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Electrochemistry

Electrochemistry. Putting redox reactions to work. Redox review. Electrons are transferred L ose E lectrons O xidation G ain E lectrons R eduction. Electrochemical Cells. Made of two half-cells Based upon two half-reactions Electrons travel between the two half-cells. Galvanic Cells.

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Electrochemistry

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  1. Electrochemistry Putting redox reactions to work

  2. Redox review • Electrons are transferred • Lose Electrons Oxidation • Gain Electrons Reduction

  3. Electrochemical Cells • Made of two half-cells • Based upon two half-reactions • Electrons travel between the two half-cells

  4. Galvanic Cells • Also called voltaic cells • Convert chemical energy into electrical energy • Spontaneous

  5. Electrolytic Cells • Convert electrical energy into chemical energy • Non-spontaneous

  6. Making a Galvanic Cell • Write the reaction for solid magnesium placed in a copper (II) sulfate solution. Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s)

  7. Making a Galvanic Cell • Balance the reaction using the half-reaction method +2 -2 +6 0 +2 -2 +6 0 Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s)

  8. Making a Galvanic Cell • Balance the reaction using the half-reaction method +2 -2 +6 0 +2 -2 +6 0 Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s) Mg (s)  Mg2+ (aq) + 2e- Cu2+(aq) + 2e- Cu(s)

  9. Making a Galvanic Cell • Balance the reaction using the half-reaction method +2 -2 +6 0 +2 -2 +6 0 Mg (s) + CuSO4 (aq) MgSO4 (aq) + Cu (s) 1(Mg (s)  Mg2+ (aq) + 2e-) 1(Cu2+(aq) + 2e- Cu(s)) Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-

  10. Making a Galvanic Cell

  11. Making a Galvanic Cell • Anode • Cathode • Salt Bridge • Flow of electrons

  12. Measuring the Output of a Galvanic Cell • Potential (either half-cell or cell) • Pull on the electrons • Electromotive force (emf) • Volt (V) • Joule/Coulomb (J/C) • Voltmeter • Analog • Digital • Potentiometer • Positive potential…spontaneous • Negative potential…nonspontaneous

  13. Measuring the Output of a Galvanic Cell • Standard Reduction Potentials Chart • Only reduction reactions • Must look up the reverse of the oxidation and flip the sign of the potential • Add standard half-cell potentials to get standard cell potential

  14. Measuring the Output of a Galvanic Cell 1(Mg (s)  Mg2+ (aq) + 2e-) E˚ox= +2.37 V 1(Cu2+(aq) + 2e- Cu(s)) E˚red= +0.342 V Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e- E˚cell = +2.71 V

  15. Writing a Line Notation • Oxidation||Reduction • X(s)|X+(aq)||Y+(aq)|Y(s) • Mg(s)|Mg2+(aq)||Cu2+(aq)|Cu(s) 1(Mg (s)  Mg2+ (aq) + 2e-) 1(Cu2+(aq) + 2e- Cu(s)) Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e-

  16. Writing a Line Notation • Cu(s)|Cu2+(aq)||Ag1+(aq)|Ag(s) 1(Cu (s)  Cu2+ (aq) + 2e-) E˚ox= -0.342V 2(Ag1+(aq) + 1e- Ag(s)) E˚red= +0.800V Cu(s)+2Ag1+(aq)+2e- Cu2+(aq)+2Ag (s)+2e- E˚cell= +0.458V

  17. Batteries • Series of electrochemical cells connected to each other • Completes the circuit • Dry cell • Flashlight battery • Watch battery • Wet Cell • Car battery

  18. Batteries • Carbon-Zinc Battery • Zinc casing…anode • Carbon rod…cathode • MnO2 is actually reduced • Alkaline battery…has KOH rather than NH4Cl

  19. Batteries • Carbon-Zinc Battery • Zn(s) Zn2+(aq) + 2e- • 2NH41+(aq) + 2MnO2(s) + 2e- Mn2O3(s) + 2NH3(g) + H2O(l)

  20. Batteries • Lead-Acid Storage Battery • Pb(s) + PbO2(s) + H2SO4(aq) PbSO4(s) + H2O(l) • Spontaneous & nonspontaneous

  21. Concentration Cells • Not 1M • Require additional calculations • Can manipulate potential to a particular V

  22. Concentration Cells • Nernst Equation • Ecell = E˚cell – {(0.0592/n)(logQ)} • n • Q

  23. Concentration Cells • A 0.500M solution of copper (II) sulfate is reacted with magnesium metal. A 0.750M solution of magnesium sulfate is one of the products. What is the cell potential? • Write two half reactions • Write balanced equation • Determine n • Determine E˚cell • Use Nernst to solve for Ecell • Mg(s)|Mg2+(aq)||Cu2+(aq)|Cu(s)

  24. Concentration Cells 1(Mg (s)  Mg2+ (aq) + 2e-) E˚ox= +2.37 V 1(Cu2+(aq) + 2e- Cu(s)) E˚red= +0.342 V Mg (s)+CuSO4 (aq)+2e- MgSO4 (aq)+Cu (s)+2e- E˚cell = +2.71 V

  25. Concentration Cells Ecell = 2.71 V – {(0.0592/2)(log([0.75]/[0.5]))} Ecell = 2.71 V – {(0.0296)(0.176)} Ecell = 2.71 V – 0.00521 Ecell = 2.70 V

  26. Concentration Cells • Cu(s)|Cu2+(0.0100M)||Ag1+(0.0250M)|Ag(s)

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