Chapter 17 Electrochemistry Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Chapter 17 Electrochemistry © 2012 Pearson Education, Inc. Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry Galvanic Cells 4/26/2017 10:43:48 AM Electrochemistry: The area of chemistry concerned with the interconversion of chemical and electrical energy Galvanic (Voltaic) Cell: A spontaneous chemical reaction which generates an electric current Electrolytic Cell: An electric current which drives a nonspontaneous reaction Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry Galvanic Cells 4/26/2017 10:43:48 AM Zn2+(aq) + Cu(s) Zn(s) + Cu2+(aq) Oxidation half-reaction: Zn2+(aq) + 2e Zn(s) Reduction half-reaction: Cu(s) Cu2+(aq) + 2e Balancing redox reactions was covered in Ch 4 (Reactions in Aqueous Solution). It’s common for people to postpone redox balancing until now. Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry Galvanic Cells 4/26/2017 10:43:48 AM Zn2+(aq) + Cu(s) Zn(s) + Cu2+(aq) Be careful about assumptions. Don’t always place the same electrode on the same side. For instance, in the past, some textbooks would commonly placed the anode on the left side which led some students to believe that the anode always belongs on the left. Students often are confused as the purpose and need of a salt bridge. Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Galvanic Cells Anode: The electrode where oxidation occurs. The electrode where electrons are produced. Is what anions migrate toward. Has a negative sign. The wire convention is used for the sign at the anode and cathode. “Anode” and “oxidation” both begin with vowels. “Cathode” and “reduction” both begin with consonants. Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Galvanic Cells Cathode: The electrode where reduction occurs. The electrode where electrons are consumed. Is what cations migrate toward. Has a positive sign. The wire convention is used for the sign at the anode and cathode. “Anode” and “oxidation” both begin with vowels. “Cathode” and “reduction” both begin with consonants. Copyright © 2011 Pearson Prentice Hall, Inc.

Chapter 17: Electrochemistry 4/26/2017 Galvanic Cells Salt Bridge: a U-shaped tube that contains a gel permeated with a solution of an inert electrolytes Maintains electrical neutrality by a flow of ions Anions flow through the salt bridge from the cathode to anode compartment Cations migrate through salt bridge from the anode to cathode compartment “Anode” and “oxidation” both begin with vowels. “Cathode” and “reduction” both begin with consonants. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Galvanic Cells Why do negative ions (anions) move toward the negative electrode (anode)? Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry Galvanic Cells 4/26/2017 10:43:48 AM Anode half-reaction: Zn(s) Zn2+(aq) + 2e Cathode half-reaction: Cu2+(aq) + 2e Cu(s) Overall cell reaction: Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s) No electrons should be appeared in the overall cell reaction Copyright © 2011 Pearson Prentice Hall, Inc.

Shorthand Notation for Galvanic Cells Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Shorthand Notation for Galvanic Cells Salt bridge Anode half-cell Cathode half-cell Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) Electron flow Phase boundary Phase boundary Copyright © 2011 Pearson Prentice Hall, Inc.

17.2 Shorthand Notation for Galvanic Cells Cell involving gas Additional vertical line due to presence of addition phase List the gas immediately adjacent to the appropriate electrode Detailed notation includes ion concentrations and gas pressure E.g Cu(s) + Cl2(g)  Cu2+(aq) + 2 Cl-(aq) Cu(s)|Cu2+(aq)||Cl2(g)|Cl-(aq)|C(s)

Example Consider the reactions below Write the two half reaction Identify the oxidation and reduction half Identify the anode and cathode Give short hand notation for a galvanic cell that employs the overall reaction Pb2+(aq) + Ni(s)  Pb(s) + Ni2+(aq)

Example Given the following shorthand notation, sketch out the galvanic cell Pt(s)|Sn2+,Sn4+(aq)||Ag+(aq)|Ag(s)

Cell Potentials and Free-Energy Changes for Cell Reactions Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Cell Potentials and Free-Energy Changes for Cell Reactions Electromotive Force (emf): The force or electrical potential that pushes the negatively charged electrons away from the anode ( electrode) and pulls them toward the cathode (+ electrode). It is also called the cell potential (E) or the cell voltage. Copyright © 2011 Pearson Prentice Hall, Inc.

