# NO2/N2O4 Equilibrium Demonstration

Heating or cooling flasks of NO2 and N2O4 shifts the equilibrium between these two species. When more NO2 is produced, the color of the gas inside the flask becomes darker brown.

The equilibrium system can be represented as

N2O4(g)  <-->  2 NO2(g)    H = 58.0 kJ

A computer animation representing what occurs at the particulate level was available but it is missing.  The computer animation needs to be re-built.

A YouTube video of this demonstration is available

Curriculum Notes

This demonstration can be used when the concept of the equilibrium concept is being introduced to illustrate 1) the dynamic aspects of equilibrium - the rate of the forward reaction is equal to the rate of the reverse reaction - if the N2O4 <-> 2NOcomputer animation is paired with this demonstration,  2) that equilibrium systems are reversible and 3) the influence of temperature on the equilibrium constant. It can also be used later on in thermodynamics to illustrate the importance of the entropy term of the Gibbs free energy equation and to show the relationship of free energy to equilibrium constant.

One day of lead time is required for this project.
Discussion

Nitrogen dioxide is an odd-electron system so it exists as a molecular radical. Its lone electron absorbs strongly in the visible range, so it is a deep maroon color. All of the electrons of dinitrogen tetraoxide are paired and it is colorless.

2 NO2(g) <--> N2O4(g)

At room temperature and normal atmospheric pressure the reactant and product are in a fairly well distributed equilibrium. At higher temperatures nitrogen dioxide is favored and at lower temperatures dinitrogen tetroxide is favored. Since there are twice as many moles of gas on the right hand side of the equation, the entropy change of this reaction is positive, so you would expect the product to be favored at higher temperatures.

Materials
• two sealed Kjeldahl flasks containing an equilibrium mixture of nitrogen dioxide and dinitrogen tetroxide
• two insulated containers - one containing hot water and the other containing ice water
• a white background box
Procedure

One flask is immersed in the ice water and another is immersed in the hot water. They are in immersed until the color change is quite noticeable (about 90 seconds). They are then withdrawn from their respective baths and the intensity of their colors is compared. The cold flask should be noticeably paler in color and the hot flask should be noticeably darker in color.

The demonstration should be performed shortly after the materials are brought into the classroom or the hot water will cool down. Allow about 10 minutes for this demonstration.

Safety Precautions

Goggles should be worn by the demonstrator. Care should be taken to ensure that the flasks are not broken. If one of the flasks break the room should be evacuated. Nitrogen dioxide and dinitrogen tetraoxide form nitric acid when they come in contact with the moist lining of the lungs when inhaled, so avoid inhaling the vapors if one of the flasks breaks.

Footnotes

References

1.  Wheeler, A.E.; Kass, J.H. (1978).  Student misconceptions in chemical equilibrium,  Science Education, 62(2), 223-232.

2. Banerjee, Anil C.  (1991). Misconception of students and teachers in chemical equilibrium.  International Journal of Science Education, Vol.13(4), p.487-494 .

3. Bergquist, Wilbur ; Heikkinen, Henry (1990).  Student ideas regarding chemical equilibrium. Journal of Chemical Education, Vol.67(12), p.1000-1003.

4. Putti, A. All things being equal. The Science Teacher. Vol 79, 7, 2012.

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© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group