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2 2 Electrochemistry (Mark=2) 76 slides940701

3 3 Electrochemistry All of Chemical reactins are related to ELECTRONS Redox reactions 76 slides940701

4 Chemical Reactions Electric Power Power consumption Power generation Electrolytic cells Galvanic cells 76 slides940701

5 5 Electrochemistry Conduction 1)Metalic 2)Electrolytic Temprature  Motion of ions  Resistance  slides940701

6 battery +- power source e-e- e-e- Ions Chemical change (-)(+) Aqueous NaCl Interionic attractions Ions Solvation Solvent viscosity Na + Cl - H2OH2O Electrolytic conduction Ion-Ion Attr. Ion- Solvent Attr. Solvent–Solvent Attr. Temprature  Attractions  & Kinetic energy  Conduction  76 slides Conduction ≈ Ions mobility

7 battery +- inert electrodes power source vessel e-e- e-e- conductive medium Electrolytic Cell Construction 76 slides

8 +- battery Na (l) electrode half-cell Molten NaCl Na + Cl - Na + Na + + e -  Na2Cl -  Cl 2 + 2e - Cl 2 (g) escapes Observe the reactions at the electrodes NaCl (l) (-) Cl - (+) 76 slides940701

9 +- battery e-e- e-e- NaCl (l) (-)(-)(+)(+) cathode anode Molten NaCl Na + Cl - Na + Na + + e -  Na 2Cl -  Cl 2 + 2e - cations migrate toward (-) electrode anions migrate toward (+) electrode At the microscopic level 76 slides940701

10 Molten NaCl Electrolytic Cell cathode half-cell (-) REDUCTION Na + + e -  Na anode half-cell (+) OXIDATION2Cl -  Cl 2 + 2e - overall cell reaction 2Na + + 2Cl -  2Na + Cl 2 X 2 Non-spontaneous reaction! 76 slides940701

11 What chemical species would be present in a vessel of aqueous sodium chloride, NaCl (aq)? Na + Cl - H2OH2O Will the half-cell reactions be the same or different? 76 slides940701

Water Complications in Electrolysis In an electrolysis, the most easily oxidized and most easily reduced reaction occurs. When water is present in an electrolysis reaction, then water (H 2 O) can be oxidized or reduced according to the reaction shown. ElectrodeIons...Anode RxnCathode Rxn E° Pt (inert)H 2 O H 2 O (l) + 2e-  H 2(g) + 2OH - (aq) V H 2 O 2 H 2 O (l)  4e - + 4H + (g) + O 2(g) V Net Rxn Occurring: 2 H 2 O  2 H 2(g) + O 2 (g) E° = V

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14 battery +- power source e-e- e-e- NaCl (aq) (-)(+) cathode different half-cell Aqueous NaCl anode 2Cl -  Cl 2 + 2e - Na + Cl - H2OH2O What could be reduced at the cathode? 76 slides 2H 2 O + 2e -  H 2 + 2OH

15 Aqueous NaCl Electrolysis possible cathode half-cells (-) REDUCTION Na + + e -  Na 2H 2 O + 2e -  H 2 + 2OH - possible anode half-cells (+) OXIDATION2Cl -  Cl 2 + 2e - 2H 2 O  O 2 + 4H + + 4e - overall cell reaction 2Cl - + 2H 2 O  H 2 + Cl 2 + 2OH slides940701

16 Aqueous CuCl 2 Electrolysis possible cathode half-cells (-) REDUCTION Cu e -  Cu 2H 2 O + 2e -  H 2 + 2OH - possible anode half-cells (+) OXIDATION2Cl -  Cl 2 + 2e - 2H 2 O  O 2 + 4H + + 4e - overall cell reaction Cu Cl -  Cu (s) + Cl 2(g) 76 slides940701

17 Aqueous Na 2 SO 4 Electrolysis possible cathode half-cells (-) REDUCTION Na + + e -  Na [2H 2 O + 2e -  H 2 + 2OH - ] possible anode half-cells (+) OXIDATION SO 4 2-  S 4 O 8 2_ + 2e - 2H 2 O  O 2 + 4H + + 4e - overall cell reaction 6H 2 O  2H 2 + O 2 +4H + + 4OH slides ×

18 Faraday’s Law The mass deposited or eroded from an electrode depends on the quantity of electricity. Quantity of electricity = coulomb (Q) Q = It coulomb current in amperes (amp) time in seconds 76 slides940701

