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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.

tempno2.gif

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°(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

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