Changing Gases

Introduction

Perform a simple demonstration that shows the effect of temperature on an unusual phase change equilibrium.

Concepts

• Le Chatelier’s principle
• Equilibrium

Materials

Safety Precautions

Nitrogen dioxide is moderately toxic by inhalation. The demonstration tubes are very fragile, handle with care. Wear safety glasses for protection from flying glass in case the tubes are dropped. Liquid nitrogen can cause skin burns on contact. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

1. Observe the color and contents of the two tubes at room temperature. They should be identical in color. Each tube may contain a small amount of liquid.
2. Carefully place one tube in the ice water bath and the other in the hot water bath (about 80 °C).
3. Observe the billowing brown gas in the tube that is in the hot water. Also, note the lightening of the tube in the ice water bath.
4. Switch the tubes and watch how quickly the lightly colored tube darkens and the dark tube lightens.
5. As an optional activity, place a tube in liquid nitrogen and observe. Depending on the exact contents of the tube, a blue solid, N₂O₃, or a white solid, N₂O₄, will form.

Discussion

During the manufacturing process, the tubes are filled with nitrogen dioxide gas, NO₂, and sealed. Nitrogen dioxide gas then reaches an equilibrium with dinitrogen tetraoxide, N₂O₄, according to Equation 1.

Le Chatelier’s principle states that when a system is subjected to stress or change due to temperature, pressure, or concentration, the system reacts in such a way as to partially relieve the stress or change while reaching a new state of equilibrium. Le Chatelier’s principle predicts that if the temperature rises, the equilibrium will shift in the endothermic direction (towards NO₂). This occurs because the heat energy is absorbed by the molecules. Likewise, if the temperature falls, the equilibrium will tend to shift in the exothermic direction (towards N₂O₄).

Another way to evaluate this reaction is to use the Gibbs-Helmholz equation: ΔG = ΔH – TΔS. The Gibbs-Helmholz equation helps us predict whether or not a reaction will proceed. There are three factors which are involved in a reaction: temperature (T), heat of reaction (ΔH) and entropy (ΔS). In this case, the reaction as written (2NO₂ → N₂O₄) has a negative ΔH value. The ΔS of the reaction is also negative. When ΔG is negative, the reaction proceeds in the forward direction.

The ΔH and ΔS compete against each other because of the negative term in front of TΔS. As an example, the values for a temperature of 300 K are as follows:

ΔG = –3.2

As the temperature rises, the (–TΔS) value becomes more positive, causing ΔG to have a less negative value. At some temperature, the ΔH and TΔS terms will be equal—making ΔG = 0. Above this temperature, the reverse reaction (N₂O₄ → 2NO₂) will proceed due to its negative ΔG. In other words, as the temperature increases and the entropy term becomes more prevalent (–TΔS goes up), the reaction proceeds in the reverse direction, towards disorder. Essentially, heat of reaction and entropy compete with each other to control the direction of this reaction. At low temperatures, the heat of reaction term is able to overcome the decrease in entropy in going from NO₂ → N₂O₄. If the tubes are heated to 140 °C, (413 K, ΔG = 16.7) the gas mixture will contain almost 100% NO₂. If the tubes are placed in liquid nitrogen, solid N₂O₄ is formed. Some tubes contain small quantities of nitrous oxide, NO, gas which forms dinitrogen trioxide, N₂O₃, upon cooling. In this case, placing this tube in liquid nitrogen will allow solid N₂O₃ to form—which is blue.

References

Special thanks to Lee Marek of Naperville North High School in Naperville, IL, for providing instructions for this activity.

Atkins, P. W. Physical Chemistry; W. H. Freeman: New York, 1990.

Ouellette, R. J. Introduction to General, Organic, and Biological Chemistry, Macmillan: New York, 1992.