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.
This demonstration can be used when the concept of the equilibrium concept is being introduced to illustrate the temperature dependence of 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. The demo 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 demo.
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 tetroxide 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.
- 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
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 They are then withdrawn from their respective baths and the intensity of their colors is compared.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 and the hot flask should be noticeably darker.
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 tetroxide 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.