Le Chatelier’s Principle (NO2 – N2O4)
Two ~ 1 ft long glass tubes containing a reddish colored gas and put in a dry-ice-isopropy alcohol bath. The tubes loose their gas and a green liquid condenses on the bottom. When one tube is returned to room temperature, its reddish color is restored.
NO2 gas exists as a mixture of NO2 and N2O4 at equilibrium. It is a reddish color. When the temperature is changed the equilibrium shifts. Cooling the gas, causes a shift towards the dimer, N2O4, which should be colorless when pure but can be blue-green due to impurities such as N2O3. The dimerization in NO2 is exothermic.