The Effect of Temperature on the NO2/N2O4 Equilibrium

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Chemical Concept Demonstrated

Temperature's effect on equilibrium

Demonstration

Four glass tubes are sealed with NO₂ gas.

The first tube is placed in a hot water bath.
The second is placed in ice-water.
The third is placed in a flask filled with dry-ice/acetone slush.
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 H°_f(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)	K_p
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

Contributors

Dr. George Bodner (Perdue University)