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Chapter 22 Electrochemistry

Chapter 22 Electrochemistry. Objectives: describe how an electrolytic cell works describe how galvanic (voltaic) cell works determine net voltage from paired standard half-cells in a galvanic cell predict products using standard reduction potentials and an activity series. Electrochemistry.

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Chapter 22 Electrochemistry

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  1. Chapter 22 Electrochemistry Objectives: describe how an electrolytic cell works describe how galvanic (voltaic) cell works determine net voltage from paired standard half-cells in a galvanic cell predict products using standard reduction potentials and an activity series Created by C. Ippolito March 2007

  2. Electrochemistry • the relationship between the flow of electric current and chemical changes • Electrolysis • changes electrical energy into chemical energy • charging a car battery • electroplating a metal • involves reactions with partial gain/loss of electrons • Electrochemical Cell • changes chemical energy into electrical energy • dry cells running iPod, cell phone etc • involves reactions with complete gain/loss of electrons Created by C. Ippolito March 2007

  3. Half Reactions • All redox reactions – reduction + oxidation 2Na(s) + Cl2(g)  2NaCl(s) • Half-reactions • oxidation (OIL) Na0 Na+ • reduction (RIG) Cl2  2Cl- Created by C. Ippolito March 2007

  4. Half-Reaction Equations • All redox reactions – reduction + oxidation 2Na(s) + Cl2(g)  2NaCl(s) • Half-reaction equations • represent chemical changes of redox • oxidation reaction 2Na0 2Na+ + 2e- (OIL) • reduction reaction Cl2 + 2e- 2Cl- (RIG) Created by C. Ippolito March 2007

  5. Electric Current • flow of electric charge • Metallic Conduction • movement of loosely held valance electrons • Ionic Conduction • movement of positive and negative ions along a path • Direct Current • electrons flow in only one direction • from negative terminal to positive terminal Created by C. Ippolito March 2007

  6. Electrolysis • electric current causes redox in the electrolyte in an electrolytic cell • source of current • electrodes • cathode (- terminal) – site of reduction • anode (+ terminal) – site of oxidation • electrolyte • aqueous or liquid permits ions to move between electrodes Created by C. Ippolito March 2007

  7. Electroplating • use of electrolysis to coat a material with a layer of metal • Copper strip – anode • coin – cathode • electrolyte – copper sulfate Created by C. Ippolito March 2007

  8. Electrochemical Cell • Galvanic (voltaic) Cells • electric current from spontaneous redox rxns • chemical energy  electrical energy • Battery • multiple voltaic cells act as a unit • Electromotive Force (emf) • voltage between the electrodes • affected by: • temperature • metals used • electrolyte concentration Created by C. Ippolito March 2007

  9. Zinc-Copper Voltaic Cell Created by C. Ippolito March 2007

  10. Zinc-Copper Voltaic Cell External Circuit Oxidation ZnoZn2+ + 2e- Reduction Cu2+ + 2e- Cuo Internal Circuit Created by C. Ippolito March 2007

  11. Zinc-Copper Voltaic Cell • Zinc-Copper Voltaic Cell • Zn(s)|ZnSO4(aq)||CuSO4(aq)|Cu(s) • oxidized half cell is always written first Created by C. Ippolito March 2007

  12. Dry Cells • Voltaic cell with “paste” electrolyte Created by C. Ippolito March 2007

  13. Lead Storage Battery Created by C. Ippolito March 2007

  14. Electric Potential • measures cell’s ability to produce current • results from a competition for electrons • reduction potential – tendency of a given half reaction to occur as reduction oxidation • reduction occurs in the cell with the greater reduction potential • cell potential – difference between the reduction potentials Created by C. Ippolito March 2007

  15. Standard Cell Potential • measured when • ion concentrations = 1M • 25oC and 1 atmosphere (101 kPa) • Standard hydrogen electrode used with others to determine reduction potentials • assigned reduction potential 0.00 V - Created by C. Ippolito March 2007

  16. Standard Reduction Potentials • determined using standard hydrogen electrode and the equation for standard cell potential Created by C. Ippolito March 2007

  17. Calculating Standard Cell Potentials • Use Table of Reduction Potentials to predict the half-cells of reduction and oxidation. • Given reaction: Zn(s) + 2Ag+(aq)  Zn2+ + 2Ag(s) • Write half-reactions and look up E0 Zn(s)  Zn2+(aq) + 2e- E0 = -0.76V Ag+ + e- Ag(s) E0 = +0.80V E0 = 0.80V-(-0.76V) = +1.56V Created by C. Ippolito March 2007

  18. Corrosion • the deterioration and wearing away of metals usually through “oxidation” • Prevention: • coat with paint to stop water and oxygen contact • electroplate with less reactive metal • alloy with another metal (stainless steel – Fe & Cr) • protect metal by making it the “cathode” Mg strips on ship hulls corrode instead of the hull Created by C. Ippolito March 2007

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