21.1 The Effect of Temperature on the NO2/N2O4 Equilibrium


Chemical Concept Demonstrated: Temperature's effect on equilibrium

Demonstration:

Four glass tubes are sealed with NO2 gas.
  1. The first tube is placed in a hot water bath.
  2. The second is placed in ice-water.
  3. The third is placed in a flask filled with dry-ice/acetone slush.
  4. The fourth is placed in liquid nitrogen.

Compare the glass tubes.

Observations:

    NO2 is brown and N2O4 is colorless.  The intensity of the brown color decreases as the temperature decreases.   Therefore, a decrease in temperature yields and increase in N2O4.   Equilibrium is shifted to the N2O4 side upon a decrease in temperature.


Explanation (including important chemical equations):

    2 NO2 (g) <=> N2O4 (g)

    The standard enthalpy (delta H = -57.2 kJ) and the entropy (delta S = -175.83 kJ) of reaction can be calculated from the follow standard-state enthalpies of formation and standard-state entropies.

Compound delta Hf (kJ/mol) delta S (J/mol-K)
NO2 33.18 240.06
N2O4 9.16 304.29

    We can then invoke the assumption that the temperature dependence of delta H for this reaction is small to estimate the equilibrium constant at various temperatures.

Temperature delta G (kJ) Kp
100 C 8.4 0.066
0 C -9.2 58
-78 C -22.9 1.3 x 106
-196 C -43.6 3.7 x 1029