Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry213Kc d= [ C ] [ D ] eq eqa b[ A] [ B]eq eqHere we include the subscript “eq” to indicate a concentration at equilibrium.Although we usually will omit the “eq” when writing equilibriumconstant expressions, it is important to remember that the value of K isdetermined by equilibrium concentrations.6.5As written, equation 6.5 is a limiting lawthat applies only to infinitely dilute solutionswhere the chemical behavior of onespecies is unaffected by the presence ofother species. Strictly speaking, equation6.5 should be written in terms of activitiesinstead of concentrations. We will returnto this point in Section 6I. For now, wewill stick with concentrations as this conventionis already familiar to you.6CManipulating Equilibrium ConstantsWe will take advantage of two useful relationships when working with equilibriumconstants. First, if we reverse a reaction’s direction, the equilibriumconstant for the new reaction is simply the inverse of that for the originalreaction. For example, the equilibrium constant for the reactionis the inverse of that for the reaction[ AB ]2A+ B ABK =[ A][ B]22 1 2AB A 2BK K + =( ) =2 2 1− 1 [ A][B][ AB ]Second, if we add together two reactions to obtain a new reaction, theequilibrium constant for the new reaction is the product of the equilibriumconstants for the original reactions.[AC]A+ C AC K =3[A][C][AC ]2AC + C AC K =2 4[AC][C][AC] [AC ] [AC]22A+ 2C AC K = K × K = × =2 5 3 42[A][C] [AC][C] [A][C]Example 6.1Calculate the equilibrium constant for the reactiongiven the following information2A+ BC+3D22