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Types of Electrochemical Cells

Types of Electrochemical Cells. Electrolytic Cells: electrical energy from an external source causes a nonspontaneous reaction to occur Voltaic Cells (Galvanic Cells): spontaneous chemical reactions produce electricity and supply it to an external circuit. Electrical Conduction .

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Types of Electrochemical Cells

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  1. Types of Electrochemical Cells • Electrolytic Cells: electrical energy from an external source causes a nonspontaneous reaction to occur • Voltaic Cells (Galvanic Cells): spontaneous chemical reactions produce electricity and supply it to an external circuit

  2. Electrical Conduction • Electric current represents charge transfer • Charges conducted through: 1. liquid electrolytes 2. metals – metalic conduction • Ionic Conduction – conduction of an electric current through motion of ions in solution

  3. Ionic Conduction Migrate + Neg. Electrode Migrate - Pos. Electrode

  4. Electrodes • Surfaces upon which oxidation and reduction half reactions occur • May or may not participate in the reaction • Inert Electrodes – do not participate Ex. Pt, C, Pd • Reduction at cathode • Oxidation at anode

  5. Electrodes RED CAT And AN OX

  6. Ted Talk • http://ed.ted.com/lessons/electric-vocabulary

  7. Voltaic or Galvanic Cells • Spontaneous oxidation – reduction reactions produce electrical energy • Two halves of redox reaction are separated • Half cell – contains the oxidized and reduced forms of an element or other complex species

  8. Voltaic or Galvanic Cells • Salt bridge – completes circuit between the two half cells • Salt bridge is any medium through which ions can flow • Agar + Salt Gelations 1. Allows electrical contact between two solutions 2. Prevents mixing of electrode solutions 3. Maintains electrical neutrality

  9. Redox Reaction

  10. Redox Reaction

  11. Redox reaction – NOTa voltaic cell • With time, Cu plates onto the Zn metal strip, and Zn strip disappears • Electrons are transferred from Zn to Cu2+, but there is no useful electric current.

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

  13. Voltaic Cell links • http://www.chembio.uoguelph.ca/educmat/chm19105/galvanic/galvanic1.htm • http://www.youtube.com/watch?v=0oSqPDD2rMA

  14. Cu - Ag Cell

  15. Sn – Cu cell

  16. Summary of Zn, Cu, Ag • Zn – Cu Cu electrode – cathode Cu+2 is more easily reduced than Zn+2 Zn is a stronger reducing agent than Cu • Ag – Cu Cu electrode – anode Ag+ is more easily reduced than Cu+2 Cu is a stronger reducing agent than Ag • Cathode – Anode are dictated by species present

  17. Summary of Zn, Cu, Ag • Strength as oxidizing agents Zn+2 < Cu+2 < Ag+ • Strength as reducing agents Zn > Cu > Ag

  18. Standard Electrode Potentials • Magnitude of a cell’s potential measures the spontaneity of its redox reaction • Higher cell potentials indicate a greater driving force • Want to separate total cell potentials into individual potentials of the two half reactions • Determine tendencies for redox reactions

  19. Standard Hydrogen Electrode • “Every oxidation needs a reduction” e- must go somewhere • Therefore it is impossible to determine experimentally the potential of a single electrode • Establish an arbitrary standard electrode Standard Hydrogen Electrode, SHE

  20. Standard Hydrogen Electrode • Metal coated with Pt immersed in a 1.0 M H+ solution. H2 gas is bubbled at 1 atm over the electrode • Assigned a potential of 0.000 V 2 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm) E° = 0.000V H2(g, 1 atm <----> 2 H+(aq, 1 M) + 2e- E° = 0.000V

  21. Cu – SHE Cell

  22. Zn – SHE Cell

  23. Zn – Cu Cell

  24. Electromotive Series • Can develop series of standard electrode potentials • When involve metals in contact with their ions – electromotive series • Zn: Std. oxidation potential = +0.763 V • Therefore, reduction potential = -0.763 V

  25. Electromotive Series • International convention is to use reduction half reactions • Indicates tendencies of electrodes to behave as cathodes toward SHE • If E° < 0.0 V, then electrode acts as anode versus SHE

  26. Uses of the Electromotive Series • Predict the spontaneity of redox reactions Question: Will Cu+2 oxidize Zn to Zn+2 or will Zn+2 oxidize Cu? Write half reactions and make sure E° is positive. Cu+2 + 2 e- Cu E° = 0.34 V Zn Zn+2 + 2 e- E° = 0.76 V Therefore, Cu+2 will oxidize Zn to Zn+2

  27. Will Cr+3 oxidize Cu to Cu+2 or will Cu+2 oxidize Cr to Cr+3?

  28. Nernst Equation • Use when you do not have standard state conditions

  29. Problem Calculate E for Fe+3/Fe+2 electrode if the [Fe+2] is 5 times that of [Fe+3].

  30. Problem Calculate E for a Al – Cu cell in which the temperature is 20.5 °C and the [Cu+2] = 0.25 M and [Al+3] is 0.75 M.

  31. Relationship of E° to DG° and Keq • DG° = -nF E° • DG = DG° + RT ln Q • DG° = -RT ln Keq

  32. “Triangle of Truth” DG° DG° = -RT ln Keq DG° = -nFE° nFE° = RT ln Keq E°cell Keq

  33. Calculate 3 Sn+4 + 2 Cr 3 Sn+2 + 2 Cr+3 Calculate DG° and Keq

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