19 e-e- Ag + Ag For every electron, an atom of silver is plated on the electrode. Ag + + e -  Ag Electrical current is expressed in terms of the ampere, which is defined as that strength of current which, when passed thru a solution of AgNO 3 (aq) under standard conditions, will deposit silver at the rate of g Ag/sec Experimentally: 1 amp = g Ag/sec 76 slides coulomb = 1 amp-sec = g Ag Q = It

20 Ag + + e -  Ag 1.00 mole Ag =1.00 mole e - = g Ag g Ag/mole e g Ag/coul = 96,485 coul/mole e - 1 Faraday ( F ) mole e - = Q/ F 76 slides C=1AS /// 1J=1CV

21 A series of solutions have 50,000 coulombs passed thru them, if the solutions were Au +3, Zn +2, and Ag +, and Au, Zn, and Ag were plated out respectively, calculate the amount of metal deposited at each anode. battery M Au M Zn M Ag + Au e -  AuZn e -  ZnAg + + e -  Ag e-e- e-e- e-e- e-e slides940701

22 Examples using Faraday’s Law 1)How many grams of Cu will be deposited in 1L of A)0.1 M CuSO 4 B) 1 M CuSO 4 After 3.00 hours electrolysis by a current of 4.00 amps?(Cu=64) Cu e -  Cu 2)The charge on a single electron is x coulomb. Calculate Avogadro’s number from the fact that 1 F = 96,487 coulombs/mole e slides940701

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25 Industrial Electrolysis Processes Slide 25 of slides940701

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27 Volta’s battery (1800) Alessandro Volta Paper moisturized with NaCl solution Cu Zn 76 slides940701

28 Cu 1.0 M CuSO 4 Zn 1.0 M ZnSO 4 Salt bridge – KCl in agar Provides conduction between half-cells Galvanic Cell Construction Observe the electrodes to see what is occurring slides940701

29 Cu 1.0 M CuSO 4 Zn 1.0 M ZnSO 4 Cu plates out or deposits on electrode Zn electrode erodes or dissolves Cu e -  Cu cathode half-cell Zn  Zn e - anode half-cell Anod - Cathod + What about half-cell reactions? What about the sign of the electrodes? What happened at each electrode? Why? 76 slides Compare with Electrolytic cells

30 +- battery e-e- e-e- NaCl (l) (-)(-)(+)(+) Cathode - Anode + Electrolytic cells sign of the electrodes? Na + Cl - Na + Na + + e -  Na 2Cl -  Cl 2 + 2e slides940701

31 Electrodes are passive (not involved in the reaction) Olmsted Williams 76 slides940701

32 H 2 input 1.00 atm inert metal How do we calculate Standard Redox Potentials? We need a standard electrode to make measurements against! The Standard Hydrogen Electrode (SHE) Pt 1.00 M H + 25 o C 1.00 M H atm H 2 Half-cell 2H + + 2e -  H 2 E o SHE = 0.0 volts 76 slides940701

E 0 is for the reaction as written E 0 red // E 0 ox The more positive E 0 the greater the tendency for the substance to be reduced The half-cell reactions are reversible The sign of E 0 changes when the reaction is reversed Changing the stoichiometric coefficients of a half-cell reaction does not change the value of E slides Strongest oxidunt Strongest reductant

34 Measuring E 0 red Cu 2+ & Zn 2+ Slide 34 of 52 cathode anode 76 slides Cu e -  Cu E=E 0 red Zn  Zn e - E=E 0 ox -E=E 0 red

35 Cu 1.0 M CuSO 4 Zn 1.0 M ZnSO 4 cathode half-cell Cu e -  Cu anode half-cell Zn  Zn e Measuring E 0 of a cell 1.1 volts 76 slides ?

36 What is the standard emf of an electrochemical cell made of a Cd electrode in a 1.0 M Cd(NO 3 ) 2 solution and a Cr electrode in a 1.0 M Cr(NO 3 ) 3 solution? Cd 2+ (aq) + 2e -  Cd (s) E 0 = V Cr 3+ (aq) + 3e -  Cr (s) E 0 = V Cd is the stronger oxidizer Cd will oxidize Cr 2e - + Cd 2+ (1 M) Cd (s) Cr (s) Cr 3+ (1 M) + 3e - Anode (oxidation): Cathode (reduction): 2Cr (s) + 3Cd 2+ (1 M)  3Cd (s) + 2Cr 3+ (1 M) x 2 x 3 E 0 cell = =0.34 cell E 0 = 0.34 V cell slides E 0 = V E 0 = 0.74 V E 0 cell = ? !!

