Nitrogen Dioxide and Dinitrogen Tetroxide Equilibrium

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★★ Requires a more experienced teacher
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Equilibrium, Gases

Alternative titles: Brown to Colorless Gas Equilibrium Demonstration

Summary

The equilibrium between brown nitrogen dioxide gas (NO2) and colorless dinitrogen tetroxide gas (N2O4) can be studied in a closed syringe system. Changes in pressure, volume, and temperature shift the equilibrium, which can be observed as changes in the intensity of the brown color.

Procedure

Generate nitrogen dioxide (NO2) (Link).
Place a sample of nitrogen dioxide gas in a sealed gas syringe to create a closed system.
Adjust the volume of the syringe by pushing or pulling the plunger to change pressure and concentration.
Observe the color changes as the system shifts toward either more brown NO2 (at lower pressures/greater volume) or more colorless N2O4 (at higher pressures/lower volume).
Place the syringe in warm water to increase temperature, and observe the mixture turning browner as more NO2 forms (endothermic direction).
Place the syringe in ice water to decrease temperature, and observe the mixture turning paler as more N2O4 forms (exothermic direction).

Links

Volume Effect on Equilibrium - LeChatelier's Principle Lab Extension - North Carolina School of Science and Mathematics:

Nitrogen Dioxide and Dinitrogen Tetroxide Equilibrium - HSC Chemistry - Science Ready:

Variations

Shine a light through the gas mixture to make color intensity changes more visible to larger groups.
Compare with other equilibrium systems such as iron(III) thiocyanate to reinforce the concept of Le Chatelier’s principle.

Safety Precautions

Perform the experiment in a fume hood, as nitrogen dioxide is toxic.
Ensure the syringe or container is securely sealed to prevent gas leaks.
Wear safety glasses and gloves.
Handle warm and ice baths carefully to avoid burns or frostbite.

Questions to Consider

Why does the color intensity change when the pressure is increased? (Because equilibrium shifts toward N2O4, which has fewer moles of gas, making the mixture less brown.)
What does the observed color change reveal about the enthalpy change of the reaction? (Formation of NO2 is endothermic; formation of N2O4 is exothermic.)
How does this experiment demonstrate Le Chatelier’s principle? (The system shifts to counteract changes in pressure, volume, or temperature, visible through the color changes.)
Why is nitrogen dioxide brown while dinitrogen tetroxide is colorless? (NO2 absorbs visible light due to its electronic structure, while N2O4 does not significantly absorb in the visible spectrum.)