Demonstration 16
Aniline + N2O4 (Attitude rocket motor).
Taking on the theme of “rockets” and “space exploration” we continue “building our rocket”. This demonstration illustrates the principle behind the rocket’s attitude correction motors where the mixing of two liquids produces spontaneous combustion and a jet of gases. The fuel used in real rockets is unsymmetrical dimethyl hydrazine. However we strongly recommend to leave this chemical to the professionals. We employ much safer “fuel” - aniline.
In theory a stoichiometric reaction of aniline with nitrogen dioxide should result in the formation of only gaseous products:
8 C6H5NH2 + 62 NO2==> 48 CO2 + 28 H2O + 35 N2 (16.1)
However, in practice, the combustion is incomplete and quite a lot of soot (carbon) is produced. If the facilities permit, aniline can be substituted for pure anhydrous methyl hydrazine (which is relatively safe to handle) - in this case essentially no soot is formed.
Preparation. To demonstrate this reaction making a jet of fire a small (250 ml) round-bottom flask with a short narrow neck is required. The flask must be thermally resistant and have stout enough walls (Pyrex Quickfit is ideal). A lecture size gas bottle with nitrogen dioxide is a very convenient source of liquid nitrogen dioxide. If this is unavailable, nitrogen dioxide can be made by reacting copper turnings with concentrated nitric acid and then condensing the gas in the flask that is cooled with ice or solid CO2.
Demonstration. Pour a few millilitres of liquid nitrogen dioxide (N2O4) into the round-bottom flask positioned on a cork ring. Then add 1-2 ml of aniline (e.g., by using a syringe with a long (at least 10 cm) needle). Flame will shoot from the neck of the flask with a characteristic roaring noise. The addition of aniline can be repeated until all nitrogen dioxide is used up. It is wise to lean backwards when adding the aniline in case the flask is shattered. Always use SAFETY SCREENS when performing this demonstration.
An alternative and somewhat safer way of demonstrating this reaction is to employ a large (ca. 5 l) flask. The rest of the procedure is the same but when the aniline is introduced into the flask containing liquid N2O4 there is only a flash with no jet firing from the neck of the flask.
Safety. This experiment is very dangerous and should be attempted only by persons who are, not only competent chemists, but also are experienced chemistry demonstrators. Apart from the explosion hazard, nitrogen dioxide is very poisonous. We present this demonstration only where there is an efficient fume extraction system as we have here, or in large halls where there is sufficient volume to dilute the gas. In the absence of extraction system we have an electric fan below the lecture table that is arranged so that it blows any NO2 away from the audience (towards us!).