Temperature Effect on the Equilibrium Between Two Gases

Brief Description: 

Demonstrate the effect of temperature on the equilibrium between N2O4 and NO2.

Purpose/Goal: 

Introduces the student to a equilibrium between two gaseous species in a closed system and the role temperature plays in shifting the equilibrium.

Explanation of Experiment: 

Nitrogen dioxide, NO2, a dark brown gas, and dintrogen tetroxide, N2O4, a colorless gas, exist in equilibrium with each other:

2 NO2(g)  ↔  N2O4 (g)

(Brown)        (Colorless)

At room temperature and above, the mixture of the two gases is normally brown in color, indicating a higher concentration of nitrogen dioxide.  Upon cooling, the forward reaction is favored which in turn increases the concentration of the dintrogen tetroxide, resulting in a yellowish colored gas mixture.  In fact, the yellow color is due to traces of nitrogen dioxide (brown) mixed with the colorless dintrogen tetroxide.  If the mixture is cooled sufficiently, the nitrogen dioxide gas concentration may be significantly reduced.

 

Materials Preparation: 

Two sealed glass tubes containing nitrogen dioxide gas

Two 1000 mL beakers - one containing an ice/water mixture, and the other containing hot water

Two ring stands with two buret clamps

Presentation: 

Present the two sealed tubes of nitrogen dioxide gas to the audience to have them confirm they are the same color.  Immerse one tube in an ice/water bath and the other in the hot water bath.  Observe the difference in the color intensity in the tubes as they equilibrate to their respective environments.

Hazards: 

Nitrogen dioxide and dintrogen tetroxide are both toxic by inhalation and poisonous.  Wear chemical splash goggles, chemical resistant gloves and lab coat.

Disposal: 

Save the gas sample tubes for future use.  Water may be flushed down the drain.

Video: 

General Concept:

Type of Reaction:

Primary Reference: 
Gross, G.R., Bilash, B. and Koob, J.K., (2010), A Demo A Day - A Year of Chemical Demonstrations, p. 202.