Analytical Chem istry - DePauw University

222 Analytical Chemistry 2.0Example 6.4Determine the value of the equilibrium constant for the reactionPbCl() s PbCl ( aq )2 2So l u t i o nWe can write this reaction as the sum of three other reactions. The first ofthese reactions is the solubility of PbCl 2 (s), described by its K sp reaction.2PbCl () s Pb + ( aq) + 2Cl− ( aq )2The remaining two reactions are the stepwise formation of PbCl 2 (aq), describedby K 1 and K 2 .2+ − +Pb ( aq) + Cl ( aq) PbCl ( aq)+ −PbCl ( aq) + Cl ( aq) PbCl 2( aq)Using values for K sp , K 1 , and K 2 from Appendix 10 and Appendix 12, wefind that the equilibrium constant is−K = K × K × K = (. 17× 10 ) × 38. 9× 1. 62 = 11 . × 10sp 1 25 −3Practice Exercise 6.3What is the equilibrium constant for the following reaction? You will findappropriate equilibrium constants in Appendix 10 and Appendix 11.2− 3− −AgBr() s + SO ( aq) Ag(S O ) ( aq) + Br ( aq )22 32Click here to review your answer to this exercise.6D.4 Oxidation–Reduction (Redox) ReactionsAn oxidation–reduction reaction occurs when electrons move from onereactant to another reactant. As a result of this electron transfer, these reactantsundergo a change in oxidation state. Those reactants that experiencean increase in oxidation state undergo oxidation, and those experiencing adecrease in oxidation state undergo reduction. For example, in the followingredox reaction between Fe 3+ and oxalic acid, H 2 C 2 O 4 , iron is reducedbecause its oxidation state changes from +3 to +2.3+2Fe ( aq) + HCO ( aq) + 2H O()l 2 2 4 22++2Fe( aq)+ 2CO( g) + 2H O6.223( aq )Oxalic acid, on the other hand, undergoes oxidation because the oxidationstate for carbon increases from +3 in H 2 C 2 O 4 to +4 in CO 2 .32