How to Solve Equilibrium Problems

As you ve seen in the previous sections, three criteria define a system at equilibrium  

let s apply equilibrium principles to quantitative situations. Many kinds of equilibrium problems arise in the real world, as well as on chemistiy exams, but we can group most of them into two types  

In one type, we are given equilibrium quantities (concentrations or partial pressures) and solve for K. 

In the other type, we are given K and initial quantities and solve for the equilibrium quantities. 

There are two common variations on the type of equilibrium problem in which we solve for K one involves a straightforward substitution of quantities, and the other requires first finding some of the quantities. 

Plan We write the expression for Q, find its value by substituting the given concentrations, and then compare its value with the given K.  

With Qc Kc, the reaction is not at equilibrium and will proceed to the left until gc = Kc-Check With [NO2] [N2O4], we expect to obtain a value for Qc that is greater than 0.21. If Qc Kc, the numerator will decrease and the denominator will increase until Qc = Kc that is, this reaction will proceed toward reactants. 

FOLLOW-UP PROBLEM 17.3 Chloromethane forms by the reaction CH4(g) + Cl2(g) CHjCIfg) + HCl(g) 

We compare the values of Q and K to determine the direction in which a reaction wiii proceed toward equilibrium. 

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