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Electrochemical Cells. Definitions Voltaic cell (battery): An electrochemical cell or group of cells in which a product-favored redox reaction is used.

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Presentation on theme: "Electrochemical Cells. Definitions Voltaic cell (battery): An electrochemical cell or group of cells in which a product-favored redox reaction is used."— Presentation transcript:

1 Electrochemical Cells

2 Definitions Voltaic cell (battery): An electrochemical cell or group of cells in which a product-favored redox reaction is used to product an electric current. Electrochemical cell: A combination of anode, cathode, and other materials arranged so that a product-favored redox reaction can cause a current to flow or an electric current can cause a reactant- favored redox reaction to occur Galvanic cell: A cell in which an irreversible chemical reaction produces electrical current Electrolytic cell: electrochemical reactions are produced by applying electrical energy

3 Outline Electrochemistry –Electrochemical cells –Modeling a electrochemical cells –Standard Hydrogen Electrodes –Using standard reduction potentials –Nernst –Quantifying current –Electrolysis

4 A Copper-Zinc battery – What Matters?

5 What occurs at Copper electrode? Copper plates out, mass increases What occurs at Zinc electrode? Zinc is oxidized and dissolves, mass decreases

6 A Copper-Zinc battery – What Matters? Consider reduction potentials: Cu e- → Cu(s) V Zn e- → Zn(s) V Place Zn electrode in Copper Sulfate Solution – What happens? Cu e- → Cu(s) V Zn(s) → Zn e V Cu +2 + Zn(s) → Zn +2 + Cu(s)1.1 V E > 0, spontaneous Note, no need for electron to flow external to cell for reaction to occur!! Copper is plated on Zn electrode

7 A Copper-Zinc battery – What Matters? Consider reduction potentials: Cu e- → Cu(s) V Zn e- → Zn(s) V Place Cu electrode in Zinc Sulfate Solution – What happens? Cu(s) → Cu e V Zn e- → Zn(s) V Zn +2 + Cu(s) → Cu +2 + Zn(s)-1.1 V E < 0, not spontaneous No reaction occurs !! Zn doesn’t plate on copper electrode?!

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9 What are the ½ reactions? What is the overall reaction? Identify the oxidation, reduction, anode, and cathode

10 SHE: Standard Hydrogen Electrode 2 H 3 O + (aq, 1.00 M) + 2e - H 2(g, 1 atm) + 2H 2 O (l) E o = 0V Standard conditions: 1M, 1atm, 25 o C

11 Measuring Relative Potentials

12 Standard Reduction Potentials

13 The half-reaction with the more positive standard reduction potential occurs at the cathode as reduction. The half-reaction with the more negative standard reduction potential occurs at the anode as oxidation. E o cell = E o cathode - E o anode

14 Is potential always the same? Standard conditions: 1 atm, 25 o C, 1 M What will influence the potential of a cell?

15 Mathematical Relationships: Nernst The Nernst Equation: E o = standard potential of the cell R = Universal gas constant = J/mol*K T = temperature in Kelvin n = number of electrons transferred F = Faraday’s constant = 96,483.4 C/mol Q = reaction quotient (concentration of anode divided by the concentration of the cathode) E = E o - RT ln Q nF Cu +2 + Zn(s) → Zn +2 + Cu(s) Q = [Zn +2 ]/[Cu +2 ]

16 Applying the Nernst Equation This cell is operating at 25 o C with 1.00x10 -5 M Zn 2+ and 0.100M Cu 2+ ? Predict if the voltage will be higher or lower. E = E o - RT ln Q nF Cu +2 + Zn(s) → Zn +2 + Cu(s)

17 E o = standard potential of the cell R = Universal gas constant = J/mol*K T = temperature in Kelvin n = number of electrons transferred F = Faraday’s constant = 96,483.4 C/mol Q = reaction quotient (concentration of anode divided by the concentration of the cathode) E = E o - RT ln Q nF Zn e - -> Zn V Cu e - -> Cu 0.34 V E o cell = E o cathode - E o anode 25 o C = K n = x10 -5 M Zn 2+ and 0.100M Cu 2 Cu +2 + Zn(s) → Zn +2 + Cu(s) Q = [Zn +2 ]/[Cu +2 ]

18 Were your predictions correct?


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