III. Gas-Phase Equilibria
III-1. Introduction and Background III-2. Sample Problem III-3. List of Problems III-4. Simple Problems III-5. Advanced Problems


The following problem is an example of determining the equilibrium partial pressures in a gas-phase equilibrium problem. For background information see the documents on gas-phase equilibria and the general solution of equilibria problems.

Sample Problem

What are the equilibrium partial pressures of N2O4 and NO2 when 0.2 atm of each gas are introduced into a 4.0 L flask at 100oC, (Keq = 11 atm)?

1. Find the pre-equilibrium partial pressures, 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

2. The balanced chemical reaction is: N2O4 (g) <--> 2 NO2 (g)

and the equilibrium constant expression is: Keq = (PNO2)2 / PN2O4

3. Now calculate the reaction quotient, Q, to determine the direction in which the reaction will proceed to reach equilibrium.

Q = (PNO2)2 / PN2O4

Q = (1.5 atm)2/1.5 atm = 1.5 atm

Q < Keq, so the reaction will proceed in the forward direction, N2O4 (g) ---> 2 NO2 (g) until it reaches equilibrium.

4. For each mol of N2O4 that dissociates, 2 moles of NO2 will form. The pre-equilibrium partial pressures, changes in partial pressures, and equilibrium partial pressures are given in the following table:

Po1.50 atm1.50 atm
DeltaP-x atm+2x atm
Peq(1.50-x) atm(1.50+2x) atm

Where Po are the pre-equilibrium partial pressures, DeltaP are the changes in partial pressures, and Peq are the equilibrium partial pressures.

5. We can now calculate the equilibrium partial pressures using the equilibrium constant expression:

Keq = 11 = (PNO2)2 / PN2O4

11 = (1.50+2x)2 / (1.50-x)

Rearranging gives:

4x2 + 17x - 14.25 =0

Find x using the quadratic equation:

x = 0.717

PN2O4 = 1.50 - 0.717 = 0.783 atm

PNO2 = 1.50 + 2(0.717) = 2.93 atm

Check results: Q = (2.93)2/0.783 = 11. Q = Keq, so the system is at equilibrium.

Does a total pressure of 3.71 atm at equilibrium make sense? Think about the total pressure before the system was at equilibrium, and the direction that we said the reaction would proceed to reach equilibrium.

Equilibrium Practice Problems
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