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1 AnodeCathode Basic Concepts of Electrochemical Cells.

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Presentation on theme: "1 AnodeCathode Basic Concepts of Electrochemical Cells."— Presentation transcript:

1 1 AnodeCathode Basic Concepts of Electrochemical Cells

2 2 CHEMICAL CHANGE ---> ELECTRIC CURRENT With time, Cu plates out onto Zn metal strip, and Zn strip disappears. Zn is oxidized and is the reducing agent Zn(s) ---> Zn 2+ (aq) + 2e-Zn is oxidized and is the reducing agent Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e- ---> Cu(s)Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e- ---> Cu(s)

3 3 Oxidation: Zn(s) ---> Zn 2+ (aq) + 2e- Reduction: Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) Electrons are transferred from Zn to Cu 2+, but there is no useful electric current. CHEMICAL CHANGE ---> ELECTRIC CURRENT

4 4 To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire.To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire. CHEMICAL CHANGE ---> ELECTRIC CURRENT This is accomplished in a GALVANIC or VOLTAIC cell. A group of such cells is called a battery.

5 5 Electrons travel thru external wire. Salt bridge allows anions and cations to move between electrode compartments.Salt bridge allows anions and cations to move between electrode compartments. Electrons travel thru external wire. Salt bridge allows anions and cations to move between electrode compartments.Salt bridge allows anions and cations to move between electrode compartments. Zn --> Zn e- Cu e- --> Cu <--AnionsCations--> OxidationAnodeNegativeOxidationAnodeNegative ReductionCathodePositiveReductionCathodePositive

6 6 The Cu|Cu 2+ and Ag|Ag + Cell Electrons move from anode to cathode in the wire. Anions & cations move thru the salt bridge. Electrons move from anode to cathode in the wire. Anions & cations move thru the salt bridge.

7 7 Anode, site of oxidation, negative Cathode, site of reduction, positive

8 8 CELL POTENTIAL, E Electrons are driven from anode to cathode by an electromotive force or emf.Electrons are driven from anode to cathode by an electromotive force or emf. For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M.For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M. Zn and Zn 2+, anode Cu and Cu 2+, cathode 1.10 V 1.0 M

9 9 CELL POTENTIAL, E For Zn/Cu cell, potential is V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M.For Zn/Cu cell, potential is V at 25 ˚C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M. This is the STANDARD CELL POTENTIAL, E oThis is the STANDARD CELL POTENTIAL, E o a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C.a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C. This means pure solids or in solution at a concentration of 1M!!!!This means pure solids or in solution at a concentration of 1M!!!!

10 10 Calculating Cell Voltage Balanced half-reactions can be added together to get overall, balanced equation.Balanced half-reactions can be added together to get overall, balanced equation. Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) If we know E o for each half-reaction, we could get E o for net reaction. Lets revisit my haiku!Lets revisit my haiku!

11 11 Oxidation Haiku! Lost an electron But now feeling positive Oxidized is cool! What is that? You want a reduction Haiku?

12 12 Reduction Haiku!!! Gained some electrons Gave me a negative mood! Now I can say Ger! Thank you… Enjoy the buffet… Dont eat the chemicals or furniture kids!

13 13 CELL POTENTIALS, E o STANDARD HYDROGEN CELL, SHE. Cant measure 1/2 reaction E o directly. Therefore, measure it relative to a STANDARD HYDROGEN CELL, SHE. 2 H + (aq, 1 M) + 2e- H 2 (g, 1 atm) E o = 0.0 V

14 14 Zn/Zn 2+ half-cell hooked to a SHE. E o for the cell = V Zn/Zn 2+ half-cell hooked to a SHE. E o for the cell = V Negative electrode Supplier of electrons Acceptor of electrons Positive electrode 2 H + + 2e- --> H 2 ReductionCathode Zn --> Zn e- OxidationAnode

15 15 Reduction of H + by Zn Figure 20.10

16 16 Overall reaction is reduction of H + by Zn metal. Zn(s) + 2 H + (aq) --> Zn 2+ + H 2 (g) E o = V Therefore, E o for Zn ---> Zn 2+ (aq) + 2e- is V Zn is a (better) (poorer) reducing agent than H 2.

