GENERAL CHEMISTRY PRINCIPLES AND MODERN APPLICATIONS ELEVENTH EDITION

Electrochemistry Slide 19 - 2 CONTENTS 19 -1 Electrode Potentials and Their Measurement 19

19 -1 Electrode Potentials and Their Measurement Cu(s) + 2 Ag+(aq) Cu(s) + Zn

Cathode FIGURE 19 -2 An electrochemical half cell Slide 19 - 4 General Chemistry:

Slide 19 - 5 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

FIGURE 19 -3 Measurement of the electromotive force of an electrochemical cell Slide 19

Slide 19 - 7 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

FIGURE 19 -4 The reaction Zn(s) + Cu 2+(aq) Slide 19 - 8 Zn

Cell Diagrams and Terminology Slide 19 - 9 General Chemistry: Chapter 19 Copyright ©

Galvanic (or voltaic) cells Produce electricity as a result of spontaneous reactions. Electrolytic cells

19 -2 Standard Electrode Potentials Cell voltages – potential differences between electrodes – are

2 H+(a = 1) + 2 e− on Pt H 2(g, 1 bar) E°

The standard electrode potential, Eº The potential difference, or voltage, of a cell formed

Cu 2+(1 M) + 2 e− → Cu(s) EºCu 2+/Cu = ? Pt|H 2(g,

Pt|H 2(g, 1 bar)|H+(a = 1) || Cu 2+(1 M)|Cu(s) Eºcell = 0. 340

Eº − E* ∆vgas = 0. 3382 × z m. V ∆vgas = ∑

FIGURE 19 -6 Measuring standard electrode potentials Slide 19 - 17 General Chemistry: Chapter

Eºcell = Eº (right) − Eº (left) = Eº (cathode) − Eº (anode) =

Slide 19 - 19 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Slide 19 - 20 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Slide 19 - 21 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

19 -3 Ecell, ΔG, and Keq Faraday constant, F = 96, 485 C mol−

2 H+(aq) + 2 e− H 2(g) or H+(aq) + e− ½ H 2(g)

Slide 19 - 24 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Combining Reduction Half-Equations Fe 3+(aq) + 3 e− → Fe(s) E°Fe 3+/Fe = ?

Spontaneous Change in Oxidation-Reduction Reactions ΔG < 0 for spontaneous change. Therefore E°cell >

FIGURE 19 -7 Reaction of Al(s) and Cu 2+(aq) Slide 19 - 27 General

The Behavior or Metals Toward Acids Oxidation: Reduction: M(s) → M 2+(aq) + 2

Certain metals that will not react with HCl will react with an acid in

The Relationship Between E°cell and Keq ΔG° = −RT ln Keq = −z. FE°cell

FIGURE 19 -8 A summary of important thermodynamic, equilibrium and electrochemical relationships under standard

19 -4 Ecell as a Function of Concentration ΔG = ΔG° − RT ln

Concentration Cells Two half cells with identical electrodes but different ion concentrations. Pt|H 2

2 H+(1 M) → 2 H+(x M) E°cell = E°H+/H − E°H+/H = 0

Measurement of Ksp Ag|Ag+(sat’d Ag. I)||Ag+(0. 10 M)|Ag(s) FIGURE 19 -12 A concentration cell

Reduction: Oxidation: Overall: Ag+(0. 100 M) + e− → Ag(s) → Ag+(sat’d Ag. I)

Alternative Standard Electrodes silver-silver chloride electrode: Ag(s)|Ag. Cl|Cl−(1. 0 M) Ag. Cl(s) + e−

Alternative Standard Electrodes FIGURE 19 -13 Schematic diagrams of some common electrodes (a) Ag|Ag.

The Glass Electrode and the Electrochemical Measurement of p. H Ag|Ag. Cl(s)|Cl−(1. 0 M),

19 -5 Batteries: Producing Electricity Through Chemical Reactions • Primary cells (or batteries). Cell

The Leclanché Dry Cell Oxidation: Reduction: Acid-base reaction: Precipitation reaction: Slide 19 - 41

FIGURE 19 -14 The Leclanché (dry) cell Slide 19 - 42 General Chemistry: Chapter

The alkaline cell Reduction: 2 Mn. O 2(s) + H 2 O(l) + 2

The Lead-Acid (Storage) Battery FIGURE 19 -15 The lead-acid (storage) cell Slide 19 -

