ch20---redox-equilibrium-II.ppt

5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 21Chapter 20Redox Equilibrium II:Electrochemical Cells20.1 Electrode Potentials 20.2 Half cells 20.3 Electrochemical cells 20.4 Standard Electrode Potentials 5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 2220.5 Uses of Standard Electrode Potentials 20.6 Primary and Secondary Cells 20.7 Corrosion of Iron and its Prevention 20.8 Socioeconomic Implications of Corrosion and Prevention 5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 23Electrode PotentialsDifferent absolute electrode potentials(separation of charges)developed at the surface of the electrodes.reductionoxidationM(s)Mn+(aq)+ne-Depend on:20.1 Electrode Potentials (SB p.186)Nature of electrodes&soln;temp and pressure;Conc.of ions in soln5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 2420.2 Half Cells(SB p.187)Half Cell involving MetalMetal IonZn2+(aq)+2e-Zn(s)Cu2+(aq)+2e-Cu(s)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 25Half Cell involving Non-metal-Non-metal Ion20.2 Half Cells(SB p.188)I2(aq)+2e-2I-(aq)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 2620.2 Half Cells(SB p.188)Half Cell involving Ions in Different Oxidation StatesFe3+(aq)+e-Fe2+(aq)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 27Electrochemical Cells20.3 Electrochemical Cells (SB p.189)Reduction:Zn(s)Zn2+(aq)+2e-Oxidation:Cu2+(aq)+2e-Cu(s)Overall equation:Zn(s)+Cu2+(aq)Zn2+(aq)+Cu(s)H is-ve5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 28Reduction:Zn(s)Zn2+(aq)+2e-Oxidation:Cu2+(aq)+2e-Cu(s)Overall equation:Zn(s)+Cu2+(aq)Zn2+(aq)+Cu(s)H is-ve20.3 Electrochemical Cells (SB p.189)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 2920.3 Electrochemical Cells (SB p.189)Reduction:Zn(s)Zn2+(aq)+2e-Oxidation:Cu2+(aq)+2e-Cu(s)Overall equation:Zn(s)+Cu2+(aq)Zn2+(aq)+Cu(s)H is-ve5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 210Measuring the e.m.f.of a Cell-by high resistance voltmeterVoltage difference across the two electrodes in a chemical cell20.3 Electrochemical Cells (SB p.189)must be with high resistance for accurate measurement of e.m.f.(otherwise there will be internal voltage drop across the cell)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 211Measuring the e.m.f.of a Cell-by potentiometerPQ is a resistance wire,the potential difference across which is 20V.Point R is located so there is no deflection in the Galvanometer reading.20.3 Electrochemical Cells (SB p.189)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 212Measuring the e.m.f.of a Cell-by digital multimeter20.3 Electrochemical Cells (SB p.189)Digitalmultimeter1.105/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 213What factors determine the e.m.f.of a chemical cell?Nature of electrodesNature of ions in solutionConc.of ions in solutionTemp.at which the celloperates20.3 Electrochemical Cells (SB p.190)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 214IUPAC cell convention20.3 Electrochemical Cells (SB p.192)Cell DiagramZn(s)Zn2+(aq,1M)Cu2+(aq,1M)Cu(s)E =+1.10V=anodeSalt bridgecathode Zn(s)|Zn2+(aq,1M)Cu2+(aq,1M)|Cu(s)E =+1.10Vanode-porous potcathode5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 215The absolute electrode potential of hydrogen is used as a reference standard.20.4 Standard Electrode Potentials (SB p.194)Standard Hydrogen ElectrodeThe standard conditions for a standard hydrogen electrode are:1.Temperature of electrode:298 K(25oC)2.Pressure of hydrogen gas:1 atm3.Concentration of hydroxonium ion:1M(or 1 mol dm-3)4.Platinum electrode with platinum black5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 21620.4 Standard Electrode Potentials (SB p.195)Metal-metal Ion SystemZn(s)+2H+(aq)Zn2+(aq)+H2(g)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 21720.4 Standard Electrode Potentials (SB p.196)Metal-metal Ion SystemCu2+(aq,1M)Cu(s)Ered=+0.34V5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 21820.4 Standard Electrode Potentials (SB p.197)Ion-ion