Effect of Temperature on Nitrogen Dioxide Equilibrium
Publication information:
Abstract
Sealed tubes of a mixture of NO2 and N2O4 gas are placed in ice water and in boiling water, and an instant change in the equilibrium system is demonstrated, according to Le Chatelier’s Principle.
Full text
What it shows
An equilibrium mixture of brown NO2 gas and colorless N2O4 gas is contained in glass tubes at room temperature. When the tubes of brown gas are placed in ice water and in boiling water, the tube in ice water becomes a light yellow color, and the tube in boiling water becomes a dark brown color.
How it works
Le Chatelier’s Principle says that when a change is made to a system at equilibrium, the system will react in a way to counteract the change and restore equilibrium. Inside the sealed glass tubes, brown NO2 gas is in equilibrium with colorless N2O4 gas according to the following chemical equation:
2NO2 (g) ⇔ N2O4 (g) + heat
Since heat is released as a product, this is an exothermic reaction as written.
When a tube is placed in boiling water: Temperature increases – more heat is added to the system. As a result, the reaction moves toward the left side of the reaction in order to use up the additional heat that is being applied. As more N2O4 decomposes into brown NO2 gas, the color turns a dark shade of reddish-brown, and a new equilibrium position is reached.
When a tube is placed in ice water: Temperature decreases – heat is removed from the system. As a result, the reaction shifts toward the right side of the reaction in order to restore the heat being taken away from the system. As more NO2 molecules combine to form colorless N2O4 molecules, the color turns a yellow color, until a new equilibrium position is reached.
Setting it up
Safety: Wear safety glasses.
Use a lightbox behind the setup to illuminate the tubes of gas. Fill a 1.5 L (or 2 L) beaker with DI water, and get it boiling on a hot plate with a watch glass on top. Fill a second 1.5 L (or 2 L) beaker with about 300 mL of water, and add ice to the very top. Get the two tubes of NO2/N2O4 gas (they are in a cylinder with cotton). If you have a 3rd tube, place that in a tall form container with some cotton as a room temperature/control tube.
Put the hot water beaker on top of the hot plate on the bench, and place the ice water beaker on a tray. Place the lightbox behind them. Put the tubes of gas next to the beaker setup. Provide thermal gloves in case the tubes get hot and some paper towels in case of spillage.
When it’s time to present the demo, remove the watch glass from the boiling water, shut off the hot plate, and place one of the tubes in the hot water. The gas should immediately start turning darker. Then take the 2nd tube and place it into the ice water – the color will start to fade (this change is a bit more gradual and there is less contrast).