ELECTROCHEMISTRY Chapter 21 Electric automobile Copyright © 1999 by Harcourt Brace & Company All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida

2 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved TRANSFER REACTIONS Atom transfer HOAc + H 2 O ---> OAc - + H 3 O + Electron transfer Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s)

3 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Electron Transfer Reactions Electron transfer reactions are oxidation- reduction or redox reactions.Electron transfer reactions are oxidation- reduction or redox reactions. Redox reactions can result in the generation of an electric current or be caused by imposing an electric current.Redox reactions can result in the generation of an electric current or be caused by imposing an electric current. Therefore, this field of chemistry is often called ELECTROCHEMISTRY.Therefore, this field of chemistry is often called ELECTROCHEMISTRY.

4 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Review of Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number.OXIDATION—loss of electron(s) by a species; increase in oxidation number. REDUCTION—gain of electron(s); decrease in oxidation number.REDUCTION—gain of electron(s); decrease in oxidation number. OXIDIZING AGENT—electron acceptor; species is reduced.OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized.REDUCING AGENT—electron donor; species is oxidized. OXIDATION—loss of electron(s) by a species; increase in oxidation number.OXIDATION—loss of electron(s) by a species; increase in oxidation number. REDUCTION—gain of electron(s); decrease in oxidation number.REDUCTION—gain of electron(s); decrease in oxidation number. OXIDIZING AGENT—electron acceptor; species is reduced.OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized.REDUCING AGENT—electron donor; species is oxidized.

5 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved OXIDATION-REDUCTION REACTIONS Direct Redox Reaction Oxidizing and reducing agents in direct contact. Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s)

6 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved OXIDATION-REDUCTION REACTIONS Indirect Redox Reaction A battery functions by transferring electrons through an external wire from the reducing agent to the oxidizing agent.

7 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Why Study Electrochemistry? BatteriesBatteries

8 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Why Study Electrochemistry? BatteriesBatteries CorrosionCorrosion A rusted car.

9 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Why Study Electrochemistry? BatteriesBatteries CorrosionCorrosion Industrial production of chemicals such as Cl 2, NaOH, F 2 and AlIndustrial production of chemicals such as Cl 2, NaOH, F 2 and Al A rusted car.

10 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Why Study Electrochemistry? BatteriesBatteries CorrosionCorrosion Industrial production of chemicals such as Cl 2, NaOH, F 2 and AlIndustrial production of chemicals such as Cl 2, NaOH, F 2 and Al Biological redox reactionsBiological redox reactions The heme group

11 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Electrochemical Cells An apparatus that allows a redox reaction to occur by transferring electrons through an external connector.An apparatus that allows a redox reaction to occur by transferring electrons through an external connector. Product favored reaction ---> voltaic or galvanic cell ----> electric currentProduct favored reaction ---> voltaic or galvanic cell ----> electric current Reactant favored reaction ---> electrolytic cell ---> electric current used to cause chemical change.Reactant favored reaction ---> electrolytic cell ---> electric current used to cause chemical change. Batteries are voltaic cells

12 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved CHEMICAL CHANGE --> ELECTRIC CURRENT With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.” Zn is oxidized and is the reducing agent Zn(s) ---> Zn 2+ (aq) + 2e-Zn is oxidized and is the reducing agent Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e- ---> Cu(s)Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e- ---> Cu(s)

13 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved CHEMICAL CHANGE --> ELECTRIC CURRENT Oxidation: Zn(s) ---> Zn 2+ (aq) + 2e- Reduction: Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s)

14 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved CHEMICAL CHANGE --> ELECTRIC CURRENT Oxidation: Zn(s) ---> Zn 2+ (aq) + 2e- Reduction: Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) Electrons are transferred from Zn to Cu 2+, but there is no useful electric current.

15 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved CHEMICAL CHANGE --> ELECTRIC CURRENT To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire.To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire. This is accomplished in a GALVANIC or VOLTAIC cell.This is accomplished in a GALVANIC or VOLTAIC cell. A group of such cells is called a battery.A group of such cells is called a battery.