SystemPt(s)H2(g,1atm)2H+(aq,1M)Fe3+(aq,1M)Pt(s)=5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 21920.4 Standard Electrode Potentials (SB p.197)Pt(s)H2(g,1 atm)H+(aq,1M)MnO4-(aq,1M)+8H+(aq,1M),Mn2+(aq,1M)+4H2O(l)Pt(s)=5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 220Very weak reducing agentVery strong Oxidizing agentVery weak oxidizing agent20.5 Uses of Standard Electrode Potentials (SB p.199)Standard reduction potentials of some common redox systems at 25oCE(V)Li+(aq)+e-Li(s)K+(aq)+e-K(s)Ca2+(aq)+2e-Ca(s)Na+(aq)+e-Na(s)Mg2+(aq)+2e-Mg(s)Al3+(aq)+3e-Al(s)Zn2+(aq)+2e-Zn(s)Fe2+(aq)+2e-Fe(s)2H+(aq)+2e-H2(g)Cu2+(aq)+2e-Cu(s)Fe3+(aq)+e-Fe2+(aq)Br2(g)+2e-2Br-(aq)Cr2O72-(aq)+6e-2Cr3+(aq)+7H2O(l)MnO4-(aq)+8H+(aq)+5e-Mn2+(aq)+4H2O(l)F2(g)+2e-2F-(aq)-3.04-2.92-2.87-2.71-2.38-1.66-0.76-0.440.00+0.34+0.77+1.07+1.33+1.52+2.87Very strong reducing agent5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 221To Calculate the e.m.f.of an Electrochemical Cell20.5 Uses of Standard Electrode Potentials (SB p.200)Ecell=Ecathode-Eanode5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 22220.5 Uses of Standard Electrode Potentials (SB p.203)To Predict Feasibility of Redox ReactionsExample 20-4Is the reaction Mg(s)+2Ag+(aq)Mg2+(aq)+2Ag(s)feasible?Given:Mg2+(aq)+2e-Mg(s)E =-2.38V Ag+(aq)+e-Ag(s)E =+0.80VSolution 2 x(Ag+(aq)+e-Ag(s)E =+0.80V-)Mg2+(aq)+2e-Mg(s)E =-2.38V 2Ag+(aq)+Mg(s)Mg2+(aq)+2Ag(s)E =+3.18VAs the E cell of the reaction has a positive value,the reaction is feasible and spontaneous.Answer5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 22320.5 Uses of Standard Electrode Potentials (SB p.204)To Predict Feasibility of Redox ReactionsIs the reaction Pb(s)+Ni2+(aq)Pb2+(aq)+Ni(s)feasible?Given:Pb2+(aq)+2e-Pb(s)E =-0.13V Ni2+(aq)+2e-Ni(s)E =-0.25V Solution Ni2+(aq)+2e-Ni(s)E =-0.25V -)Pb2+(aq)+2e-Pb(s)E =-0.13V Ni2+(aq)+Pb(s)Ni(s)+Pb2+(aq)E =-0.12VAs the Ecell of the reaction has a negative value,the reaction is not feasible.Answer5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 224Remarks1.The above predictions are based on standard reduction potential.2.The predictions may not be applicable directly to other conditions.3.Generally speaking,if the magnitude of the cell e.m.f.Calculated is greater than 0.4 V,it is quite safe to say that the prediction is valid.20.5 Uses of Standard Electrode Potentials (SB p.204)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 22520.6 Primary and Secondary Cells (SB p.207)Primary CellAt anode:Zn(s)Zn2+(aq)+2e-At cathode:2MnO2(s)+2NH4+(aq)+2e-Mn2O3(s)+2NH3(aq)+H2O(l)The overall reaction is:Zn(s)+2MnO2(s)+2NH4+(aq)Zn2+(aq)+Mn2O3(s)+2NH3(aq)+H2O(l)E =+1.50V5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 22620.6 Primary and Secondary Cells (SB p.207)Secondary CellAt anode:Pb(s)+SO42-(aq)PbSO4(s)+2e-At cathode:PbO2(s)+4H+(aq)+SO42-(aq)+2e-PbSO4(s)+2H2O(l)The overall reaction is:Pb(s)+PbO2(s)+4H+(aq)+2SO42-(aq)2PbSO4(s)+2H2O(l)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 22720.6 Primary and Secondary Cells (SB p.209)Fuel CellAt anode:H2(g)+2OH-(aq)2H2O(l)+2e-At cathode:O2(g)+2H2O(l)+4e-4OH-(aq)The overall reaction is:2H2(g)+O2(g)2H2O(l)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 22820.7 Corrosion of Iron and its Prevention (SB p.210)Electrochemical Process involved in RustingAt anodic region:Fe(s)Fe2+(aq)+2e-At cathodic region:O2(g)+2H2O(l)+4e-4OH-(aq)Fe2+(aq)+2OH-(aq)Fe(OH)2(s)2Fe(OH)2(s)+O2(g)+(x-2)H2O(l)Fe2O3 xH2O(s)rust5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 2291.Coating2.Sacrificial protection3.Alloying4.Cathodic protection20.7 Corrosion of Iron and its Prevention (SB p.211)Prevention of Corrosion5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 230Socioeconomic Implication of Corrosion and PreventionFactors concerned:1.Direct cost of rusting2.Indirect cost of rusting3.Wastage of natural resource4.Inconvenience to daily life5.Safety20.8 Socioeconomic Implications of Corrosion and Prevention (SB p.213)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 231