17 17 Cu/Cu 2+ and H 2 /H + Cell E o = V Acceptor of electrons Supplier of electrons Cu e- --> Cu ReductionCathode H 2 --> 2 H + + 2e- OxidationAnode Positive Negative

18 18 Cu/Cu 2+ and H 2 /H + Cell Overall reaction is reduction of Cu 2+ by H 2 gas. Cu 2+ (aq) + H 2 (g) ---> Cu(s) + 2 H + (aq) Measured E o = V Therefore, E o for Cu e- ---> Cu is V

19 19 Zn/Cu Electrochemical Cell Zn(s) ---> Zn 2+ (aq) + 2e-E o = V Cu 2+ (aq) + 2e- ---> Cu(s)E o = V Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) E o (calcd) = V Cathode, positive, sink for electrons Anode, negative, source of electrons +

20 20 Yes It is finally time for a DEMO!! Do you feel like bridging that salt?

21 21 TABLE OF STANDARD REDUCTION POTENTIALS 2 E o (V) Cu e- Cu H + + 2e- H0.00 Zn e- Zn-0.76 oxidizing ability of ion reducing ability of element

22 22 Potential Ladder for Reduction Half-Reactions Figure 20.11

23 23 Table 21.1 Page 970

24 24 Standard Redox Potentials, E o Cu e- Cu H + 2e- H Zn e- Zn Northwest-southeast rule: product-favored reactions occur between reducing agent at southeast corner (anode) and oxidizing agent at northwest corner (cathode). Any substance on the right will reduce any substance higher than it on the left.

25 25 Standard Redox Potentials, E o Any substance on the right will reduce any substance higher than it on the left. Zn can reduce H + and Cu 2+.Zn can reduce H + and Cu 2+. H 2 can reduce Cu 2+ but not Zn 2+H 2 can reduce Cu 2+ but not Zn 2+ Cu cannot reduce H + or Zn 2+.Cu cannot reduce H + or Zn 2+.

26 26 Using Standard Potentials, E o Table 20.1 In which direction do the following reactions go?In which direction do the following reactions go? Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s)Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s) 2 Fe 2+ (aq) +2 Fe 2+ (aq) + Sn 2+ (aq) ---> 2 Fe 3+ (aq) + Sn(s) What is E o net for the overall reaction?

27 27 Standard Redox Potentials, E o E˚ net = distance from top half-reaction (cathode) to bottom half-reaction (anode) E˚ net = E˚ cathode - E˚ anode E o net for Cu/Ag+ reaction = V

28 28 Cd --> Cd e- or Cd e- --> Cd Fe --> Fe e- or Fe e- --> Fe E o for a Voltaic Cell All ingredients are present. Which way does reaction proceed?

29 29 From the table, you see Fe is a better reducing agent than CdFe is a better reducing agent than Cd Cd 2+ is a better oxidizing agent than Fe 2+Cd 2+ is a better oxidizing agent than Fe 2+ E o for a Voltaic Cell Overall reaction Fe + Cd > Cd + Fe 2+ E o = E˚ cathode - E˚ anode = (-0.40 V) - (-0.44 V) = V

30 30 More About Calculating Cell Voltage Assume I - ion can reduce water. 2 H 2 O + 2e- ---> H OH - Cathode 2 I - ---> I 2 + 2e- Anode I H 2 O --> I OH - + H 2 2 H 2 O + 2e- ---> H OH - Cathode 2 I - ---> I 2 + 2e- Anode I H 2 O --> I OH - + H 2 Assuming reaction occurs as written, E˚ net = E˚ cathode - E˚ anode = ( V) - ( V) = V Minus E˚ means rxn. occurs in opposite direction

31 31 If you have reached this far, you need a break!


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