Reaction Quotient

Introduction

The reaction quotient, Q, has the same form 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.

Example:

N₂O_{4 (g)} 2 NO_{2 (g)}      K_eq = 11 atm (T = 373 K)

mix 0.2 mol of N₂O₄ with 0.2 mol of NO₂ in a 4.0 L flask at 100^oC.

Q = (P_NO2)² / P_N2O4

First find P_NO2 and P_N2O4 using PV = nRT

P_NO2 = P_N2O4 = (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, N₂O_{4 (g)} 2 NO_{2 (g)} until it reaches equilibrium.

See the gas-phase equilibrium problem to determine the equilibrium partial pressures of N₂O₄ and NO₂