Created by C. Ippolito March 2007 Updated March 2007 Chapter 22 Electrochemistry Objectives: 1.describe how an electrolytic cell works 2.describe how galvanic (voltaic) cell works 3.determine net voltage from paired standard half-cells in a galvanic cell 4.predict products using standard reduction potentials and an activity series

Created by C. Ippolito March 2007 Updated March 2007 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 Updated March 2007 Half Reactions All redox reactions – reduction + oxidation 2Na(s) + Cl 2 (g)  2NaCl(s) –Half-reactions oxidation (OIL) Na 0  Na + reduction (RIG) Cl 2  2Cl -

Created by C. Ippolito March 2007 Updated March 2007 Half-Reaction Equations All redox reactions – reduction + oxidation 2Na(s) + Cl 2 (g)  2NaCl(s) –Half-reaction equations represent chemical changes of redox –oxidation reaction 2Na 0  2Na + + 2e - (OIL) –reduction reaction Cl 2 + 2e -  2Cl - (RIG)

Created by C. Ippolito March 2007 Updated March 2007 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 Updated March 2007 Electrolysis electric current causes redox in the electrolyte in an electrolytic cell 1.source of current 2.electrodes –cathode (- terminal) – site of reduction –anode (+ terminal) – site of oxidation 3.electrolyte –aqueous or liquid permits ions to move between electrodes

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

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

Created by C. Ippolito March 2007 Updated March 2007 Zinc-Copper Voltaic Cell Oxidation Zn o  Zn e - Reduction Cu e -  Cu o External Circuit Internal Circuit

Created by C. Ippolito March 2007 Updated March 2007 Zinc-Copper Voltaic Cell –Zn(s)|ZnSO 4 (aq)||CuSO 4 (aq)|Cu(s) –oxidized half cell is always written first

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

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

Created by C. Ippolito March 2007 Updated March 2007 Electric Potential measures cell’s ability to produce current results from a competition for electrons –reduction potential –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 –cell potential – difference between the reduction potentials

Created by C. Ippolito March 2007 Updated March 2007 Standard Cell Potential measured when –ion concentrations = 1M –25 o C 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 Updated March 2007 Standard Reduction Potentials determined using standard hydrogen electrode and the equation for standard cell potential

Created by C. Ippolito March 2007 Updated March 2007 Calculating Standard Cell Potentials Use Table of Reduction Potentials to predict the half- cells of reduction and oxidation. Given reaction: Zn(s) + 2Ag + (aq)  Zn Ag(s) Write half-reactions and look up E 0 Zn(s)  Zn 2+ (aq) + 2e - E 0 = -0.76V Ag + + e -  Ag(s) E 0 = +0.80V E 0 = 0.80V-(-0.76V) = +1.56V

Created by C. Ippolito March 2007 Updated March 2007 Corrosion the deterioration and wearing away of metals usually through “oxidation” Prevention: 1.coat with paint to stop water and oxygen contact 2.electroplate with less reactive metal 3.alloy with another metal (stainless steel – Fe & Cr) 4.protect metal by making it the “cathode” Mg strips on ship hulls corrode instead of the hull