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ICS Redox potentials Electrochemistry. ICS Redox Reactions Oxidation loss of electrons Reduction gain of electrons oxidizing agent substance that cause.

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Presentation on theme: "ICS Redox potentials Electrochemistry. ICS Redox Reactions Oxidation loss of electrons Reduction gain of electrons oxidizing agent substance that cause."— Presentation transcript:

1 ICS Redox potentials Electrochemistry

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

3 ICS 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.

4 ICS

5 Voltaic Cells harnessed chemical reaction which produces an electric current

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

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

8 ICS Voltaic Cells

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

10 ICS

11

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

13 ICS Some Standard Reduction Potentials Table 18-1, pg 837 Li + + e - ---> Li v Zn e - ---> Zn-0.763v Fe e - ---> Fe-0.44v 2 H + (aq) + 2 e - ---> H 2(g) 0.00v Cu e - ---> Cu+0.337v O 2(g) + 4 H + (aq) + 4 e - ---> 2 H 2 O (l) v F 2 + 2e - ---> 2 F v

14 ICS 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

15 ICS 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

16 ICS 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 25 o C

17 ICS 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) /ZnSO 4(aq) //CuSO 4(aq) /Cu (s) anode cathode

18 ICS 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

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

20 ICS Gibbs Free Energy and Cell Potential G = - nFE wheren => number of electrons changed F => Faradays constant E => cell potential

21 ICS 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

22 ICS Applications of Electrochemical Cells Batteries Primary Cells "dry" cell & alkaline cell 1.5 v/cell mercury cell 1.34 v/cell fuel cell 1.23v/cell Secondary Cells lead-acid (automobile battery) 2 v/cell NiCad 1.25 v/cell

23 ICS Dry Cell

24 ICS Dry Cell

25 ICS Flash Light Batteries "Dry" Cell Zn (s) + 2 MnO 2(s) + 2 NH > Zn +2 (aq) + 2 MnO(OH) (s) + 2 NH 3 Alkaline Cell Zn (s) + 2 MnO 2(s) ---> ZnO (s) + Mn 2 O 3 (s)

26 ICS New Super Iron Battery Mfe(VI)O 4 + 3/2 Zn 1/2 Fe(III) 2 O 3 + 1/2 ZnO + MZnO 2 (M = K 2 or Ba) Environmentally friendlier than MnO 2 containing batteries.

27 ICS

28 Lead-Acid (Automobile Battery)

29 ICS Lead-Acid (Automobile Battery) Pb (s) + PbO 2(s) + 2 H 2 SO 4 = 2 PbSO 4(s) + 2 H 2 O 2 v/cell

30 ICS Nickel-Cadmium (Ni-Cad) Cd (s) + 2 Ni(OH) 3(s) = Cd(OH) 2(s) + 2 Ni(OH) 2(s) NiCad 1.25 v/cell

31 ICS

32 Automobile Oxygen Sensor

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

34 ICS pH = (E glass electrode - constant)/ pH Meter

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

36 ICS 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 E o = (0.34 V) Cu couple + (-(-0.76 V) Zn couple n = 2, 2 electron changeE cell = E o cell - (0.0257/n)ln Q thusE cell = ( (0.0257/2)ln 10) V E cell = ( (0.0257/2)2.303) V E cell = ( ) V = 1.07 V

37 ICS Nernst Equation [H + ] acid side [H + ] base side E = E o – RT nF ln Q = – 2.3 RT F log [H + ] base side [H + ] acid side [h + ] p-type side [h + ] n-type side E (in volts) = – 2.3 RT F log [h + ] n-type side [h + ] p-type side


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