37 Calculating the cell potential, E o cell, at standard conditions Fe e -  Fe E o = v O 2 (g) + 2H 2 O + 4e -  4 OH - E o = v This is spontaneoues corrosion or the oxidation of a metal. Consider a drop of oxygenated water on an iron object Fe H 2 O with O 2 Fe  Fe e - -E o = v2x 2Fe + O 2 (g) + 2H 2 O  2Fe(OH) 2 (s) E o cell = v reverse 76 slides Fe + O 2 (g) + H 2 O  Fe(OH) 2 (s) Which one is oxidunt ?

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39  G o = -n F E o cell Free Energy and the Cell Potential Cu  Cu e - E o = Ag + + e -  Ag E o = v 2x Cu + 2Ag +  Cu Ag E o cell = v where n is the number of electrons for the balanced reaction What is the free energy for the cell? 1 F = 96,500 J/v 76 slides Cu + 2Ag +  Cu Ag  G o = -2×96500×0.46= J

40 - E depends on: -Related half reaction - Concentration -kinetic e - +2H +  H 2 E 0 = Fe  3e - +Fe 3+ E 0 = Fe +H +  Fe 3+ +H 2 E 0 = Spontaneous redox reaction ????? !!!!!!!No =========================================================================================== 76 slides V V

V 76 slides e - +Cu +  Cu E 0 = V Cu +  Cu 2+ +e - E 0 = V Cu +  Cu 2+ +Cu E 0 = 0.368V 2Cu +  Cu 2+ +Cu Auto redox=Dis proportionation

slides V Auto redox=Dis proportionation ?????? 2e - +Fe 2+  Fe E 0 = V Fe 2+  Fe 3+ +e - E 0 = V × Fe 2+  2Fe 3+ +Fe E 0 = V NO

slides V e +Fe 3+  Fe E0= ? No e isn’t a function state 1) e +Fe 3+  Fe 2+ E0= ) 2e +Fe 2+  Fe E0=-0.440

44  G 0 =-nE 0 f= -3E 0 f 76 slides  G 0 =-nE 0 f 2) 2e +Fe 2+  Fe E 0 = ) e +Fe 3+  Fe 2+ E 0 =  G 0 =-1(+0.771) F=-0.771f  G 0 =-2(-0.440) F=+0.880f 3e +Fe 3+  Fe  G 0 =+0.109f =+0.109f 3E 0 = E 0 = v

Free Energy and Chemical Reactions 45 W W q q ΔGΔG ΔGΔG ΔHΔH ΔHΔH TΔSTΔS TΔSTΔS Spontaneous reaction Ideal reverse cell Operating cell slides ΔG = ΔH - T·ΔS W = ΔH - q

slides Ni (s) | Ni 2+ (XM) || Sn 2+ (YM) | Sn (s) A cell 2 e - + Sn 2+ → Sn (s) Ni (s) → 2 e - + Ni 2+ Ni (s) + Sn 2+ → Ni 2+ + Sn (s) Redox reaction Cathode Anode Representation of a cell

slides Ni (s) | Ni 2+ (1M) || Sn 2+ (1M) | Sn (s) Ni (s) → 2 e - + Ni 2+ Eº =0.230 V Ni (s) + Sn 2+ (1M) → Ni 2+ (1M) + Sn (s) CathodeAnode Emf of a standard cell Eº = =0.090V 2 e - + Sn 2+ → Sn (s) Eº=-0.140V

48 Effect of Concentration on Cell EMF A voltaic cell is functional until E = 0 at which point equilibrium has been reached. The point at which E = 0 is determined by the concentrations of the species involved in the redox reaction. The Nernst Equation 76 slides E = E o – RT ln Q nf /-nf E = E o log Q n

49 Effect of Concentration on Cell EMF at 25 o C: E = E o log Ni 2+ / Sn 2+ n Calculate the E red for the hydrogen electrode where 0.1 M H + and 0.2 atm H slides Ni (s) | Ni 2+ (XM) || Sn 2+ (YM) | Sn (s) Ni (s) + Sn 2+ (YM) → Ni 2+ (XM) + Sn (s) Eº= V Q= Ni 2+ / Sn 2+ E= /2×logx/y E= /2×logpH2/[H + ] 2 2H + +2e →H 2 Q=X/Y

