Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry227Example 6.6What happens to the solubility of AgCl if we add HNO 3 to the equilibriumsolution defined by reaction 6.29?So l u t i o nNitric acid is a strong acid, which reacts with ammonia as shown here+ −HNO ( aq) + NH ( aq) NH ( aq) + NO ( aq)3 3 4 3Adding nitric acid lowers the concentration of ammonia. Decreasing ammonia’sconcentration causes reaction 6.29 to move from products to reactants,decreasing the solubility of AgCl.Increasing or decreasing the partial pressure of a gas is the same as increasingor decreasing its concentration. Because the concentration of a gasdepends on its partial pressure, and not on the total pressure of the system,adding or removing an inert gas has no effect on a reaction’s equilibriumposition.Most reactions involve reactants and products dispersed in a solvent.If we change the amount of solvent by diluting or concentrating the solution,then the concentrations of all reactants and products either decreaseor increase. The effect of simultaneously changing the concentrations of allreactants and products is not as intuitively obvious as when changing theconcentration of a single reactant or product. As an example, let’s considerhow diluting a solution affects the equilibrium position for the formationof the aqueous silver-amine complex (reaction 6.28). The equilibrium constantfor this reaction isThe relationship between pressure andconcentration can be deduced using theideal gas law. Starting with PV = nRT, wesolve for the molar concentrationnmolarconcentration = =VPRTOf course, this assumes that the gas is behavingideally, which usually is a reasonableassumption under normal laboratoryconditions.[ Ag(NH )]+3 2 eqβ 2=+2[ Ag ] [ NH ]eq 3 eq6.30where we include the subscript “eq” for clarification. If we dilute a portionof this solution with an equal volume of water, each of the concentrationterms in equation 6.30 is cut in half. The reaction quotient, Q, becomes+05 .[ Ag(NH )]3 2 eq 05 .Q = =+ 2 205 .[ Ag ] ( 05 .)[ NH ] ( 05 . )eq3eq3+× [ Ag(NH )]3 2 eq4[ ] [ ]= β+Ag NHeq32 2eqBecause Q is greater than β 2 , equilibrium is reestablished by shifting thereaction to the left, decreasing the concentration of Ag(NH 3 ) 2 + . Note thatthe new equilibrium position lies toward the side of the equilibrium reactionhaving the greatest number of solute particles (one Ag + ion and twomolecules of NH 3 versus a single metal-ligand complex). If we concentratethe solution of Ag(NH 3 ) 2 + by evaporating some of the solvent, equilibriumis reestablished in the opposite direction. This is a general conclusion thatwe can apply to any reaction. Increasing volume always favors the direc-