Theory of this Experiment
 

The spiral of platinum glows because the oxidation of ammonia is an exothermic process. The faster the reaction proceeds, the faster heat is produced on the catalyst surface. If this is faster than the spiral can radiate it away, the spiral will heat up.  The small explosions in the flask, which produce yellow flames, become cyclic – the spiral glows, the mixture explodes, cooling the spiral which then heats up until a further explosion occurs.

  • The reaction occurring in the flask under the conditions of a solid platinum catalyst, moderate temperature and excess oxygen is:

4NH3 (g) + 5O2 (g) --> 4NO (g) + 6H2O (l)
ammonia + oxygen --> nitrogen monoxide + water

  • This is then followed by:

2NO (g) + O2 (g) --> 2NO2 (g)
nitrogen monoxide + oxygen <--> nitrogen dioxide

  • The brown vapour observed in the reaction is nitrogen dioxide. In addition to being given off as a gas, the nitrogen dioxide reacts with the water to form nitric acid:

2NO2 (g) + H2O (g) -->HNO2 (aq) + HNO3 (aq)
nitrogen dioxide + water --> nitrous acid + nitric acid

  • These acids react with ammonia to give ammonium salts. White fumes of ammonium nitrite and ammonium nitrate were observed in the flask during the experiment. These reactions are similar to those which occur in the Ostwald Process for the manufacture of nitric acid:

4NO2 (g) + 2H2O (g) + O2 (g) --> 4HNO3 (aq)
nitrogen dioxide + water + oxygen --> nitric acid

  • The nitric acid produced in this process is used for making fertilisers, explosives and nylon.
 

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