Reduction: Oxidation: Overall: Pb. O 2(s) + 3 H+(aq) + HSO 4− (aq) +

The Silver-Zinc Cell: A Button Battery Zn(s), Zn. O(s)|KOH(sat’d)|Ag 2 O(s), Ag(s) FIGURE 19

Reduction: Ag 2 O(s) + H 2 O(l) + 2 e− → 2 Ag(s)

The Nickel-Cadmium Cell: A Rechargeable Battery A rechargeable nickel-cadmium cell, or nicad battery Cd(s)

Reduction: Oxidation: Overall: 2 Ni. O(OH)(s) + 2 H 2 O(l) + 2 e−

The Lithium-Ion Battery FIGURE 19 -17 The electrodes of a lithium-ion battery Slide 19

Reserve Battery Anode: Mg(s) → Mg 2+(aq) + 2 e− Cathode: Cu. Cl(s) +

Fuel Cells Reduction: O 2(g) + 2 H 2 O(l) + 4 e− →

FIGURE 19 -18 A hydrogen-oxygen fuel cell Slide 19 - 53 General Chemistry: Chapter

Reduction: Oxidation: Overall: 2 {O 2(g) + 4 H+(aq) + 4 e− → 2

Air Batteries Reduction: Oxidation: 3 {O 2(g) + 2 H 2 O(l) + 4

FIGURE 19 -19 A simplified aluminum-air battery Slide 19 - 56 General Chemistry: Chapter

19 -6 Corrosion: Unwanted Voltaic Cells In neutral solution: Reduction: Oxidation: O 2(g) +

FIGURE 19 -20 Demonstration of corrosion and methods of corrosion protection Slide 19 -

FIGURE 19 -21 Protection of iron against electrolytic corrosion Slide 19 - 59 General

Magnesium sacrificial anodes The small cylindrical bars of magnesium attached to the steel ship

19 -7 Electrolysis: Causing Non-spontaneous Reactions to Occur Voltaic Cell: Zn(s) + Cu 2+(aq)

Predicting Electrolysis Reaction FIGURE 19 -22 An electrolytic cell Slide 19 - 62 General

1. A voltage significantly in excess of the calculated value may be necessary –

In aqueous Na. Cl with inert electrodes there are two possible reduction and two

Unless the overpotential for H 2(g) is unusually high, there are two possibilities for

Quantitative Aspects of Electrolysis 1 mol e− = 96485 C Charge (C) = current

19 -8 Industrial Electrolysis Processes The refining of copper by electrolysis. Slide 19 -

Electrorefining Electroplating Electrosynthesis A rack of metal parts being lifted from the electrolyte solution

Chlor-Alkali Process FIGURE 19 -24 A diaphragm chlor-alkali cell Slide 19 - 69 FIGURE

2 Na+ (aq) + 2 Cl−(aq) → 2 Na(in Hg) + Cl 2(g) E°cell

End of Chapter Slide 19 - 71 General Chemistry: Chapter 19 Copyright © 2017

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GENERAL CHEMISTRY PRINCIPLES AND MODERN APPLICATIONS ELEVENTH EDITION PETRUCCI HERRING MADURA Electrochemistry BISSONNETTE 19 PHILIP DUTTON UNIVERSITY OF WINDSOR DEPARTMENT OF CHEMISTRY AND BIOCHEMISTRY Slide 19 - 1 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Electrochemistry Slide 19 - 2 CONTENTS 19 -1 Electrode Potentials and Their Measurement 19 -2 Standard Electrode Potentials 19 -3 Ecell, ΔG, and K 19 -4 Ecell as a Function of Concentrations 19 -5 Batteries: Producing Electricity Through Chemical Reactions 19 -6 Corrosion: Unwanted Voltaic Cells 19 -7 Electrolysis: Causing Nonspontaneous Reactions to Occur 19 -8 Industrial Electrolysis Processes General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

19 -1 Electrode Potentials and Their Measurement Cu(s) + 2 Ag+(aq) Cu(s) + Zn 2+(aq) Cu 2+(aq) + 2 Ag(s) No reaction FIGURE 19 -1 Behavior of Ag+(aq) and Zn+(aq) in the presence of copper Slide 19 - 3 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Galvanic (or voltaic) cells Produce electricity as a result of spontaneous reactions. Electrolytic cells Non-spontaneous chemical change driven by electricity. Slide 19 - 10 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

19 -2 Standard Electrode Potentials Cell voltages – potential differences between electrodes – are among the most precise scientific measurements possible. The potential of an individual electrode is difficult to establish. Arbitrary zero is chosen. The standard hydrogen electrode (SHE) Slide 19 - 11 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

