metal hydride Alkaline Lithium ion Lithium ion polymer Lead acid
  46. 46. In a secondary cell, the chemicalreactions taking place are reversibleand can be reversed by passingelectricity.Since these cells can berecharged, they can be used again andagain.A battery consists of two or more
  47. 47. Lead Storage BatteryAnode (-) Pb + HSO4- ---> PbSO4 + H+ + 2e- Eo = +0.36 VCathode (+) PbO2 + HSO4- + 3 H+ + 2e- ---> PbSO4 + 2 H2O E0 = +1.68 V
  48. 48. Lead Storage BatteryThe lead acid accumulator used in automobiles consists of 3 to 6 cellsto get a voltage of 6 to 12.The cell has anode made of spongy lead presses in to grids andcathode made of lead dioxide, PbO2 presses into grid made of lead.A number of lead plates are connected in parallel and a number of leaddioxide plates are also connected in parallel.The plates are arranged alternately, separated by thin perforatedplastic or fibre glass sheets.The whole arrangement is suspended in the electrolyte which isdilutee sulphuric acid of density 1.31 gml-1, taken in a plastic or hardrubber vessel.
  49. 49. Working of Lead Storage Cell When discharging a lead storage cell, At Anode Lead loses electrons which flow through the wire to the cathode. Pb Pb2+ + 2e- The lead ions combine with the sulphate ions from sulphuric acid forming a precipitate of lead sulphate. Pb2+ + SO42- PbSO4 At Cathode The electrons flowing from anode react with PbO2 of cathode and PbO2 is reduced to Pb2+ in presence of H+ ions from H2SO4. PbO2 + 4H+ + 2e- Pb2+ + 2 H2 O The lead ions formed at the cathode react with sulphate ions forming a precipitate of lead sulphate. Pb2+ + SO42- PbSO4
  50. 50. Overall ReactionPb + PbO2 + 4H+ + 2SO42- 2PbSO4 + 2H2O + Energy Total EMF = 2V
  51. 51. When you charge a battery, you are forcing the electrons backwards (from the + to the -). To do this, you will need a higher voltage backwards than forwards. This is why the ammeter in your car often goes slightly higher while your battery is charging, and then returns to normal.In your car, the battery charger iscalled an alternator. If you have adead battery, it could be thebattery needs to be replaced ORthe alternator is not charging thebattery properly.
  52. 52. Charging of Lead Storage CellDuring discharging, both the electrodes get covered with PbSO4 and thedilute sulphuric acid is consumed and its density falls from 1.31 to 1.2 g/ml.When recharging an external e.m.f greater than 2 volts is passed from a generator torecharge the cell. The positive pole of the generator is connected to positive pole ofthe storage cell.At Anode (+ve terminal)PbSO4 + 2H2O PbO2 + 4H+ + SO42- + 2e- At Cathode (-ve terminal)PbSO4 + 2e- Pb + SO42- Overall reaction2PbSO4 + 2H2O + Energy Pb + PbO2 + 4H+ + 2SO42-
  53. 53. Ni-Cad BatteryAnode (-) : Cd + 2 OH- ---> Cd(OH)2 + 2e-Cathode (+) : NiO2 + 2H2O + 2e- ---> Ni(OH)2 + 2OH-Cell representation: Cd CdO KOH NiO2 Ni
  54. 54. 1.The metal atoms convert to metal ions. M Mn+ + ne- (Oxidation) - - - - - + - + + - + + - + + +
  55. 55. 1.The metal ions in solution gains electrons from metal leaving a positive charge on the metal. Mn+ + ne- M (Reduction) + + + - + - - + - - + - - + - - -
  56. 56. Whatever may be the process, an electrical double layergenerates in between the metal and the solution. This electricaldouble layer generates a potential difference.The potential difference set up between the metal and its ions inthe solution is called electrode potential.It is a measure of the tendency of an electrode to lose or gainelectrons when it is in contact with its own ions in solution.i) If oxidation takes place at the electrode, thepotential is called oxidation potential.ii) If reduction takes place at the electrode, thepotential is called reduction potential.
