chapter21_2.ppt

CHEM 132 Layman chapter 20 Learning Objectives Distinguish between voltaic and electrolytic cells Calculate standard cell potential, E?cell Relationship of E?cell to DG? and K. The Nernst equation Why the @!%&$%%# do we care about thermo and E-chem? Macintosh Portable 1989 15.8 lbs. Lead-acid battery MacBook Air 2008 3.0 lbs. Lithium-ion battery WE GET USEFUL & PORTABLE ENERGY FROM CHEMICAL REACTIONS! BATTERIES GET THEIR ENERGY FROM REDOX (CHEMICAL) REACTIONS! Redox reactions give lots of energy! Fuel Cells C8H18(l) + 8.5O2(g) ? 4CO2(g) + 9H2O(g) 2H2(g) + O2(g) ? 2H2O(g) Secondary (Rechargeable) Batteries LixC6(s) +Li1-xMn2O4(s) ? LiMn2O4(s) + C6(s) Voltaic vs. Electrolytic cells Voltaic Cell Notation Sn(s) + Cu2+ (aq) ? Sn2+ (aq) + Cu (s) RULES: 1. Anode on left Single line: Phase boundary Coma: Separates half-cell components in same phase Double line: Separates half cells Order: Keep same as order in half-reaction Electrodes appear on far left and far right Voltaic Cell Notation - Practice From Fig 21.6 Also try sample problem 21.2 Voltaic Cell Notation ? Practice 2 Cr(s) + 3Ag+(aq) ? Cr3+(aq) + 3Ag(s) Cr(s)?Cr3+(aq)? Ag+(aq)?Ag(s) Cr3+(aq)?Cr(s) ? Ag(s)?Ag+(aq) Cr3+(aq)?Cr(s) ? Ag+(aq)?Ag(s) Ag+(aq)?Ag(s)? Cr(s)?Cr3+(aq) Ag(s)?Ag+(aq)? Cr3+(aq)?Cr(s) Selected Standard Potentials (298 K) Half-Reaction Eºhalf-cell (V) F2(g) + 2e- ? 2F-(aq) +2.87 Cl2(g) + 2e- ? 2Cl-(aq) +1.36 MnO2(s) + 4H+(aq) + 2e- ? Mn2+(aq) + 2H2O(l) +1.23 Ag+(aq) + e- ? Ag(s) +0.80 Fe3+(aq) + e- ? Fe2+(aq) +0.77 O2(g) + 2H2O(l) +4e- ? 4OH-(aq) +0.40 Cu2+(aq) + 2e- ? Cu(s) +0.34 2H+(aq) + 2e- ? H2(g) 0.00 Fe2+(aq) + 2e- ? Fe(s) -0.44 Zn2+(aq) + 2e- ? Zn(s) -0.76 2H2O(l) + 2e- ? H2(g) + 2OH-(aq) -0.83 Na+(aq) + e- ? Na(s) -2.71 Li+(aq) + e- ? Li(s) -3.05 Strength of oxidizing agent Strength of reducing agent Ecell ___ 0 for a spontaneous process What is the redox reaction a voltaic cell containing Li and Fe? Li+ (aq) + e? Li(s) E? = -3.05 V Fe2+ (aq) + 2e? Fe(s) E? = -0.44 V Standard Cell Potentials Why didn?t we double E°lithium? What is the standard cell potentials for Zn (s) + Cu2+(aq) Zn2+ (aq) + Cu(s) ? -0.42 V -1.10 V 0.42 V 1.10 V None of these From Appendix D: Cu2+ + 2e- Cu(s) E? = +0.34V Zn2+ + 2e- Zn(s) E? = -0.76V How much free energy can we extract from the system? How much work can we do? ?G° = -nFE° n = F = Voltaic cells convert DG of a spontaneous reaction into electrical energy! How many moles of electrons are transferred in the following reaction? 4 Ag(s) + O2(g) + 4H+ ? 4 Ag+(aq) + 2 H2O (l) Extra data: O2(g) + 4H+ + 4 e- ? 2 H2O (l) E° = +1.23V Ag+(aq) + e- ? Ag(s) E? = +0.80V 1 mol e- 2 mol e- 3 mol e- 4 mol e- 6 mol e- What is the DG° (standard free energy change) of the following reaction? 4 Ag(s) + O2(g) + 4H+ ? 4 Ag+(aq) + 2 H2O (l) Extra data: O2(g) + 4H+ + 4 e- ? 2 H2O (l) E° = +1.23V Ag+(aq) + e- ? Ag(s) E? = +0.80V -165.9 kJ -738.58 kJ 165.9 kJ 738.58 kJ I don?t know Cell Potentials at Standard and Nonstandard Conditions ?G = ?G° + RT ln Q ?G = - nFE E = E° - RT ln Q nF ?G° = - RT ln K ?G° = - nFE ° Standard Conditions Nonstandard Conditions Cell potentials at nonstandard conditions How many electrons are transferred in the reaction: 2Al(s) + 3Ni2+(aq) ? 2Al3+(aq) + 3Ni(s) E° = +1.32 V 1 mol e- 2 mol e- 3 mol e- 4 mol e- 6 mol e- Cell potentials at nonstandard conditions 2Al(s) + 3Ni2+(aq) ? 2Al3+(aq) + 3Ni(s) E° = +1.32 V; n = 6 What is the cell potential at 300 K if [Ni2+] = [Al3+] = 1.0 M? -1.32 V 0 V 1.32 V I don?t know = 1.32 V Cell potentials at nonstandard conditions 2Al(s) + 3Ni2+(aq) ? 2Al3+(aq) + 3Ni(s) E° = +1.32 V; n = 6 What is the cell potential at 300 K if [Ni2+] = 1.0 M and [Al3+] = 0.25 M? 1.31 V 1.32 V 1.33 V 1.34 V 1.35 V = 1.33 V How does potential change during cell operation? Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s) [Zn2+] [Cu2+] Q = As voltaic cells operate, their potential decreases. Everything goes to equilibrium, Q = K, ?G = 0, E = 0. Energy transfer (work) stops. Concentration cells Everything goes to equilibrium, Q = K, ?G = 0, E = 0. Concentration cells: concentrations are equal at equilib.