2 H+(a = 1) + 2 e− on Pt H 2(g, 1 bar) E° = 0 volt (V) H+(a = 1)|H 2(g, 1 bar)|Pt FIGURE 19 -5 The standard hydrogen electrode (SHE) Slide 19 - 12 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

The standard electrode potential, Eº The potential difference, or voltage, of a cell formed from two standard electrodes. Difference is always taken as Eºcell = Eº (right) − Eº (left) (cathode) (anode) Measures the tendency for a reduction process to occur at an electrode. All ionic species present at a = 1 (approximately 1 M). All gases are at 1 bar (approximately 1 atm). Where no metallic substance is indicated, the potential is established on an inert metallic electrode (ex. Pt). Slide 19 - 13 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Cu 2+(1 M) + 2 e− → Cu(s) EºCu 2+/Cu = ? Pt|H 2(g, 1 bar)|H+(1 M) || Cu 2+(1 M)|Cu(s) anode (19. 6) Eºcell = 0. 340 V (19. 7) cathode Standard cell potential: the potential difference of a cell formed from two standard electrodes. Eºcell = Eº (right) − Eº (left) (cathode) Slide 19 - 14 (anode) General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Pt|H 2(g, 1 bar)|H+(a = 1) || Cu 2+(1 M)|Cu(s) Eºcell = 0. 340 V Eºcell = Eºcathode − Eºanode Eºcell = EºCu 2+/Cu − EºH+/H 2 = 0. 340 V = EºCu 2+/Cu − 0 V = 0. 340 V Cu 2+(1 M) + 2 e− → Cu(s) H 2(g, 1 atm) + Cu 2+(1 M) → H+(1 M) + Cu(s) Slide 19 - 15 EºCu 2+/Cu = +0. 340 V (19. 8) Eºcell = 0. 340 V (19. 9) General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Eºcell = Eº (right) − Eº (left) = Eº (cathode) − Eº (anode) = Eº (reduction half-cell) − Eº (oxidation half-cell) = EºCu 2+/Cu − EºZn 2+/Zn = 0. 340 V − (− 0. 763 V) = 1. 103 V Slide 19 - 18 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

19 -3 Ecell, ΔG, and Keq Faraday constant, F = 96, 485 C mol− 1 When products are in their standard states Slide 19 - 22 elec = −z. FEcell (19. 13) ΔG = −z. FEcell (19. 14) ΔG° = −z. FE°cell (19. 15) General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

2 H+(aq) + 2 e− H 2(g) or H+(aq) + e− ½ H 2(g) Pt(s)|H 2(g)|H+(aq)||Cu 2+(aq)|Cu(s) Eºcell = Eº (right) − Eº (left) = E°Cu 2+/Cu − 0 V = 0. 340 V 96485 C ΔG° = −z. FE°cell = 2 × × 0. 340 V mol = − 6. 5610× 104 J mol− 1 = − 65. 6 k. J mol− 1 Slide 19 - 23 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Combining Reduction Half-Equations Fe 3+(aq) + 3 e− → Fe(s) E°Fe 3+/Fe = ? Fe 2+(aq) + 2 e− → Fe(s) ΔG° = − 2 × F × (− 0. 440 V) Fe 3+(aq) + 1 e− → Fe 2+(aq) ΔG° = − 1 × F × ( 0. 771 V) Fe 3+(aq) + 3 e− → Fe(s) ΔG° = (0. 880 F)V − (0. 771 F)V =(0. 109 F)V ΔG° = −z. FE°Fe 3+/Fe 2+ = − 3 FE°Fe 3+/Fe 2+ = (0. 109 F)V E°Fe 3+/Fe = −(0. 109 V /3 F)V = − 0. 0363 V E°Fe 3+/Fe = +0. 331 V Slide 19 - 25 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Spontaneous Change in Oxidation-Reduction Reactions ΔG < 0 for spontaneous change. Therefore E°cell > 0 because ΔGcell = −n. FE°cell > 0 Reaction proceeds spontaneously as written. E°cell = 0 Reaction is at equilibrium. E°cell < 0 Reaction proceeds in the reverse direction spontaneously. Slide 19 - 26 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