Cell Potentials and Free-Energy Changes for Cell Reactions Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Cell Potentials and Free-Energy Changes for Cell Reactions 1 J = 1 C x 1 V joule (J) SI unit of energy volt (V) SI unit of electric potential coulomb (C) Electric charge 1 coulomb is the amount of charge transferred when a current of 1 ampere (A) flows for 1 second. Copyright © 2011 Pearson Prentice Hall, Inc.

Cell Potentials and Free-Energy Changes for Cell Reactions Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Cell Potentials and Free-Energy Changes for Cell Reactions faraday or Faraday constant The electric charge on 1 mol of electrons and is equal to 96,500 C/mol e DG = nFE or DG° = nFE° Free-energy change The “°” refers to standard free-energy change and standard cell potential. Cell potential Number of moles of electrons transferred in the reaction Copyright © 2011 Pearson Prentice Hall, Inc.

Cell Potentials and Free-Energy Changes for Cell Reactions Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Cell Potentials and Free-Energy Changes for Cell Reactions The standard cell potential at 25 °C is 1.10 V for the reaction: Zn2+(aq) + Cu(s) Zn(s) + Cu2+(aq) Calculate the standard free-energy change for this reaction at 25 °C. Is the reaction spontanous at this condition? Worked example 17.3 p689. The sign for a spontaneous reaction for free-energy: - The sign for a spontaneous reaction for cell potential: + Copyright © 2011 Pearson Prentice Hall, Inc.

Examples Calculate the cell potential at standard state (Eocell) for the following reaction. Then write the half reactions I2(s) + 2 Br-(aq)  2I-(aq) + Br2(l) Go = 1.1 x 105J

Standard Reduction Potentials Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Standard Reduction Potentials Anode half-reaction: H2(g) 2H+(aq) + 2e Cathode half-reaction: Cu2+(aq) + 2e Cu(s) Overall cell reaction: H2(g) + Cu2+(aq) 2H1+(aq) + Cu(s) The standard potential of a cell is the sum of the standard half-cell potentials for oxidation at the anode and reduction at the cathode: E°cell = E°ox + E°red Individual half-cell potentials can’t be measured. They must be measured in pairs. The measured potential for this cell: E°cell = 0.34 V Copyright © 2011 Pearson Prentice Hall, Inc.

Standard Reduction Potentials Eocell is the standard cell potential when both products and reactants are at their standard states: Solutes at 1.0 M Gases at 1.0 atm Solids and liquids in pure form Temp = 25.0oC

Standard Reduction Potentials Spotaniety of the reaction can be determined by the positive Eocell value The cell reaction is spontaneous when the half reaction with the more positive Eo value is cathode Note: Eocell is an intensive property; the value is independent of how much substance is used in the reaction Ag+(aq) + e-  Ag(s) Eored = 0.80 V 2 Ag+(aq) + 2e-  2 Ag(s) Eored = 0.80V

Standard Reduction Potentials Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Standard Reduction Potentials The standard hydrogen electrode (S.H.E.) has been chosen to be the reference electrode. Potentials can only be measured in pairs which is why we need to establish a reference point for our half-cell potentials. Much like what was done for standard enthalpies and standard free energies. Copyright © 2011 Pearson Prentice Hall, Inc.

Standard Reduction Potentials Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry Standard Reduction Potentials 4/26/2017 10:43:48 AM The standard hydrogen electrode (S.H.E.) has been chosen to be the reference electrode. H2(g, 1 atm) 2H+(aq, 1 M) + 2e E°ox = 0 V E°red = 0 V 2H+(aq, 1 M) + 2e H2(g, 1 atm) The half-cell potentials for the S.H.E. have been defined to be 0 V. Much like the atomic mass of carbon-12 has been defined to 12 u. Copyright © 2011 Pearson Prentice Hall, Inc.

