Ammonia is catalytically oxidized at the platinum surface according to the following reaction.
5 NH3 (g) + 5 O 2 (g) ® 4 NO (g) + 6 H2O (g)
The reaction is exothermic and as written releases 920 kJ of energy. A second reaction proceeds automatically from the first reaction.
2 NO (g) + O 2 (g) ® 2 NO2 (g)
The second reaction is also exothermic and as written releases 112 kJ of energy. These two reactions taken together make up two thirds of the Ostwald process for the synthesis of nitric acid. If the NO2 (g) were put into contact with liquid water, nitric acid and NO (g) would be produced. The initial heating of the wire with the lighter provides the actvation energy for the reaction. Once the reaction begins, the released energy will provide more than enough activation energy for subsequent reactions to occur.
An interesting variation of this demonstration is to substitute a thin copper wire, approximately 0.2 mm in diameter for the platimun wire (2). The heat from the reaction is sufficient to melt the copper wire. The molten copper will spatter and make a spectacular effect. Additionally, some of the copper will oxidize and turn the ammonia solution blue by forming a complex ion between the Cu 2+ ion and the ammonia [Cu(NH 3) 4] 2+. This variation does not work as consistently well as the platinum version does.