The reaction quotient, Q, is the same as the equilibrium constant expression, but for partial pressures or concentrations of the reactants and products before the system reaches equilibrium.
If Q < K then the reaction is proceeding in the forward direction.
If Q > K then the reaction is proceeding in the reverse direction.
N2O4 (g) 2 NO2 (g) Keq = 11 atm (T = 373 K)
mix 0.2 mol of N2O4 with 0.2 mol of NO2 in a 4.0 L flask at 100oC.
Q = (PNO2)2 / PN2O4
First find PNO2 and PN2O4 using PV = nRT
PNO2 = PN2O4 = (0.20 mol)(0.0821 L atm/mol K)(373 K)/(4.0 L) = 1.5 atm
Q = (1.5 atm)2/1.5 atm = 1.5 atm
Q < K, so the reaction will proceed in the forward direction, N2O4 (g) --> 2 NO2 (g) until it reaches equilibrium.
See the gas-phase equilibrium problem to determine the equilibrium partial pressures of N2O4 and NO2