The Behavior or Metals Toward Acids Oxidation: Reduction: M(s) → M 2+(aq) + 2 e− 2 H+(aq) + 2 e− → H 2(g) E° = −E°M 2+/M E°H+/H 2 = 0 V 2 H+(aq) + M(s) → H 2(g) + M 2+(aq) Overall: E°cell = E°H+/H 2 − E°M 2+/M = − E°M 2+/M When E°M 2+/M < 0, E°cell > 0. Therefore ΔG° < 0. Metals with negative reduction potentials react with acids. Slide 19 - 28 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Certain metals that will not react with HCl will react with an acid in the presence of an anion that is a better oxidiing agent than H+. 3 Ag+(s) + NO 32− + 2 H+(aq) → 3 Ag+(aq) + NO(g) + H 2 O(l) NO 32− is a good oxidizing agent in acidic solution Slide 19 - 29 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

The Relationship Between E°cell and Keq ΔG° = −RT ln Keq = −z. FE°cell RT E°cell = ln Keq z. F 0. 025693 V ln K E°cell = eq z Slide 19 - 30 General Chemistry: Chapter 19 (19. 17) Copyright © 2017 Pearson Canada Inc.

FIGURE 19 -8 A summary of important thermodynamic, equilibrium and electrochemical relationships under standard conditions. Slide 19 - 31 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

19 -4 Ecell as a Function of Concentration ΔG = ΔG° − RT ln Q −z. FEcell = −z. FE°cell − RT ln Q Ecell = E°cell − RT z. F ln Q Convert to log 10 and calculate constants. Ecell = E°cell − Slide 19 - 32 0. 0257 V z log Q FIGURE 19 -9 Variation of Ecell with ion concentrations The Nernst Equation General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Concentration Cells Two half cells with identical electrodes but different ion concentrations. Pt|H 2 (1 atm)|H+(x M)||H+(1. 0 M)|H 2(1 atm)|Pt(s) 2 H+(1 M) + 2 e− → H 2(g, 1 atm) → 2 H+(x M) + 2 e− 2 H+(1 M) → 2 H+(x M) FIGURE 19 -11 A concentration cell Slide 19 - 33 E°cell = E°H+/H − E°H+/H = 0 2 General Chemistry: Chapter 19 2 Copyright © 2017 Pearson Canada Inc.

2 H+(1 M) → 2 H+(x M) E°cell = E°H+/H − E°H+/H = 0 2 2 but we can calculate using the Nernst Equation 0. 0257 V Ecell = E°cell − ln Q z 0. 0257 V Ecell = E°cell − ln 2 x 2 12 0. 0257 V x Ecell = 0 − × 2 ln 1 2 Ecell = − 0. 0257 V ln x Ecell = − 0. 0592 V log x Ecell = (0. 0592 V) p. H Slide 19 - 34 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Measurement of Ksp Ag|Ag+(sat’d Ag. I)||Ag+(0. 10 M)|Ag(s) FIGURE 19 -12 A concentration cell for determining Ksp of Ag. I Slide 19 - 35 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Reduction: Oxidation: Overall: Ag+(0. 100 M) + e− → Ag(s) → Ag+(sat’d Ag. I) + e− Ag+(0. 100 M) → Ag+(sat’d Ag. I) Work Example 19 -11 as an exercise to understand the process. Slide 19 - 36 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Alternative Standard Electrodes silver-silver chloride electrode: Ag(s)|Ag. Cl|Cl−(1. 0 M) Ag. Cl(s) + e− → Ag(s) + Cl−(aq) mercurous chloride or calomel electrode: E° = 0. 2223 V Hg(l)|Hg 2 Cl 2(s)|Cl−(1. 0 M) ½ Hg 2 Cl 2(s) + e− → Hg(l) + Cl−(aq) E° = 0. 2680 V Slide 19 - 37 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

The Glass Electrode and the Electrochemical Measurement of p. H Ag|Ag. Cl(s)|Cl−(1. 0 M), H+(1. 0 M)|glass membrane|H+(unknown)|| Cl−(1. 0 M)|Ag. Cl(s)|Ag(s) + Cl− Ag. Cl(s) + e− H+(1. 0 M) Ag. Cl(s) + e− H+(unknown) Ag(s) + Cl−(aq) ΔGm = Gm (unknown) − Gm (1. 0 M) = Gm° + RTln[unknown] − Gm° − RTln(1. 0) =RTln[unknown] E°cell = −RTln[unknown]/z. F When z = 1 and at 298 K: = RT/(2. 303 z. F) × −log[unknown] = 0. 0592 V × p. H Slide 19 - 39 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

