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Redox potentials Electrochemistry ICS.

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Presentation on theme: "Redox potentials Electrochemistry ICS."— Presentation transcript:

1 Redox potentials Electrochemistry ICS

2 Redox Reactions Oxidation loss of electrons Reduction
gain of electrons oxidizing agent substance that cause oxidation by being reduced reducing agent ICS

3 Electrochemistry In the broadest sense, electrochemistry is the study of chemical reactions that produce electrical effects and of the chemical phenomena that are caused by the action of currents or voltages. ICS


5 Voltaic Cells harnessed chemical reaction which produces an electric current ICS

6 Voltaic Cells Cells and Cell Reactions Daniel's Cell
Zn(s) + Cu+2(aq) ---> Zn+2(aq) + Cu(s) oxidation half reaction anode Zn(s) ---> Zn+2(aq) e- reduction half reaction cathode Cu+2(aq) e- ---> Cu(s) ICS

7 Voltaic Cells copper electrode dipped into a solution of copper(II) sulfate zinc electrode dipped into a solution of zinc sulfate ICS

8 Voltaic Cells ICS

9 Hydrogen Electrode consists of a platinum electrode covered with a fine powder of platinum around which H2(g) is bubbled. Its potential is defined as zero volts. Hydrogen Half-Cell H2(g) = 2 H+(aq) e- reversible reaction ICS

10 ICS

11 ICS

12 Standard Reduction Potentials
the potential under standard conditions (25oC with all ions at 1 M concentrations and all gases at 1 atm pressure) of a half-reaction in which reduction is occurring ICS

13 Some Standard Reduction Potentials Table 18-1, pg 837
Li+ + e- ---> Li v Zn e- ---> Zn v Fe e- ---> Fe v 2 H+(aq) e- ---> H2(g) v Cu e- ---> Cu v O2(g) H+(aq) e- ---> 2 H2O(l) v F e- ---> 2 F v ICS

14 If the reduction of mercury (I) in a voltaic cell is desired, the half reaction is:
Which of the following reactions could be used as the anode (oxidation)? A, B ICS

15 Cell Potential the potential difference, in volts, between the electrodes of an electrochemical cell Direction of Oxidation-Reduction Reactions positive value indicates a spontaneous reaction ICS

16 Standard Cell Potential
the potential difference, in volts, between the electrodes of an electrochemical cell when the all concentrations of all solutes is 1 molar, all the partial pressures of any gases are 1 atm, and the temperature at 25oC ICS

17 Zn(s)/ZnSO4(aq)//CuSO4(aq)/Cu(s)
Cell Diagram the shorthand representation of an electrochemical cell showing the two half-cells connected by a salt bridge or porous barrier, such as: Zn(s)/ZnSO4(aq)//CuSO4(aq)/Cu(s) anode cathode ICS

18 Metal Displacement Reactions
solid of more reactive metals will displace ions of a less reactive metal from solution relative reactivity based on potentials of half reactions metals with very different potentials react most vigorously ICS

19 Ag+ + e- --->Ag E°= 0.80 V Cu2+ + 2e- ---> Cu E°= 0.34 V
Will Ag react with Cu2+? yes, no Will Cu react with Ag+? ICS

20 Gibbs Free Energy and Cell Potential
DG = - nFE where n => number of electrons changed F => Faraday’s constant E => cell potential ICS

21 Applications of Electrochemical Cells
Batteries device that converts chemical energy into electricity Primary Cells non-reversible electrochemical cell non-rechargeable cell Secondary Cells reversible electrochemical cell rechargeable cell ICS

22 Applications of Electrochemical Cells
Batteries Primary Cells "dry" cell & alkaline cell 1.5 v/cell mercury cell v/cell fuel cell 1.23v/cell Secondary Cells lead-acid (automobile battery) 2 v/cell NiCad v/cell ICS

23 “Dry” Cell ICS

24 “Dry” Cell ICS

25 “Flash Light” Batteries
"Dry" Cell Zn(s) MnO2(s) NH > Zn+2(aq) MnO(OH)(s) NH3 Alkaline Cell Zn(s) MnO2(s) ---> ZnO(s) + Mn2O3(s) ICS

26 “New” Super Iron Battery
Mfe(VI)O4 + 3/2 Zn /2 Fe(III)2O3 + 1/2 ZnO + MZnO2 (M = K2 or Ba) Environmentally friendlier than MnO2 containing batteries. ICS

27 ICS

28 Lead-Acid (Automobile Battery)

29 Lead-Acid (Automobile Battery)
Pb(s) + PbO2(s) H2SO4 = 2 PbSO4(s) H2O 2 v/cell ICS

30 Nickel-Cadmium (Ni-Cad)
Cd(s) Ni(OH)3(s) = Cd(OH)2(s) Ni(OH)2(s) NiCad v/cell ICS

31 ICS

32 Automobile Oxygen Sensor

33 Automobile Oxygen Sensor
see Oxygen Sensor Movie from Solid-State Resources CD-ROM ICS

34 pH Meter pH = (Eglass electrode - constant)/0.0592 ICS

35 Effect of Concentration on Cell Voltage: The Nernst Equation
Ecell = Eocell - (RT/nF)ln Q Ecell = Eocell - (0.0592/n)log Q where Q => reaction quotient Q = [products]/[reactants] ICS

36 EXAMPLE: What is the cell potential for the Daniel's cell when the [Zn+2] = 10 [Cu+2] ? Q = ([Zn+2]/[Cu+2] = (10 [Cu+2])/[Cu+2] = 10 Eo = (0.34 V)Cu couple + (-(-0.76 V)Zn couple n = 2, 2 electron change Ecell = Eocell - (0.0257/n)ln Q thus Ecell = ( (0.0257/2)ln 10) V Ecell = ( (0.0257/2)2.303) V Ecell = ( ) V = V ICS

37 Nernst Equation Q [H ] ® E = – RT nF ln – 2.3 F log [h ] ® E
+ ] acid side base side E = o RT nF ln Q – 2.3  F log base side acid side [h + ] p-type side n-type side E (in volts) = – 2.3  RT F log n-type side p-type side © 1993 American Chemical Society In an electrochemical concentration cell, the cell potential depends on the concentrations of ions (in this case, H+ ion) in the two half-cells. In semiconductors the voltage obtainable with a p-n junction depends on the relative concentration of electrons or holes on the two sides of the junction. ICS

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