50 Ni (s) + Sn 2+ → Ni 2+ + Sn (s) Eº= V 76 slides Ni (s) | Ni 2+ (0.600M) || Sn 2+ (0.300M) | Sn (s) Emf of a cell

51 Emf of a cell Sn (s) | Sn 2+ (1.0M) || Pb 2+ (0.0010M) | Pb (s) 2 e - + Pb 2+ → Pb (s) Eº= V Sn (s) → 2 e - + Sn 2+ Eº=0.136V 76 slides E=E º /2log[Sn 2+ ]/[Pb 2+ ] E= !!!= Reversed cell Sn (s) + Pb 2+ (0.0010M) → Sn 2+ (1.0M) + Pb (s) Eº cell =0.010 V pb (s) | pb 2+ (1.0M) || sn 2+ (0.0010M) | sn (s) E= (Electrolytic cell) (Galvanic cell)

52 equilibrium constant of a cell at equilibrium E = 0 Nernst Equation: 76 slides E = E o log B n A A BA B

slides Ni (s) | Ni 2+ (0.600M) || Sn 2+ (0.300M) | Sn (s) Ni (s) + Sn 2+ → Ni 2+ + Sn (s) Eº= V equilibrium constant of a cell

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55 Electrod potential and electrolysis Theoritical emf of a Voltaic cell is maximum voltage. (Practically is less) Theoritical emf of an electrolysis cell is minimum voltage. (Practically is more) Emf is related to: Resistance Concentration Overvoltage 76 slides

56 Electrod potential and electrolysis E° = V) 2H 2 O + 2e -  H 2 + 2OH - (E° = V) In aqueous salts electrolysis [OH - ] =1× M 76 slides E = E° - log Q n V E = E° - log [OH - ] 2 pH V E = log [1*10 -7 ] 2 *1=

57 Electrod potential and electrolysis E° = V) 2H 2 O  O 2 + 4H + + 4e - (E° = V) In aqueous salts electrolysis [H + ] =1× M 76 slides E = E° - log Q n V E = E° - log [H + ] 4 pO V E = log [1*10 -7 ] 4 *1=

58 Effect of concentration in aqueous Na 2 SO 4 electrolysis possible cathode half-cells (-) E° = V) REDUCTION Na + + e -  Na (E° = V) E° = V) [2H 2 O + 2e -  H 2 + 2OH - ] (E° = V) E = V) (E = V) possible anode half-cells (+) E° = V) OXIDATION SO 4 2-  S 4 O 8 2_ + 2e - (E° = V) E° = V) 2H 2 O  O 2 + 4H + + 4e - (E° = V) E = V) (E = V) overall cell reaction 6H 2 O  2H 2 + O 2 + 4H + + 4OH - E = = slides ×

59 Electrod potential and electrolysis Overvoltage(OV): (Because of slow rate of reaction) OV of deposition of metals are low OV of liberation of gases are appreciable (O 2 & H 2 >Cl 2 ) 76 slides

60 Effect of overvoltage & concentration in aqueous NaCl Electrolysis possible cathode half-cells (-) E° = V) REDUCTION Na + + e -  Na (E° = V) E° = V) [2H 2 O + 2e -  H 2 + 2OH - ] (E° = V) E = V) (E = V) possible anode half-cells (+) E° = V) OXIDATION2Cl -  Cl 2 + 2e - (E° = V) E° = V) 2H 2 O  O 2 + 4H + + 4e - (E° = V) E = V) (E = V) OVERVOLTAGE H2 & O2 > Cl 2 overall cell reaction 2Cl - + 2H 2 O  H 2 + Cl 2 + 2OH slides940701

61 Effect of overvoltage & concentration in aqueous CuCl 2 Electrolysis possible cathode half-cells (-) E° = V) REDUCTION Cu e -  Cu (E° = V) E° = V) 2H 2 O + 2e -  H 2 + 2OH - (E° = V) E = V) (E = V) possible anode half-cells (+) E° = V) OXIDATION2Cl -  Cl 2 + 2e - (E° = V) E° = V) 2H 2 O  O 2 + 4H + + 4e - (E° = V) E = V) (E = V) OVERVOLTAGE H2 & O2 > Cl 2 overall cell reaction Cu Cl -  Cu (s) + Cl 2(g) 76 slides

62 Cu CuSO 4 Cu Cu e -  CuCu  Cu e - What happened at each electrode? 76 slides battery Impure Cu pure Cu Anode + Cathode - Pure Cu deposit on cathode = (Pure cathodic Cu) What happens if aqueous CuSO 4 electrolyze between 2 Cu electrodes ?=purification of Cu

