Redox Reactions and ElectrochemistryChapter 19

Cell Potentials

Ecell = Ered (cathode) − Ered (anode)

= +0.34 V − (−0.76 V)= +1.10 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 thatG = 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 − RTnF ln Q

or, using base-10 logarithms,

E = E − 2.303 RTnF log Q

Nernst Equation

At room temperature (298 K),

Thus the equation becomes

E = E − 0.0592n log Q

2.303 RTF = 0.0592 V

Nernst - 0.0592 Vn log QE0E =

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.Ecell

• Therefore, as long as the concentrations are different, E will not be 0.

- 0.0592 Vn log QE0E =

Concentration CellsIon 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 CellsElectrocardiography: measuring voltage changes during heartbeats at the surface of the body

Applications of Oxidation-Reduction Reactions

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

BatteriesGalvanic 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) Zn2+ (aq) + 2e-Anode:

Cathode: 2NH4 (aq) + 2MnO2 (s) + 2e- Mn2O3 (s) + 2NH3 (aq) + H2O (l)+

Zn (s) + 2NH4 (aq) + 2MnO2 (s) Zn2+ (aq) + 2NH3 (aq) + H2O (l) + Mn2O3 (s)