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Topic: Electrochemical Cells

Topic: Electrochemical Cells. Do Now : 5 color pencils. Electrochemistry – the study of the relationship between chemical potential energy and electrical energy . All this occurs Via flow of electrons. Chemical rxns can produce electricity OR

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Topic: Electrochemical Cells

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  1. Topic: Electrochemical Cells Do Now: 5 color pencils

  2. Electrochemistry – the study of the relationship between chemical potential energy and electrical energy

  3. All this occurs Via flow of electrons • Chemical rxns can produce electricity OR • Electrical energy can be used carry out chemical reactions A Chemical reactions can generate electrical energy if it occurs spontaneously

  4. How does a spontaneous reaction generate electricity? Electrochemical Cells AKA voltaic cell or galvanic cell • Used when the chemical rxn is spontaneous • It produces electricity (energy) via flow of electrons through a wire therefore is exothermic • Oxidation and Reduction Reaction have to be separated

  5. Remember the lab SR labAgNO3 + Cu  _______ +_______ +5 0 +1 +5 -2 0 -2 +1 CuNO3 Ag Was this spontaneous? Was this a redox? So why didn’t we produce electricity? yes Yes, Ag was Reduced & Cu was Oxidized Half reactions weren’t separated

  6. Electrochemical Cell Requirements • 2 half-cells containing aqueous solution& an electrode • Need to be connected by a wire for the electrons to flow through. • Need to be connected by a salt bridgeto maintain electrical neutrality. (if electrons are moving ions need to move also = + ions will flow in same direction as electrons) Zn+2aq SO4-2 aq Cu+2aq SO4-2 aq

  7. Voltmeter = measures electrical currentWhen the cells reach equilibriumvoltage = 0 =no more electrical current

  8. Electrode Surface at which oxidation or reduction half-reaction occurs. Anode – Oxidation The anode = location for the oxidation half-reaction. Reduction – Cathode The cathode = location for the reduction half-reaction.

  9. MOVIE CLIP • MEMORIZE • FatRed Cat ateAn Ox • Anode oxidation • Fat (getting bigger) Reduction Cathode Crap  yummy

  10. Since Cu is the cathode – a fat red cat = gain mass • Where does the mass come from? • Reduction of Cu+2 is gaining 2 e- to make Cu • So the concentration of Cu+2 ions decreases • Since Zn is the anode – An ox was eaten = lost e- = getting smaller • Oxidation = Lose electrons, who is going to lose Zn or Zn+2 • Zn Zn+2 + 2e- • So the concentration of Zn+2 increases

  11. Is this reaction at equilibrium? How can you tell? What is happening to the concentration of Zn+2 ions? What is happening to the concentration of Cu+2 ions? No – volts not a 0 increasing decreasing

  12. How do you know which electrode is which? • Use Table J to predict which electrode is the anode / cathode. • higher in Table J. gets oxidized • Anode = Oxidation = Electron Donor • lower in Table J. gets reduced • Cathode = Reduction = Electron Acceptor

  13. Label anode and cathode anode cathode

  14. e-  e-  e-  e-  e-  e- • Electrons flow from Anode to Cathode • Cathode is (+) Anode is (-) • Positive Ions flow in same direction as electrons via Salt bridge (too offset the negative electrons) • Which direction would the negative ions flow? +  +  +  + anode cathode Anode = site of oxidation =getting smaller cathode = site of reduction =getting larger

  15. What’s wrong with this picture? No salt bridge = needed to complete the circuit so ions can flow

  16. Now you tryCreate an galvanic (electrochemical) cell with Al and Pb and AlNO3 and Pb(NO3)2Label anode and cathode(Use Table J), direction of electron flow in wire, direction of positive ion flow in salt bridge, positive electrode, negative electrode. Pb Al

  17. Create an galvanic (electrochemical) cell with Al and Pb and AlNO3 and Pb(NO3)2Label anode and cathode(Use Table J), direction of electron flow in wire, direction of positive ion flow in salt bridge, positive electrode, negative electrode. e-  e-  e-  e-  e-  e- Pb Al Al cathode anode Al+3& NO3-1 Pb+2& NO3-1

  18. What half-reactions occured?REMEMBER Al got smaller and Pb got bigger Al  Al+3 + 3e- Pb+2 + 2e-  Pb Al was the anode, it was oxidized, it lost e- Pb got bigger. How? By gaining electrons. The Pb+2 ions gained 2 e- to make Pb.

  19. Overall Spontaneous Rxn 2(Al  Al+3 + 3e-) 3(Pb+2 + 2e-  Pb) + _______________________ 2Al + 3Pb+2 2Al+3 + 3Pb

  20. 2Al + 3Pb+2 2Al+3 + 3Pb • Which electrode is losing mass? • Which electrode is gaining mass? • What’s happening to the [Al+3]? • What’s happening to the [Pb+2]? Al because now Al+3 ions in solution Pb, Pb+2 gains 2e- an makes solid Pb Increasing Decreasing

  21. Notation for Cells

  22. Application: Batteries

  23. Dry Cell

  24. Mercury battery

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