Redox Reactions and Electrochemistry Chapter 19

Cell Potentials E cell  = E red  (cathode) − E red  (anode) = V − (−0.76 V) = V

Oxidizing and Reducing Agents The strongest oxidizers have the most positive reduction potentials. The strongest reducers have the most negative reduction potentials. Remember that the oxidant occurs on the left side of the equation, and the reductant occurs on the right side of the equation

Oxidizing and Reducing Agents The greater the difference between the two half- reaction potentials, the greater the voltage of the cell.

Free Energy  G for a redox reaction can be found by using the equation  G = −nFE where n is the number of moles of electrons transferred, and F is a constant, the Faraday. 1 F = 96,485 C/mol = 96,485 J/V-mol

Free Energy Under standard conditions,  G  = −nFE 

Nernst Equation Remember that  G =  G  + RT ln Q This means −nFE = −nFE  + RT ln Q

Nernst Equation Dividing both sides by −nF, we get the Nernst equation: E = E  − RT nF ln Q or, using base-10 logarithms, E = E  − RT nF log Q

Nernst Equation At room temperature (298 K), Thus the equation becomes E = E  − n log Q RT F = V

Nernst V n log Q E 0 E =

Concentration Cells Notice that the Nernst equation implies that a cell could be created that has the same substance at both electrodes. For such a cell, would be 0, but Q would not.E cell  Therefore, as long as the concentrations are different, E will not be V n log Q E 0 E =

Concentration Cells Ion concentration and emf in the human heart: variation of the electrical potential caused by changes of ion concentrations in the pacemaker cells of the heart

Concentration Cells Electrocardiography: measuring voltage changes during heartbeats at the surface of the body

Applications of Oxidation-Reduction Reactions

Batteries Portable, self-contained electrochemical power source; vary greatly in both size and in the electrochemical reaction used to generate electricity

Batteries Galvanic cell, or a series of combined galvanic cells, that can be used as a source of direct electric current at a constant voltage

Batteries The battery required to start a car must be capable of delivering a large electrical current for a short period of time The battery that powers a heart pace-maker must be very small and capable of delivering a small but steady current over an extended time period Some batteries are primary cells, meaning they cannot be recharged Some batteries are secondary cells, meaning they can be recharged from an external power source after their emf has dropped Different applications require batteries with different properties

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)