H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 1 Chapter 20 Redox Equilibrium II: Electrochemical Cells 20.1Electrode Potentials 20.2Half cells 20.3Electrochemical cells 20.4Standard Electrode Potentials

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Uses of Standard Electrode Potentials 20.6Primary and Secondary Cells 20.7Corrosion of Iron and its Prevention 20.8Socioeconomic Implications of Corrosion and Prevention Corrosion and Prevention

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 3 Electrode Potentials Different absolute electrode potentials (separation of charges) developed at the surface of the electrodes. reduction oxidation M(s) M n+ (aq) + ne - Depend on: 20.1 Electrode Potentials (SB p.186) Nature of electrodes & soln; temp and pressure ; Conc. of ions in soln

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Half Cells (SB p.187) Half Cell involving Metal–Metal Ion Zn 2+ (aq) + 2e - Zn(s) Cu 2+ (aq) + 2e - Cu(s)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 5 Half Cell involving Non-metal-Non-metal Ion 20.2 Half Cells (SB p.188) I 2 (aq) + 2e - 2I - (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Half Cells (SB p.188) Half Cell involving Ions in Different Oxidation States Fe 3+ (aq) + e - Fe 2+ (aq)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 7 Electrochemical Cells 20.3 Electrochemical Cells (SB p.189) Reduction: Zn(s) Zn 2+ (aq) + 2e - Oxidation: Cu 2+ (aq) + 2e - Cu(s) Overall equation: Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) ΔH is -ve

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 8 Reduction: Zn(s) Zn 2+ (aq) + 2e - Oxidation: Cu 2+ (aq) + 2e - Cu(s) Overall equation: Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) ΔH is -ve 20.3 Electrochemical Cells (SB p.189)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Electrochemical Cells (SB p.189) Reduction: Zn(s) Zn 2+ (aq) + 2e - Oxidation: Cu 2+ (aq) + 2e - Cu(s) Overall equation: Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) ΔH is -ve

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 10 Measuring the e.m.f. of a Cell ---- by high resistance voltmeter Voltage difference across the two electrodes in a chemical cell 20.3 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)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 11 Measuring the e.m.f. of a Cell ---- by potentiometer PQ is a resistance wire, the potential difference across which is 20V. Point R is located so there is no deflection in the Galvanometer reading Electrochemical Cells (SB p.189)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 12 Measuring the e.m.f. of a Cell ---- by digital multimeter 20.3 Electrochemical Cells (SB p.189) Digital multimeter 1.10

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 13 What factors determine the e.m.f. of a chemical cell? Nature of electrodes Nature of ions in solution Conc. of ions in solution Temp. at which the cell operates 20.3 Electrochemical Cells (SB p.190)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 14 IUPAC cell convention 20.3 Electrochemical Cells (SB p.192) Cell Diagram Zn(s) Zn 2+ (aq, 1M) Cu 2+ (aq, 1M) Cu(s) E = V ø = = = anodeSalt bridgecathode Zn(s) | Zn 2+ (aq, 1M) Cu 2+ (aq, 1M) | Cu(s) E = +1.10V ø anode --- porous potcathode

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 15 The absolute electrode potential of hydrogen is used as a reference standard Standard Electrode Potentials (SB p.194) Standard Hydrogen Electrode The standard conditions for a standard hydrogen electrode are: 1.Temperature of electrode: 298 K (25 o C) 2.Pressure of hydrogen gas: 1 atm 3.Concentration of hydroxonium ion: 1M ( or 1 mol dm -3 ) 4.Platinum electrode with platinum black

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Standard Electrode Potentials (SB p.195) Metal-metal Ion System Zn(s) + 2H + (aq) Zn 2+ (aq) + H 2 (g)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Standard Electrode Potentials (SB p.196) Metal-metal Ion System Cu 2+ (aq, 1M) Cu(s) E red = +0.34V ø

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Standard Electrode Potentials (SB p.197) Ion-ion System Pt(s) H 2 (g, 1atm) 2H + (aq, 1M) Fe 3+ (aq, 1M) Pt(s) = = =

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Standard Electrode Potentials (SB p.197) Pt(s) H 2 (g, 1 atm) H + (aq, 1M) [MnO 4- (aq, 1M) + 8H + (aq, 1M)], [Mn 2+ (aq, 1M) + 4H 2 O(l)] Pt(s) = = =