Standard Reduction Potentials Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry Standard Reduction Potentials 4/26/2017 10:43:48 AM Anode half-reaction: H2(g) 2H+(aq) + 2e Cu2+(aq) + 2e Cathode half-reaction: Cu(s) Overall cell reaction: H2(g) + Cu2+(aq) 2H+(aq) + Cu(s) E°cell = E°ox + E°red 0.34 V = 0 V + E°red Recall that the 0.34 V was measured for the cell with the half-cells as described above. The oxidation half-cell potential is defined to be 0 V since it is the S.H.E. Note also that the “ox” and “red” subscripts for oxidation and reduction have been dropped. While this can be very confusing to students, it is common usage. A standard reduction potential can be defined: Cu(s) Cu2+(aq) + 2e E° = 0.34 V Copyright © 2011 Pearson Prentice Hall, Inc.

Standard Reduction Potentials Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM Standard Reduction Potentials Copyright © 2011 Pearson Prentice Hall, Inc.

Examples Of the two standard reduction half reactions below, write the net equation and determine which would be the anode and which would be the cathode of a galvanic cell. Calculate Eocell Cd2+(aq) + 2e-  Cd(s) Eored = -0.40 V Ag+(aq) + e-  Ag(s) Eored = 0.80 V Fe2+(aq) + 2e-  Fe(s) Eored = -0.44 V Al3+(aq) + 3e-  Al(s) Eored = -1.66 V

Standard Cell Potentials and Equilibrium Constants Chapter 17: Electrochemistry 4/26/2017 Standard Cell Potentials and Equilibrium Constants Using DG° = -nFE° and DG° = -RT ln K -nFE° = -RT ln K nF RT nF 2.303 RT E° = ln K = log K DG° = -RT ln K came from Ch 16 (Thermodynamics: Entropy, Free Energy, and Equilibrium). The mathematical reduction from 2.303RT/nF to 0.0592V/n is the same as what was done for the Nernst equation. n 0.0592 V E° = log K in volts, at 25°C Copyright © 2008 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry 4/26/2017 10:43:48 AM The Nernst Equation DG = DG° + RT ln Q Using: DG = nFE and DG° = nFE° nF RT Nernst Equation: E = E°  ln Q or log Q nF 2.303RT E = E°  n is the number of moles of transferred electrons from the balanced reaction (balanced number of electrons from the half-reactions). Also, just the number is used (“2” instead of “2 mol e-”). 0.0592 V E = E°  log Q in volts, at 25 oC n Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 17: Electrochemistry The Nernst Equation 4/26/2017 10:43:48 AM Consider a galvanic cell that uses the reaction: Cu(s) + 2Fe3+(aq) Cu2+(aq) + 2Fe2+(aq) What is the potential of a cell at 25 oC that has the following ion concentrations? [Fe3+] = 1.0 × 104 M [Cu2+] = 0.25 M [Fe2+] = 0.20 M Problem 17.10 p696. Copyright © 2011 Pearson Prentice Hall, Inc.

Example Use the tabulated half-cell potentials to calculate K for the following oxidation of copper by H+ 2Cu(s) + 2H+(aq)  Cu2+(aq) + H2(g)

Example Calculate the concentration of cadmium ion in the galvanic cell below Cd(s)|Cd2+(aq)(?M)||Ni2+(aq)(0.100M)|Ni(s)

Standard Cell Potentials and Equilibrium Constants Chapter 17: Electrochemistry Standard Cell Potentials and Equilibrium Constants 4/26/2017 Copyright © 2008 Pearson Prentice Hall, Inc.

Examples Calculate the equilibrium constant, Keq, for the reaction below Zn2+(aq) + 2e-  Zn(s) Eored = -0.76 V Sn2+(aq) + 2e-  Sn(s) Eored = -0.14 V

Standard Cell Potentials and Equilibrium Constants Chapter 17: Electrochemistry 4/26/2017 Standard Cell Potentials and Equilibrium Constants Three methods to determine equilibrium constants: K = [A]a[B]b [C]c[D]d K from concentration data: RT -DG° ln K = K from thermochemical data: ln K nF RT E° = K from electrochemical data: or RT nFE° ln K = Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.