II. General Solution to Equilibria Problems
II-1. Introduction II-2. Equilibrium Constant II-3. Le Chatelier's Principle II-4. Reaction Quotient II-5. General Solution

Introduction

The expression for the reaction quotient, Q, is the same as the expression for the equilibrium constant (see II-2.), 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.


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