16 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Electrons travel thru external wire. Salt bridge allows anions and cations to move between electrode compartments.Salt bridge allows anions and cations to move between electrode compartments. This maintains electrical neutrality.This maintains electrical neutrality. Electrons travel thru external wire. Salt bridge allows anions and cations to move between electrode compartments.Salt bridge allows anions and cations to move between electrode compartments. This maintains electrical neutrality.This maintains electrical neutrality.

17 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Electrochemical Cell Electrons move from anode to cathode in the wire. Anions & cations move thru the salt bridge.

CELL POTENTIAL, E Electrons are “driven” from anode to cathode by an electromotive force or emf.Electrons are “driven” from anode to cathode by an electromotive force or emf. For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M.For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25  C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M. Zn and Zn 2+, anode Cu and Cu 2+, cathode Copyright © 1999 by Harcourt Brace & Company All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida

19 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved CELL POTENTIAL, E For Zn/Cu cell, voltage is 1.10 V at 25 C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M.For Zn/Cu cell, voltage is 1.10 V at 25  C and when [Zn 2+ ] and [Cu 2+ ] = 1.0 M. This is the STANDARD CELL POTENTIAL, E oThis is the STANDARD CELL POTENTIAL, E o —a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 C.—a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25  C.

20 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Calculating Cell Voltage Balanced half-reactions can be added together to get overall, balanced equation.Balanced half-reactions can be added together to get overall, balanced equation. If we know E o for each half-reaction, we could get E o for net reaction.If we know E o for each half-reaction, we could get E o for net reaction. 2 I - ---> I 2 + 2e- 2 H 2 O + 2e- ---> 2 OH - + H I H 2 O --> I OH - + H 2 2 I - ---> I 2 + 2e- 2 H 2 O + 2e- ---> 2 OH - + H I H 2 O --> I OH - + H 2

21 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved CELL POTENTIALS, E o Can’t measure 1/2 reaction E o directly. Therefore, measure it relative to a STANDARD HALF CELL, SHE. 2 H + (aq, 1 M) + 2e- --> H 2 (g, 1 atm) E o = 0.0 V

22 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Zn/Zn 2+ half-cell hooked to a SHE. E o for the cell = V Zn/Zn 2+ half-cell hooked to a SHE. E o for the cell = V

23 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Zn/Zn 2+ half-cell hooked to a SHE. E o for the cell = V Zn/Zn 2+ half-cell hooked to a SHE. E o for the cell = V

24 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Overall reaction is reduction of H + by Zn metal. Zn(s) + 2 H + (aq) --> Zn 2+ + H 2 (g) E o = V

25 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Overall reaction is reduction of H + by Zn metal. Zn(s) + 2 H + (aq) --> Zn 2+ + H 2 (g) E o = V Therefore, E o for Zn ---> Zn 2+ (aq) + 2e- is

26 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Overall reaction is reduction of H + by Zn metal. Zn(s) + 2 H + (aq) --> Zn 2+ + H 2 (g) E o = V Therefore, E o for Zn ---> Zn 2+ (aq) + 2e- is V.

27 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Overall reaction is reduction of H + by Zn metal. Zn(s) + 2 H + (aq) --> Zn 2+ + H 2 (g) E o = V Therefore, E o for Zn ---> Zn 2+ (aq) + 2e- is V. Zn is a (better) (poorer) reducing agent than H 2.

28 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Overall reaction is reduction of H + by Zn metal. Zn(s) + 2 H + (aq) --> Zn 2+ + H 2 (g) E o = V Therefore, E o for Zn ---> Zn 2+ (aq) + 2e- is V. Zn is a better reducing agent than H 2.

29 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Cu/Cu 2+ and H 2 /H + Cell

30 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Cu/Cu 2+ and H 2 /H + Cell E o = V

31 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Cu/Cu 2+ and H 2 /H + Cell Overall reaction is reduction of Cu 2+ by H 2 gas.Overall reaction is reduction of Cu 2+ by H 2 gas.

