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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:

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


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