Analytical Chem istry - DePauw University

224 Analytical Chemistry 2.0ln(x) = 2.303log(x)RTE = Eo −nFlnQwhere E o is the potential under standard-state conditions. Substituting appropriatevalues for R and F, assuming a temperature of 25 o C (298 K), andswitching from ln to log gives the potential in volts as0.05916E = Eo − logQ6.24nSt a n d a r d Po t e n t i a l sA standard potential is the potential whenall species are in their standard states. Youmay recall that we define standard stateconditions as: all gases have partial pressuresof 1 atm, all solutes have concentrationsof 1 mol/L, and all solids and liquidsare pure.A redox reaction’s standard potential, E o , provides an alternative way ofexpressing its equilibrium constant and, therefore, its equilibrium position.Because a reaction at equilibrium has a ∆G of zero, the potential, E, alsomust be zero at equilibrium. Substituting these values into equation 6.24and rearranging provides a relationship between E o and K.Eo = 0. 05916 log K6.25nWe generally do not tabulate standard potentials for redox reactions.Instead, we calculate E o using the standard potentials for the correspondingoxidation half-reaction and reduction half-reaction. By convention,standard potentials are provided for reduction half-reactions. The standardpotential for a redox reaction, E o , iso oE = E −Eredooxwhere E o red and E o ox are the standard reduction potentials for the reductionhalf-reaction and the oxidation half-reaction.Because we cannot measure the potential for a single half-reaction, wearbitrarily assign a standard reduction potential of zero to a reference halfreactionand report all other reduction potentials relative to this reference.The reference half-reaction is+ −2HO ( aq) + 2e 2HO() l + H ( g)3 2 2Appendix 13 contains a list of selected standard reduction potentials. Themore positive the standard reduction potential, the more favorable the reductionreaction under standard state conditions. Thus, under standardstate conditions the reduction of Cu 2+ to Cu (E o = +0.3419 V) is morefavorable than the reduction of Zn 2+ to Zn (E o = –0.7618 V).Example 6.5Calculate (a) the standard potential, (b) the equilibrium constant, and (c)the potential when [Ag + ] = 0.020 M and [Cd 2+ ] = 0.050 M, for the followingreaction at 25 o C.