Calculate the percentage change in Kx for the reaction CH3OH(g) + NoCl(g) HCl(g) + CH3NO2(g) when the total pressure is increased from 1.0 bar to 2.0 bar at constant temperature. Strategy To solve this exercise well begin with the definition of equilibrium constant (Equation 6.13 pg 216) K = j a vj j equilibrium As this reactions is for gas, the activity of each reactant and product is taken as proportional to its partial pressure by aj = pj/p Hence the expression for K of this reaction is K = (pHCl/p) 1 (pCH3NO2 /p) 1 (pCH3OH/p) 1 (pNoCl/p) 1 = pHCl pCH3NO2 pCH3OH pNoCl We also know that the partial pressure pj of each j-th compound can be related to the mole fraction of compound j j in the gas mixture and the total pressure p. pj = jp This relationship allows us to determine K in terms of total pressure p and mole fractions. K = pHCl pCH3NO2 pCH3OH pNoCl = HClp CH3NO2 p CH3OHp NoClp = HCl CH3NO2 CH3OH NoCl Here we see that in this expression for K the total pressures p in the numerator and denominator cancel. Therefore K is independent of the total pressure and will not change with a change in pressure. This could also be discerned from simple physical intuition about this specific reaction. As there are the same number of moles of gas on both the reactants and products side, the total moles of gas is independent of the extent of reaction. Converting more moles of reactants into products (or vice versa) doesnt effect the total moles of gas and the equilibrium in this reaction therefore isnt effected by the total pressure. 8 CHEM 3411, Fall 2010 Solution Set 6 Solution K will not change