32 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Cu/Cu 2+ and H 2 /H + Cell Overall reaction is reduction of Cu 2+ by H 2 gas.Overall reaction is reduction of Cu 2+ by H 2 gas. Cu 2+ (aq) + H 2 (g) ---> Cu(s) + 2 H + (aq)Cu 2+ (aq) + H 2 (g) ---> Cu(s) + 2 H + (aq) Measured E o = VMeasured E o = V

33 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Cu/Cu 2+ and H 2 /H + Cell Overall reaction is reduction of Cu 2+ by H 2 gas.Overall reaction is reduction of Cu 2+ by H 2 gas. Cu 2+ (aq) + H 2 (g) ---> Cu(s) + 2 H + (aq)Cu 2+ (aq) + H 2 (g) ---> Cu(s) + 2 H + (aq) Measured E o = VMeasured E o = V Therefore, E o for Cu e- ---> Cu isTherefore, E o for Cu e- ---> Cu is

34 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Cu/Cu 2+ and H 2 /H + Cell Overall reaction is reduction of Cu 2+ by H 2 gas.Overall reaction is reduction of Cu 2+ by H 2 gas. Cu 2+ (aq) + H 2 (g) ---> Cu(s) + 2 H + (aq)Cu 2+ (aq) + H 2 (g) ---> Cu(s) + 2 H + (aq) Measured E o = VMeasured E o = V Therefore, E o for Cu e- ---> Cu isTherefore, E o for Cu e- ---> Cu is V+0.34 V

35 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Cu/Cu 2+ and H 2 /H + Cell E o for Cu e- ---> Cu is VE o for Cu e- ---> Cu is V Now we can calculate E o for the Zn/Cu cell.Now we can calculate E o for the Zn/Cu cell.

36 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Zn/Cu Electrochemical Cell Zn(s) ---> Zn 2+ (aq) + 2e-E o = V Cu 2+ (aq) + 2e- ---> Cu(s)E o = V Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) E o (calc’d) = V Cathode, positive, sink for electrons Anode, negative, source of electrons

37 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Uses of E o Values These experiments show we can a)decide on relative ability of elements to act as reducing agents (or oxidizing agents) b)assign a voltage to a half-reaction that reflects this ability.

38 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved TABLE OF STANDARD POTENTIALS

39 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Standard Redox Potentials, E o Any substance on the right will reduce any substance higher than it on the left.Any substance on the right will reduce any substance higher than it on the left. Zn can reduce H + and Cu 2+.Zn can reduce H + and Cu 2+. H 2 can reduce Cu 2+ but not Zn 2+H 2 can reduce Cu 2+ but not Zn 2+ Cu cannot reduce H + or Zn 2+.Cu cannot reduce H + or Zn 2+.

40 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Using Standard Potentials, E o See Table 21.1 or Screen Which is the best oxidizing agent: O 2, H 2 O 2, or Cl 2 ? _________________ Which is the best reducing agent: Hg, Al, or Sn? ____________________ In which direction does the following reaction go? Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s)

41 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved E o for a Voltaic Cell Cd --> Cd e- or Cd e- --> Cd Fe --> Fe e- or Fe e- --> Fe

42 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved From the table, you see Fe is a better reducing agent than CdFe is a better reducing agent than Cd Cd 2+ is a better oxidizing agent than Fe 2+Cd 2+ is a better oxidizing agent than Fe 2+ Overall reaction Fe + Cd > Cd + Fe 2+ E o = V E o for a Voltaic Cell

43 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved E o and G o E o and  G o E o is related to G o, the free energy change for the reaction. E o is related to  G o, the free energy change for the reaction. G o = - n F E o  G o = - n F E o where F = Faraday constant = x 10 4 J/Vmol and n is the number of moles of electrons transferred Michael Faraday

44 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved Michael Faraday Originated the terms anode, cathode, anion, cation, electrode. Discoverer of electrolysiselectrolysis magnetic props. of mattermagnetic props. of matter electromagnetic inductionelectromagnetic induction benzene and other organic chemicalsbenzene and other organic chemicals Was a popular lecturer.

45 Copyright (c) 1999 by Harcourt Brace & Company All rights reserved E o and G o E o and  G o G o = - n F E o  G o = - n F E o For a product-favored reaction Reactants ----> Products Reactants ----> Products G o 0  G o 0 E o is positive For a reactant-favored reaction Reactants <---- Products Reactants <---- Products G o > 0 and so E o 0 and so E o < 0 E o is negative