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 20 Very weak reducing agent Very strong Oxidizing agent Very weak oxidizing agent 20.5 Uses of Standard Electrode Potentials (SB p.199) Standard reduction potentials of some common redox systems at 25 o C E (V) ø Li + (aq) + e - Li(s) K + (aq) + e - K(s) Ca 2+ (aq) + 2e - Ca(s) Na + (aq) + e - Na(s) Mg 2+ (aq) + 2e - Mg(s) Al 3+ (aq) + 3e - Al(s) Zn 2+ (aq) + 2e - Zn(s) Fe 2+ (aq) + 2e - Fe(s) 2H + (aq) + 2e - H 2 (g) Cu 2+ (aq) + 2e - Cu(s) Fe 3+ (aq) + e - Fe 2+ (aq) Br 2 (g) + 2e - 2Br - (aq) Cr 2 O 7 2- (aq) + 6e - 2Cr 3+ (aq) + 7H 2 O(l) MnO 4 - (aq) + 8H + (aq) + 5e - Mn 2+ (aq) + 4H 2 O(l) F 2 (g) + 2e - 2F - (aq) Very strong reducing agent

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 21 To Calculate the e.m.f. of an Electrochemical Cell 20.5 Uses of Standard Electrode Potentials (SB p.200) E cell = E cathode - E anode ø ø ø

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Uses of Standard Electrode Potentials (SB p.203) To Predict Feasibility of Redox Reactions Example 20-4 Is the reaction Mg(s) + 2Ag + (aq) Mg 2+ (aq) + 2Ag(s) feasible? Given: Mg 2+ (aq) + 2e - Mg(s) E = -2.38V Ag + (aq) + e- Ag(s) E = +0.80V ø ø Solution 2 x (Ag + (aq) + e - Ag(s) E = +0.80V -) Mg 2+ (aq) + 2e - Mg(s) E = -2.38V 2Ag + (aq) + Mg(s) Mg 2+ (aq) + 2Ag(s) E = +3.18V As the E cell of the reaction has a positive value, the reaction is feasible and spontaneous. ø ø ø ø Answer

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Uses of Standard Electrode Potentials (SB p.204) To Predict Feasibility of Redox Reactions Is the reaction Pb(s) + Ni 2+ (aq) Pb 2+ (aq) + Ni(s) feasible? Given: Pb 2+ (aq) + 2e - Pb(s) E = -0.13V Ni 2+ (aq) + 2e - Ni(s) E = -0.25V ø ø Solution Ni 2+ (aq) + 2e - Ni(s) E = -0.25V -) Pb 2+ (aq) + 2e - Pb(s) E = -0.13V Ni 2+ (aq) + Pb(s) Ni(s) + Pb 2+ (aq) E = -0.12V As the E cell of the reaction has a negative value, the reaction is not feasible. ø ø ø ø Answer

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 24 Remarks 1. 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 Uses of Standard Electrode Potentials (SB p.204)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Primary and Secondary Cells (SB p.207) Primary Cell At anode: Zn(s) Zn 2+ (aq) + 2e - At cathode: 2MnO 2 (s) + 2NH 4+ (aq) + 2e - Mn 2 O 3 (s) + 2NH 3 (aq) + H 2 O(l) The overall reaction is: Zn(s) + 2MnO 2 (s) + 2NH 4+ (aq) Zn 2+ (aq) + Mn 2 O 3 (s) + 2NH 3 (aq) + H 2 O(l) E = +1.50V ø

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Primary and Secondary Cells (SB p.207) Secondary Cell At anode: Pb(s) + SO 4 2- (aq) PbSO 4 (s) + 2e - At cathode: PbO 2 (s) + 4H + (aq) + SO 4 2- (aq) + 2e - PbSO 4 (s) + 2H 2 O(l) The overall reaction is: Pb(s) + PbO 2 (s) + 4H + (aq) + 2SO 4 2- (aq) 2PbSO 4 (s) + 2H 2 O(l)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Primary and Secondary Cells (SB p.209) Fuel Cell At anode: H 2 (g) + 2OH - (aq) 2H 2 O(l) + 2e - At cathode: O 2 (g) + 2H 2 O(l) + 4e - 4OH - (aq) The overall reaction is: 2H 2 (g) + O 2 (g) 2H 2 O(l)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Corrosion of Iron and its Prevention (SB p.210) Electrochemical Process involved in Rusting At anodic region: Fe(s) Fe 2+ (aq) + 2e - At cathodic region: O 2 (g) + 2H 2 O(l) + 4e - 4OH - (aq) Fe 2+ (aq) + 2OH - (aq) Fe(OH) 2 (s) 2Fe(OH) 2 (s) + ½ O 2 (g) + (x-2) H 2 O(l) Fe 2 O 3 · xH 2 O(s) rust

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Coating 2. Sacrificial protection 3. Alloying 4. Cathodic protection 20.7 Corrosion of Iron and its Prevention (SB p.211) Prevention of Corrosion

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 30 Socioeconomic Implication of Corrosion and Prevention Factors concerned: 1. Direct cost of rusting 2. Indirect cost of rusting 3. Wastage of natural resource 4. Inconvenience to daily life 5. Safety 20.8 Socioeconomic Implications of Corrosion and Prevention (SB p.213)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 31 The END