1 ELECTROCHEMICAL CELLS Chapter 20 : D8 C20

21.2 Half-Cells and Cell Potentials > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrical Potential What causes the electrical potential of an electrochemical cell? The electrical potential of a voltaic cell is a measure of the cell’s ability to produce an electric current. Electrical potential is usually measured in volts (V).

3 Electrons are transferred from Al to Cu 2+, but there is no useful electric current. Energy released as HEAT. Electrochemical Reactions

4 If Al and Cu are separated work is done by the electrons. Voltmeter is used to measure the energy. What determines the measured value?

5 Terms Used for Voltaic Cells Figure 20.3

6 The Cu|Cu 2+ and Ag|Ag + Cell

21.2 Half-Cells and Cell Potentials > 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrical Potential The potential of an isolated half- cell cannot be measured. You cannot measure the electrical potential of a zinc half-cell or of a copper half-cell separately. When these two half-cells are connected to form a voltaic cell, however, the difference in potential can be measured.

8 CELL POTENTIAL, E 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 CELL POTENTIAL, E STANDARD CELL POTENTIAL, E o is when [Zn 2+ ] and [Cu 2+ ] = 1.0 M. For Zn/Cu cell, potential is V 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.

21.2 Half-Cells and Cell Potentials > 10 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrical Potential The standard hydrogen electrode is used with other electrodes so the reduction potentials of the other cells can be measured. The standard reduction potential of the hydrogen electrode has been assigned a value of 0.00 V.

11 CELL POTENTIALS, E o. Can’t measure 1/2 reaction E o directly. Therefore, measure it relative to a STANDARD HYDROGEN CELL. 2 H + (aq, 1 M) + 2e- H 2 (g, 1 atm) E o = 0.0 V SHE

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21.2 Half-Cells and Cell Potentials > 13 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrical Potential The difference between the reduction potentials of the two half-cells is called the cell potential. cell potential = – reduction potential of half-cell in which reduction occurs reduction potential of half-cell in which oxidation occurs or E cell = E red – E oxid –

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 Reduction of H + by Zn No current

16 Zn(s) + 2H + (aq) --> Zn 2+ + H 2 (g) E o = V Therefore, oxidation potential E o for Zn ---> Zn 2+ (aq) + 2e- is V Overall: reduction of H + by Zn metal.

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

19 Calculating Cell Voltage 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.

20 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 (calc’d) = V Cathode, positive, sink for electrons Anode, negative, source of electrons +

21.2 Half-Cells and Cell Potentials > 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrical Potential The tendency of a given half-reaction to occur as a reduction is called the reduction potential. The half-cell in which reduction occurs has a greater reduction potential than the half-cell in which oxidation occurs.

22 Using Standard Potentials, E o Table 20.1 Which is the best oxidizing agent (reduced): O 2, H 2 O 2, or Cl 2 ? _________________ Which is the best reducing agent (oxidized): Hg, Al, or Sn? ____________________

21.2 Half-Cells and Cell Potentials > 23 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrical Potential The standard cell potential (E 0 cell ) is the measured cell potential when the ion concentrations in the half-cells are 1M, any gases are at a pressure of 101 kPa, and the temperature is 25°C. E 0 cell = E 0 red – E 0 oxid

21.2 Half-Cells and Cell Potentials > 24 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculating Standard Cell Potentials How can you determine if a redox reaction is spontaneous? Positive Cell Potentials

21.2 Half-Cells and Cell Potentials > 25 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculating Standard Cell Potentials If the cell potential for a given redox reaction is positive, then the reaction is spontaneous as written. If the cell potential is negative, then the reaction is non-spontaneous.

26 Uses of E o Values Organize half- reactions by relative ability to act as oxidizing agents (reduced) Table 20.1Table 20.1 Use this to predict cell potentials and direction of redox reactions.

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28 REDUCTION POTENTIALS

21.2 Half-Cells and Cell Potentials > 29 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Oxidation:Al(s) → Al 3+ (aq) + 3e – (at anode) Reduction:Fe 3+ (aq) + 3e – → Fe(s)(at cathode) Al(s) + Fe 3+ (aq) → Al 3+ (aq) + Fe(s) Determine the cell reaction for a voltaic cell composed of the following half-cells. Fe 3+ (aq) + 3e – → Fe(s) E 0 cell = –0.036 V Al 3+ (aq) + 3e – → Al(s) E 0 cell = –1.66 V

21.2 Half-Cells and Cell Potentials > 30 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Show that the following redox reaction between zinc metal and silver ions is spontaneous. Sample Problem 21.1 Determining Reaction Spontaneity Zn(s) + 2Ag + (aq) → Zn 2+ (aq) + 2Ag(s) If E 0 cell is positive, the reaction is spontaneous.

21.2 Half-Cells and Cell Potentials > 31 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. First identify the half-reactions. Sample Problem 21.1 Oxidation: Zn(s) → Zn 2+ (aq) + 2e – Reduction:Ag + (aq) + e – → Ag(s) Write both half-cells as reductions with their standard reduction potentials. Zn 2+ (aq) + 2e – → Zn(s)E 0 Zn 2+ = –0.76 V Ag + (aq) + e – → Ag(s)E 0 Ag + = V

21.2 Half-Cells and Cell Potentials > 32 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculate the standard cell potential. Sample Problem 21.1 E 0 cell = E 0 red – E 0 oxid = E 0 Ag + – E 0 Zn 2+ = V – (–0.76 V) = V E 0 cell > 0, so the reaction is spontaneous.

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

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

35 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

36 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?

21.2 Half-Cells and Cell Potentials > 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. In a voltaic cell in which one half-reaction is Fe e – → Fe, which of the following would occur as oxidation ? A.2H + + 2e – → H 2 B.Al e – → Al C.Br 2 + 2e – → 2Br – D.Fe 3+ + e – → Fe 2+ B.

21.2 Half-Cells and Cell Potentials > 38 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts The electrical potential of a cell results from a competition for electrons between two half-cells. You can determine the standard reduction potential of a half-cell by using a standard hydrogen electrode and the equation for standard cell potential.

21.2 Half-Cells and Cell Potentials > 39 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts & Key Equation If the cell potential for a given redox reaction is positive, then the reaction is spontaneous as written. If the cell potential is negative, then the reaction is nonspontaneous. E 0 cell = E 0 red – E 0 oxid

21.2 Half-Cells and Cell Potentials > 40 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms electrical potential: the ability of a voltaic cell to produce an electric current reduction potential: a measure of the tendency of a given half-reaction to occur as a reduction (gain of electrons) in an electrochemical cell cell potential: the difference between the reduction potentials of two half-cells

21.2 Half-Cells and Cell Potentials > 41 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms standard cell potential (E 0 cell ): the measured cell potential when the ion concentrations in the half-cells are 1.00M at 1 atm of pressure and 25°C standard hydrogen electrode: an arbitrary reference electrode (half-cell) used with another electrode (half-cell) to measure the standard reduction potential of that cell; the standard reduction potential of the hydrogen electrode is assigned a value of 0.00 V

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43 RED = REDUCED BLACK = OXIDIZED e-