Chapter 22 Electrochemistry Objectives: 1. describe how an electrolytic cell works 2. describe how galvanic (voltaic) cell works 3.
Download ReportTranscript Chapter 22 Electrochemistry Objectives: 1. describe how an electrolytic cell works 2. describe how galvanic (voltaic) cell works 3.
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 Updated March 2007 Created by C. Ippolito 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 Updated March 2007 Created by C. Ippolito March 2007 Half Reactions • All redox reactions – reduction + oxidation 2Na(s) + Cl2(g) 2NaCl(s) – Half-reactions • oxidation (OIL) Na0 Na+ • reduction (RIG) Cl2 2Cl- Updated March 2007 Created by C. Ippolito March 2007 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) Updated March 2007 Created by C. Ippolito 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 Updated March 2007 Created by C. Ippolito 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 Updated March 2007 move between electrodes Created by C. Ippolito March 2007 Electroplating • use of electrolysis to coat a material with a layer of metal – Copper strip – anode – coin – cathode – electrolyte – copper sulfate Updated March 2007 Created by C. Ippolito 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 Updated March 2007 Created by C. Ippolito March 2007 Zinc-Copper Voltaic Cell Updated March 2007 Created by C. Ippolito March 2007 Zinc-Copper Voltaic Cell External Circuit Oxidation Reduction ZnoZn2+ + 2e- Cu2+ + 2e- Cuo Internal Circuit Updated March 2007 Created by C. Ippolito March 2007 Zinc-Copper Voltaic Cell • Zinc-Copper Voltaic Cell – Zn(s)|ZnSO4(aq)||CuSO4(aq)|Cu(s) – oxidized half cell is always written first Updated March 2007 Created by C. Ippolito March 2007 Dry Cells • Voltaic cell with “paste” electrolyte Updated March 2007 Created by C. Ippolito March 2007 Lead Storage Battery Updated March 2007 Created by C. Ippolito March 2007 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 E cell E reduction E oxidation 0 Updated March 2007 0 0 Created by C. Ippolito March 2007 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 - Updated March 2007 Created by C. Ippolito March 2007 0 E H Standard Reduction Potentials • determined using standard hydrogen electrode and the equation for standard cell potential E cell E reduction E oxidation 0 0 E cell E 0 0 0 H E Zn 2 0 0 .76V 0 .00V E Z n 2 0 0 .7 6V Updated March 2007 0 E Zn 2 Created by C. Ippolito March 2007 Calculating Standard Cell Potentials • Use Table of Reduction Potentials to predict the halfcells of reduction and oxidation. 0 E cell is positive reaction SP O N TA N E O U S 0 E cell is negative reaction N O N SP O N TA N E O U S • Given reaction: Zn(s) + 2Ag+(aq) Zn2+ + 2Ag(s) • Write half-reactions and look up E0 Zn(s) Zn2+(aq) + 2eAg+ + e- Ag(s) E cell E reduction 0 Updated March 2007 0 E0 = -0.76V E0 = +0.80V 0 E oxidation E0 = 0.80V-(-0.76V) = +1.56V Created by C. Ippolito March 2007 Corrosion • • the deterioration and wearing away of metals usually through “oxidation” Prevention: 1. 2. 3. 4. 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 Updated March 2007 Created by C. Ippolito March 2007