Chances are you’ll be making use of christmas crackers over the holiday period. Chances also are that you’ve never really given a lot of thought to the chemical compounds contained therein, or the chemical reaction that makes christmas crackers go bang. Well, allow me to elaborate.
Christmas crackers owe their crack to a compound called silver fulminate. This compound has the molecular formula AgCNO, and can be prepared relatively simply by reacting concentrated nitric acid with silver and ethanol. Fulminates contain the fulminate ion, CNO–, the instability of which leads to fulminate salts being friction sensitive explosives. Silver fulminate is at the finer end of this sensitivity; it can detonate with very little provocation, and is in fact so sensitive that, if prepared in a quantity more than a few milligrams, it can easily self-detonate under its own weight.
Having such an explosive nature understandably rather limits the uses of silver fulminate. Since its discovery in 1800, its applications have been largely limited to their use in novelty noisemakers – and christmas crackers. In crackers, it is embedded in one of the two thin strips of card that run inside, whilst the other strip contains an abrasive. When the cracker is pulled, the friction generated by this abrasive detonates the silver fulminate, producing an audible crack.
In terms of the chemistry involved in the sensitivity of the fulminates, it arises due to the instability of the fulminate ion. The nitrogen-oxygen bond is weak, and nitrogen can easily form a much more stable triple bond with another nitrogen atom, leading to the decomposition of the fulminate compound, typically to nitrogen gas, carbon dioxide, and more stable metal salts.
Silver fulminate was considered for military applications in the 1970s, but research concluded that the compound was too sensitive to have any real practical use. Another fulminate, however, does have applications in explosives: mercury fulminate. This compound is frequently used as a primary explosive in detonators; that is, to trigger larger amounts of a less sensitive secondary explosive. It’s also found some exposure in recent popular culture through Breaking Bad, in which Walter White uses a small amount to cause an explosion in a drug dealer’s office – though this was proven by Mythbusters to be a little on the optimistic side.