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Electrochemistry

Electrochemistry. Chapter 17. Electrochemistry. _____________________ – A process in which an element attains a more positive oxidation state Na(s)  Na + + e - ______________________ – A process in which an element attains a more negative oxidation state Cl 2 + 2e -  2Cl -.

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Electrochemistry

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  1. Electrochemistry Chapter 17

  2. Electrochemistry • _____________________– A process in which an element attains a more positive oxidation state Na(s)  Na+ + e- • ______________________– A process in which an element attains a more negative oxidation state Cl2 + 2e- 2Cl-

  3. Electrochemistry An old memory device for oxidation and reduction goes like this… LEOsaysGER LoseElectrons =____________ GainElectrons=____________

  4. Electrochemistry • _______________________ The substance that is reduced is the oxidizing agent • _______________________ The substance that is oxidized is the reducing agent

  5. Electrochemistry • _______________ The electrode where oxidation occurs • _______________ The electrode where reduction occurs Memory device: Reduction at the Cathode

  6. Galvanic Cells

  7. Galvanic Cells • A device in which chemical energy is changed to ______________________ energy. (Opposite is called electrolysis) • The solutions must be connected so ions can flow to keep the net charge zero. A __________________________(a U-tube filled with an electrolyte) is used to connect the solutions. • ______________________ is used to measure the cell potential (Ecell)

  8. Table of Reduction Potentials Measured against the StandardHydrogenElectrode

  9. Measuring Standard Electrode Potential Potentials are measured against a hydrogen ion reduction reaction, which is arbitrarily assigned a potential of _______________________________.

  10. Standard Reduction Potentials 2H+(aq) + Zn(s)  Zn2+(aq) + H2(g) The voltage measured for this cell is 0.76V. It is impossible to measure each separate half-reaction but if the standard hydrogen reduction (1 M H+ and 1 atm) is assigned a zero voltage then the standard zinc reaction occurring at the anode has a voltage of 0.76V.

  11. Galvanic (Electrochemical) Cells Spontaneous redox processes have: A ________________ cell potential, E0 A _________________ free energy change, (-G)

  12. Zn - Cu Galvanic Cell Zn2+ + 2e- Zn E = Cu2+ + 2e-  Cu E = From a table of reduction potentials:

  13. Zn - Cu Galvanic Cell The less positive, or more negative reduction potential becomes the oxidation…

  14. Example Problem Al3+ (aq) + Mg(s)  Al(s) + Mg2+ (aq) Give the balanced chemical equation and Eo for the cell.

  15. Line Notation An abbreviated representation of an electrochemical cell Anode material Cathode material Anode solution Cathode solution | || |

  16. Calculating G0 for a Cell n= moles of electrons in balanced redox equation F=Faraday constant = ______________ coulombs/mol e- Zn + Cu2+  Zn2+ + Cu E0= + 1.10 V

  17. The Nernst Equation Standard potentials assume a concentration of 1 M. The Nernst equation allows us to calculate potential when the two cells are not 1.0 M. R= 8.31 J/(molK) T = Temperature in K n = moles of electrons in balanced redox equation F = Faraday constant = 96,485 coulombs/mol e-

  18. Nernst Equation Simplified At 25 C (298 K) the Nernst Equation is simplified this way:

  19. Equilibrium Constants and Cell Potential At equilibrium, forward and reverse reactions occur at equal rates, therefore: • The battery is __________________ • The ____________________, E, is zero volts Modifying the Nernst Equation (at 25 C):

  20. Concentration Cell ??? Both sides have the same components but at different concentrations. Step 1: Determine which side undergoes oxidation, and which side undergoes reduction.

  21. Concentration Cell ??? Both sides have the same components but at different concentrations. Anode Cathode The 1.0 M Zn2+ must decrease in concentration, and the 0.10 M Zn2+ must increase in concentration

  22. Concentration Cell Concentration Cell ??? Both sides have the same components but at different concentrations. Anode Cathode Step 2: Calculate cell potential using the Nernst Equation (assuming 25 C).

  23. Nernst Calculations Zn2+ (1.0M)  Zn2+ (0.10M)

  24. Electrolytic Processes Electrolytic processes are NOT spontaneous. They have:

  25. Electrolysis of Water In acidic solution Anode rxn: -1.23 V Cathode rxn: -0.83 V

  26. Electroplating of Silver Anode reaction: Cathode reaction: Electroplating requirements: 1. Solution of the plating metal 2. Anode made of the plating metal 3. Cathode with the object to be plated 4. Source of current

  27. Solving an Electroplating Problem How many seconds will it take to plate out 5.0 grams of silver from a solution of AgNO3 using a 20.0 Ampere current? Ag+ + e-  Ag

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