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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:*

N_{2}O_{4 (g)} 2 NO_{2 (g)} K_{eq} = 11 atm (T = 373 K)

mix 0.2 mol of N_{2}O_{4} with 0.2 mol of NO_{2} in a 4.0 L flask at 100^{o}C.

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, N_{2}O_{4 (g)} 2 NO_{2 (g)} until it reaches equilibrium.

See the gas-phase equilibrium problem to determine the equilibrium partial pressures of N_{2}O_{4} and NO_{2}

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