Analytical Chem istry - DePauw University

214 Analytical Chemistry 2.0Rxn1: A+ B DK = 040 .Rxn2: A+ EC+ D+ F K = 010 .Rxn 3: C+ E BK = 20 .Rxn4: F+ C D+ B K = 50 .1234So l u t i o nThe overall reaction is equivalent toRxn1+ Rxn2− Rxn3+Rxn4Subtracting a reaction is equivalent to adding the reverse reaction; thus,the overall equilibrium constant isKK K K= × × 1 2 4040 . × 0. 10×50 .== 010 .K20 .3Practice Exercise 6.1Calculate the equilibrium constant for the reactionC+ D+ F 2A+3Busing the equilibrium constants from Example 6.1.Click here to review your answer to this exercise.Another common name for an oxidation–reduction reaction is a redox reaction,where “red” is short for reduction and “ox”is short for oxidation.6DEquilibrium Constants for Chemical ReactionsSeveral types of chemical reactions are important in analytical chemistry,either in preparing a sample for analysis or during the analysis. The mostsignificant of these are: precipitation reactions, acid–base reactions, complexationreactions, and oxidation–reduction reactions. In this section wereview these reactions and their equilibrium constant expressions.6D.1 Precipitation ReactionsIn a precipitation reaction, two or more soluble species combine to forman insoluble precipitate. The most common precipitation reaction is ametathesis reaction, in which two soluble ionic compounds exchange parts.For example, if we add a solution of lead nitrate, Pb(NO 3 ) 2 , to a solution ofpotassium chloride, KCl, the result is a precipitate of lead chloride, PbCl 2 .We usually write a precipitation reaction as a net ionic equation, showingonly the precipitate and those ions forming the precipitate. Thus, the precipitationreaction for PbCl 2 is2+ −Pb ( aq) + Cl ( aq) PbCl () s22