19.4 Spontaneity of Redox Reactions  G = -nFE cell  G 0 = -nFE cell 0 n = number of moles of electrons in reaction F = 96,500 J V mol = 96,500 C/mol  G 0 = -RT ln K = -nFE cell 0 E cell 0 = RT nF ln K (8.314 J/K mol)(298 K) n (96,500 J/V mol) ln K = = V n ln K E cell 0 = V n log K E cell 0 F (Faraday constant)= electrical charge contained in 1 mole of electrons 1 F = 96,485.3 coulombs or 96,500 coulombs or 96,500 J/V mol e - ∆G = change in free energy; max. amount of work that can be obtained K = equlibrium constant of a redox reaction

Spontaneity of Redox Reactions 19.4

2e - + Fe 2+ Fe 2Ag 2Ag + + 2e - Oxidation: Reduction: What is the equilibrium constant for the following reaction at 25 0 C? Fe 2+ (aq) + 2Ag (s) Fe (s) + 2Ag + (aq) = V n ln K E cell E 0 = – (0.80) E 0 = V V x nE0E0 cell ln K = n = V x 2x V ln K = K = 1.23 x E 0 = E Fe /Fe – E Ag /Ag K = e -96.5

The Effect of Concentration on Cell Emf  G =  G 0 + RT ln Q  G = -nFE  G 0 = -nFE 0 -nFE = -nFE 0 + RT ln Q E = E 0 - ln Q RT nF Nernst equation At V n ln Q E 0 E = V n log Q E 0 E = Q is the reaction quotient

Will the following reaction occur spontaneously at 25 0 C if [Fe 2+ ] = 0.60 M and [Cd 2+ ] = M? Fe 2+ (aq) + Cd (s) Fe (s) + Cd 2+ (aq) 2e - + Fe 2+ 2Fe Cd Cd e - Oxidation: Reduction: n = 2 E 0 = – (-0.40) E 0 = V E 0 = E Fe /Fe – E Cd /Cd V n ln Q E 0 E = V 2 ln VE = E = E > 0Spontaneous 19.5

Batteries 19.6 Leclanché cell Dry cell Zn (s) Zn 2+ (aq) + 2e - Anode: Cathode: 2NH 4 (aq) + 2MnO 2 (s) + 2e - Mn 2 O 3 (s) + 2NH 3 (aq) + H 2 O (l) + Zn (s) + 2NH 4 (aq) + 2MnO 2 (s) Zn 2+ (aq) + 2NH 3 (aq) + H 2 O (l) + Mn 2 O 3 (s)

Batteries Zn(Hg) + 2OH - (aq) ZnO (s) + H 2 O (l) + 2e - Anode: Cathode: HgO (s) + H 2 O (l) + 2e - Hg (l) + 2OH - (aq) Zn(Hg) + HgO (s) ZnO (s) + Hg (l) Mercury Battery 19.6

Batteries 19.6 Anode: Cathode: Lead storage battery PbO 2 (s) + 4H + (aq) + SO 2- (aq) + 2e - PbSO 4 (s) + 2H 2 O (l) 4 Pb (s) + SO 2- (aq) PbSO 4 (s) + 2e - 4 Pb (s) + PbO 2 (s) + 4H + (aq) + 2SO 2- (aq) 2PbSO 4 (s) + 2H 2 O (l) 4

Batteries 19.6 Solid State Lithium Battery

Batteries 19.6 A fuel cell is an electrochemical cell that requires a continuous supply of reactants to keep functioning Anode: Cathode: O 2 (g) + 2H 2 O (l) + 4e - 4OH - (aq) 2H 2 (g) + 4OH - (aq) 4H 2 O (l) + 4e - 2H 2 (g) + O 2 (g) 2H 2 O (l)

Corrosion 19.7

Cathodic Protection of an Iron Storage Tank 19.7

19.8 Electrolysis is the process in which electrical energy is used to cause a nonspontaneous chemical reaction to occur.

Electrolysis of Water 19.8

Electrolysis and Mass Changes charge (C) = current (A) x time (s) 1 mole e - = 96,500 C 19.8

How much Ca will be produced in an electrolytic cell of molten CaCl 2 if a current of A is passed through the cell for 1.5 hours? Anode: Cathode: Ca 2+ (l) + 2e - Ca (s) 2Cl - (l) Cl 2 (g) + 2e - Ca 2+ (l) + 2Cl - (l) Ca (s) + Cl 2 (g) 2 mole e - = 1 mole Ca mol Ca = C s x 1.5 hr x 3600 s hr96,500 C 1 mol e - x 2 mol e - 1 mol Ca x = mol Ca = 0.50 g Ca 19.8

Chemistry In Action: Dental Filling Discomfort Hg 2 /Ag 2 Hg V 2+ Sn /Ag 3 Sn V 2+ Sn /Ag 3 Sn V 2+