Producing nitrogen oxides

Nitrogen and oxygen, the main constituents of air, can chemically combine to nitrogen oxide (NO) in an endothermic reaction.

At room temerature, equilibrium is completeley on the left hand side of this reaction. Only at very high temperatures (above 3000K) there is a significant fraction (>5%) of NO in equilibrium with air. In order to maintain this fraction at room temperature, it is necessary to cool the reaction products down very fast, faster than the proper equilibrium for room temperature can be established.

An electric arc fulfills both conditions, as it maintains a very high temperature but only locally, and with a steep gradient to the surrounding air. Actually, the "burning of air" in an electric arc used to be a standard (though very energy consuming) technique in the production of nitric acid. (Nowadays, it has been superseded by the method of ammonia combustion.)

NO is a colorless poisonous gas. At room temperature, it immediately grasps another oxygen atom to form nitrogen dioxide (NO₂). This is an exothermic reaction, therefore equilibrium shifts towards the left hand side at higher temperatures.

NO₂ is a reddish-brown gas with a characteristic smell. It is extremely poisonous and corrosive. It is in equilibrium with its dimer N₂O₄, which is colorless. The monomeric fraction increases with temperature, from about 20% at room temperature to nearly 100% at 140°C.

Glass vessel with NO₂ (brown gas). Between the electrodes visible at the center, an arc has been running for about 30s. This setup is described below.

Click on image for larger version.

A very simple way to do this experiment at home is this: take two identical drinking-glasses. Form two flat triangular electrodes by folding aluminum foil (several layers). Put the glasses together to form a vessel, with the electrodes between them. The part of the electrodes inside of the vessel forms a jacobs ladder, the part outside is used to make the connection to some high voltage source (obit, flyback). The electrodes must be close enough together that an arc forms without help from outside. Fix and seal everything with (preferrably transparent) tape. After running an arc for a few 10 seconds, the reddish-brown NO₂ should become clearly visible.

Nitrogen oxide (NO) is poisonous, nitrogen dioxide (NO₂) is extremely poisonous and corrosive. Avoid exposure, in particular inhaling. This experiment should be done in a well ventilated area. The reaction products from the glass vessel should be released outdoors. Take care not to overheat the vessel.

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