63 What happens if aqueous CuSO 4 electrolyze between 2 Cu electrodes ?=purification of Cu possible anode half-cells (+) (Impure Cu) E° = V) OXIDATION Cu  Cu e - (E° = V) E° = V) 2H 2 O  O 2 + 4H + + 4e - (E° = V) E = V) (E = V) 2SO 4 2- E° = -2.01V ) 2SO 4 2-  S 2 O e - (E° = -2.01V ) possible cathode half-cells (-) (Purified Cu) E° = V) REDUCTION Cu e -  Cu (E° = V) E° = V) 2H 2 O + 2e -  H 2 + 2OH - (E° = V) E = V) (E = V) 76 slides (((Purified cathodic Cu))) overall cell reaction Cu 2+ + Cu (s) Anod  Cu 2+ + Cu (s) Cathode

64 Cu 1.0 M CuSO 4 Cu 1.0 M CuSO 4 A galvanic cell with the similar electrods and electrolytes slides volts

65 Cu 1.0M CuSO 4 Cu 0.1 M CuSO 4 A galvanic cell with the similar electrods but different concentration electrolytes ؟؟ adipour 76 slides volts

66 Electrolysis of Copper Concentration Cells A concentration cell based on the Cu/Cu2+ half-reaction. A, Even though the half-reactions involve the same components, the cell operates because the half-cell concentrations are different. B, The cell operates spontaneously until the half-cell concentrations are equal. Note the change in electrodes (exaggerated here for clarity) and the equal color of solutions slides940701

76 slides67 Cu │ Cu 2+ (0.1M) ‖ Cu 2+ (1.0 M) │Cu Anod cathod E=E /2Log(0.1/1.0) = Concentration Cells Cu+Cu 2+ (1.0 M)  Cu 2+ (0.1M)+Cu

68 pH meter, A concentration Cell slides Slide 68 of 52 2 H + (1 M) → 2 H + (x M) Pt | H 2 (1 atm)|H + (x M) ||H + (1.0 M) |H 2 (1 atm) | Pt(s) 2 H + (1 M) + 2 e - → H 2 (g, 1 atm) H 2 (g, 1 atm) → 2 H + (x M) + 2 e - H 2 (g, 1 atm) +2 H + (1 M) → 2 H + (x M) + H 2 (g, 1 atm)

69 Slide 69 of 52 E cell = E cell ° - log n V x2x E cell = 0 - log V x2x2 1 E cell = V log x E cell = (0.059 V) pH 2 H + (1 M) → 2 H + (x M) E cell = E cell ° - log Q n V 76 slides pH = E cell /(0.059)

70 The pH Meter In practice, a special pH electrode is much more convenient than using platinum electrodes and a tank of hydrogen gas! A stable reference electrode and a glass-membrane electrode are contained within a combination pH electrode. The electrode is merely dipped into a solution, and the potential difference between the electrodes is displayed as pH slides940701

71 Corrosion of Fe: Unwanted Voltaic Cells O2+2H 2 O+4e - →4OH - Rust formation: Fe 2+ →Fe 3+ +e E 0 = V O 2 (g) + 4 H + (aq) + 4 e - → 2 H 2 O (aq) E 0 = V Fe 2+ (aq) + O 2 (g) + 4H + (aq)  4Fe 3+ (aq) + 2H 2 O(l) E 0 =0.458V 2Fe 3+ (aq) + 4H 2 O(l)  Fe 2 O 3  H 2 O(s) + 6H + (aq) 76 slides E0=0.440 VE0=1.229 V E 0 =0.401 V

72 Prevention of Corrosion Cover the Fe surface with a protective coating Paint Tin (Tin plate) Zn (Galvanized iron) 76 slides940701

73 Corrosion Protection Slide 73 of slides Fe →Fe 2+ +2e E 0 =0.440 V Cu →Cu 2+ +2e E 0 =0.337 V Fe →Fe 2+ +2e E 0 =0.440 V Zn →Zn 2+ +2e E 0 =0.763 V

74 Corrrosion Protection (cathode) (electrolyte) (anode) 76 slides Fe →Fe 2+ +2e E 0 =0.440 V Mg →Mg 2+ +2e E 0 =2.363 V Steel pipe don’t rust Fe →Fe 2+ +2e E 0 =0.440 V E 0 =0.401 V

75 galvanicelectrolytic need power source two electrodes produces electrical current anode (-) cathode (+) anode (+) cathode (-) salt bridge vessel conductive medium Comparison of Electrochemical Cells  G < 0  G > slides940701

76 Cathodic Protection In cathodic protection, an iron object to be protected is connected to a chunk of an active metal. The iron serves as the reduction electrode and remains metallic. The active metal is oxidized. Water heaters often employ a magnesium anode for cathodic protection slides940701