# Reaction Quotient

### Introduction

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.

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

General Solution to Equilibria Problems
1. Introduction 2. Equilibrium Constant 3. Reaction Quotient 4. General Solution