  57. 57. Electromotive Force (EMF)When two half cells are connected, due to thedifference in potential an electric current flowsfrom the electrode of higher potential to theelectrode of lower potential.The difference in potentials of two half cells of acell is known as electromotive force or emf ofthe cell or cell potential. EMF = Ecathode - Eanode
  58. 58. Electrochemical SeriesIt is an arrangement ofelements in the increasingorder of their standardreduction potential.
  59. 59. Metal SRP, Eo Lithium----------------- -3.05 V Potassium Calcium SodiumDecreasing Magnesium Increasingtendency Aluminum order of stdto loose Zinc reductionelectrons Nickel potential Tin Hydrogen--------------- 0.00 Copper Silver Platinum Gold---------------------- +1.15 V
  60. 60. Characteristics of ECS1. Metals lying above hydrogen are easily rusted.2. Iron and metals above it decomposes steam, liberating hydrogen gas.3. Oxides of iron and metals below it are decomposed easily.4. Oxides of mercury and metals below it are decomposed on heating.
  61. 61. Applications ofElectrochemical Series
  62. 62. 1. It gives an idea regarding thetendency of elements to lose or gainelectrons.Elements with lower reduction potential have a tendency tolose electrons, that is greater tendency to get oxidized.So they are good reducing agents.Elements with higher reduction potential have a tendency toreceive electrons, that is greater tendency to get reduced.So they are good oxidizing agents.
  63. 63. 2. Displacement ReactionAn element above in the series candisplace an element below it.In otherwords, an element with lowerreduction potential can displace an elementwith higher reduction potential.Eg: Zinc has lower reduction potential thanCopper. Hence zinc displaces copper fromCuSO4 solution.
  64. 64. 3. When a cell is constructed, anodeshould be a metal higher in the seriesand cathode a metal lower in theseries.Eg: When a cell is constructed using zinc andcopper, Zn which is higher in the series will be theanode and copper will be the cathode. + - Zn Cu Anode Cathode
  65. 65. 4. A metal above hydrogen in theseries can displace H2 gas from diluteacid. But a metal below hydrogencannot liberate H2 gas from acid. 2Na + H2SO4 Na2SO4 + H2 2K + H2SO4 K2SO4 + H2 Ca + H2SO4 CaSO4 + H2 Mg + H2SO4 MgSO4 + H2
  66. 66. Fuel cells are galvanic cells in whichchemical energy from combustion offuel such as H2, CO, CH4 (gases)alcohols (liquids) can be convertedinto electrical energy.About 75% of the chemical energycan be converted into electricalenergy.
  67. 67. H2/O2 as a FuelCars can use electricity generated by H2/O2 fuelcells.H2 carried in tanks or generated fromhydrocarbons (fuel)
  68. 68. The cell consists of two electrodes madeof porous graphite impregnated with acatalyst Pt, Ag or CuO.They are placed in aqueous concentrated(35%) solution of NaOH or KOH.H2 gas and O2 gas are continuouslybubbled through the porous electrodes atthe anode and cathode respectively at apressure of 50 atm.
  69. 69. The reaction at the electrodes are, At anode 2H2 + 4OH- 4H2O + 4e- At cathode O2 + 2H2O + 4e- 4OH- Overall reaction 2H2 + O2 2H2O + energyThe cell will produce an emf of about 1 volt. It is used in militaryequipments, manned space crafts and submarines.
  70. 70. Fuel: 1 to 2 molar methanol in water.To keep the concentration of methanol constant amixture of effluent and fresh methanol is recycled.
  71. 71. Anode: Oxidation of methanolCH3-OH + H2O CO2 + 6H+ + 6e-The protons H+ move from anode to the cathode via the electrolyte.Cathode: Reduction of oxygen3/2 O2 + 6H+ + 6e- 3H2OThe overall reactionCH3-OH + 3/2 O2 CO2 + 2H2O This type of cells are used as energy source in (i) Space vehicles (ii) Submarines (iii) military vehicles (iv) automobiles.
  72. 72. Advantages of Fuel cells1.It converts energy of the fuel directly to electrical energy.2.Do not cause pollution problems.3.Fuel cells are light and compact.4.Efficiency is very high (60-75%)5.Energy supply is continuous and without any drop.