19 -5 Batteries: Producing Electricity Through Chemical Reactions • Primary cells (or batteries). Cell reaction is not reversible. • Secondary cells. Cell reaction can be reversed by passing electricity through the cell (charging). Several hundred or more cycles are possible. • Reserve batteries. Electrolyte is isolated from the rest of the battery to reduce selfdischarge or chemical degradation. Designed for long term storage and to deliver high power over a relatively short time. • Flow batteries and fuel cells. Materials pass through the battery which converts chemical energy to electric energy. Can be run indefinitely when supplied with electrolytes. Slide 19 - 40 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

The Leclanché Dry Cell Oxidation: Reduction: Acid-base reaction: Precipitation reaction: Slide 19 - 41 Zn(s) → Zn 2+(aq) + 2 e− 2 Mn. O 2(s) + H 2 O(l) + 2 e− → Mn 2 O 3(s) + 2 OH− NH 4+(aq) + OH−(aq) → NH 3(g) + H 2 O(l) Zn 2+(aq) + 2 NH 3(g) + Cl−(aq) → [Zn(NH 3)2]Cl 2(s) General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

The alkaline cell Reduction: 2 Mn. O 2(s) + H 2 O(l) + 2 e− → Mn 2 O 3(s) + 2 OH− Oxidation: can be thought of as occurring in two steps Zn(s) → Zn 2+(aq) + 2 e− Zn 2+(aq) + 2 OH−→ Zn (OH)2(s) Zn (s) + 2 OH− → Zn (OH)2(s) + 2 e− E°cell is an intensive property Slide 19 - 43 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Reduction: Oxidation: Overall: Pb. O 2(s) + 3 H+(aq) + HSO 4− (aq) + 2 e− → Pb. SO 4(s) + 2 H 2 O(l) Pb (s) + HSO 4− (aq) → Pb. SO 4(s) + H+(aq) + 2 e− Pb. O 2(s) + Pb(s) + 2 H+(aq) + HSO 4− (aq) → 2 Pb. SO 4(s) + 2 H 2 O(l) Ecell = EPb. O 2/Pb. SO 4 − EPb. SO 4/Pb = 1. 74 V − (− 0. 28 V) = 2. 02 V Charging: 2 Pb. SO 4(s) + 2 H 2 O(l) → Pb. O 2(s) + Pb(s) + 2 H+(aq) + HSO 4− (aq) Slide 19 - 45 General Chemistry: Chapter 19 Ecell = − 2. 02 V Copyright © 2017 Pearson Canada Inc.

The Silver-Zinc Cell: A Button Battery Zn(s), Zn. O(s)|KOH(sat’d)|Ag 2 O(s), Ag(s) FIGURE 19 -16 The silver-zinc button (miniature) cell Slide 19 - 46 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Reduction: Ag 2 O(s) + H 2 O(l) + 2 e− → 2 Ag(s) + 2 OH−(aq) Oxidation: Zn(s) + 2 OH−(aq) → Zn. O(s) + H 2 O(l) + 2 e− Overall: Zn(s) + Ag 2 O(s) → Zn. O(s) + 2 Ag(s) (19. 25) Ecell = 1. 8 V Slide 19 - 47 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

The Nickel-Cadmium Cell: A Rechargeable Battery A rechargeable nickel-cadmium cell, or nicad battery Cd(s) + 2 Ni. O(OH)(s) + 2 H 2 O(l) → 2 Ni(OH)2(s) + Cd(OH)2(s) Slide 19 - 48 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Reduction: Oxidation: Overall: 2 Ni. O(OH)(s) + 2 H 2 O(l) + 2 e− → 2 Ni(OH)2(s) + 2 OH−(aq) Cd(s) + 2 OH−(aq) → Cd(OH)2(s) + 2 e− Cd(s) + 2 Ni. O(OH)(s) + 2 H 2 O(l) → 2 Ni(OH)2(s) + Cd(OH)2(s) Ecell = 1. 8 V Slide 19 - 49 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Reserve Battery Anode: Mg(s) → Mg 2+(aq) + 2 e− Cathode: Cu. Cl(s) + 2 e−(aq) → 2 Cu(s) + 2 Cl−(aq) Overall: Mg(s) + Cu. Cl(s) → Mg 2+(aq) + 2 Cl−(aq) + 2 Cu(s) The voltage is 0. 00 V until the reserve battery comes in contact with a solution containing ions. Slide 19 - 51 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Fuel Cells Reduction: O 2(g) + 2 H 2 O(l) + 4 e− → 4 OH−(aq) Oxidation: 2{H 2(g) + 2 OH−(aq) → 2 H 2 O(l) + 2 e−} Overall: 2 H 2(g) + O 2(g) → 2 H 2 O(l) (19. 26) E°cell = E°O 2/OH− − E°H 2 O/H 2 = 0. 401 V − (− 0. 828 V) = 1. 229 V efficiency value: = ΔG°/ ΔH° = − 474. 4 KJ/− 571. 6 KJ = 0. 83 Slide 19 - 52 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Reduction: Oxidation: Overall: 2 {O 2(g) + 4 H+(aq) + 4 e− → 2 H 2 O(l)} CH 4(g) + 2 H 2 O(l) → 2 CO 2(g) + 8 H+ + 8 e− CH 4(g) + O 2(g) → 2 CO 2(g) + 4 H 2 O(l) Δ r H° = − 890 k. J mol− 1 Slide 19 - 54 Δr. G° = − 818 k. J mol− 1 General Chemistry: Chapter 19 = 0. 92 (19. 26) (19. 27) Copyright © 2017 Pearson Canada Inc.

