Chemical Concept Demonstrated

• LeChatelier's Principle

Demonstration

Two glass tubes are filled with gas, one with NO2 and one with NO and NO2. Both of these tubes are immersed in liquid nitrogen. As an alternative, fill two clear sealed syringes with NO2 gas, place both on an overhead projector, and press one of the plungers in.

Observations

The glass tube with NO and NO₂ in it produces a blue liquid.   The flask with NO₂ in it produces a white solid.  If the syringes are used, the compressed gas first becomes darker, then becomes lighter than the control syringe as the plunger is depressed.

Explanation

NO and NO₂ in the same tube are in equilibrium with the compound N₂O₃, which is a blue liquid.  At room temperature, the equilibrium is shifted far to the end of the gases.  However, when the temperature is lowered, LeChatelier's Principle comes into play.  Because the formation of N₂O₃ is exothermic, lowering the temperature of the system makes the reaction more favorable.

A similar application of the principle explains why pure NO₂ forms N₂O₄ (a white powder) at low temperatures.

In the syringe, the gas becomes darker at first simply because the gas is compressed into a smaller space.  The gas lightens because the resulting system is not in equilibrium.  Because different gases take up the same space with the same number of particles, it is advantageous for two NO₂ molecules to form a single molecule under high-pressure conditions.  N₂O₄ is produced, fewer particles are in the syringe, and, as a result, the intensity of the color caused by the NO₂ goes down.

Contributors

• Dr. George Bodner (Perdue University)