Air Batteries Reduction: Oxidation: 3 {O 2(g) + 2 H 2 O(l) + 4 e− → 4 OH−(aq)} 4 {4 Al(s) + 4 OH−(aq) → [Al(OH)4]−(aq) + 3 e−} Overall: 4 Al(s) + 3 O 2(g) + 6 H 2 O(l) + 4 OH−(aq) → 4 [Al(OH)4]−(aq) (19. 28) Slide 19 - 55 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

19 -6 Corrosion: Unwanted Voltaic Cells In neutral solution: Reduction: Oxidation: O 2(g) + 2 H 2 O(l) + 4 e− → 4 OH−(aq) 2 Fe(s) → 2 Fe 2+(aq) + 4 e− Ecell = EO 2/OH− − EFe/Fe 2+ = 0. 401 V − (− 0. 440 V) = 0. 841 V Corrosion is especially significant in acidic solution: O 2(g) + 4 H+(aq) + 4 e− → 4 H 2 O (aq) Slide 19 - 57 E°O 2/H 2 O = 1. 229 V General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Magnesium sacrificial anodes The small cylindrical bars of magnesium attached to the steel ship provide cathodic protection against corrosion. Slide 19 - 60 Galvanized Nails General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

19 -7 Electrolysis: Causing Non-spontaneous Reactions to Occur Voltaic Cell: Zn(s) + Cu 2+(aq) → Zn 2+(aq) + Cu(s) EO 2/OH− = 1. 103 V Electolytic Cell: Reduction: Oxidation: Overall: Zn 2+(aq) + 2 e− → Zn(s) Cu(s) → Cu 2+(aq) + 2 e− Cu(s) + Zn 2+(aq) → Cu 2+(aq) + Zn(s) E°cell = EZn 2+/Zn − ECu 2+/Cu = − 0. 763 V − (0. 340 V) = − 1. 103 V Slide 19 - 61 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

1. A voltage significantly in excess of the calculated value may be necessary – overpotential. 2. Competing electrode reactions may occur. 3. Reactants are often in nonstandard states. 4. The nature of the electrodes matters. In the electrolysis of molten Na. Cl with inert electrodes only one oxidation and one reduction are possible. Slide 19 - 63 Reduction: 2 Na+(aq) + 2 e− → 2 Na(l) Oxidation: 2 Cl−(s) → Cl 2(g) + 2 e− General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Unless the overpotential for H 2(g) is unusually high, there are two possibilities for the cell reaction Half-reaction (19. 30) + half-reaction (19. 31) Slide 19 - 65 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Quantitative Aspects of Electrolysis 1 mol e− = 96485 C Charge (C) = current (C/s) × time (s) number of mole e− = current ( C ) × time (s) × s or ne− = 1 mol e− 96, 485 C I×t F C/s → C → mol e− → mol product → g product Slide 19 - 66 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

Electrorefining Electroplating Electrosynthesis A rack of metal parts being lifted from the electrolyte solution after electroplating. Slide 19 - 68 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.

2 Na+ (aq) + 2 Cl−(aq) → 2 Na(in Hg) + Cl 2(g) E°cell = − 3. 20 V 2 Na+ (aq) + H 2 O(l) → 2 Na+ (aq) + 2 OH−(aq) + H 2 (g) + Hg(l) Slide 19 - 70 General Chemistry: Chapter 19 Copyright © 2017 Pearson Canada Inc.