Fulminate vs Cyanate Stability

Silver fulminate and silver cyanate are heated with a candle flame to demonstrate the instability of the fulminate ion, which explodes loudly.  This shows the ability of formal charges to predict structural stability since the formal charges of the fulminate ion are higher than those of the cyanate ion.

Equipment

  • 2 rings stands with a foil dish to support the compounds
  • long stick/candle to apply flame to the underside of the foil at a safe distance
  • goggles and ear protection

Reagents

  • silver fulminate, AgCNO
  • silver cyanate, AgNCO

Presentation

  1. The two molecular structures are presented and formal charges calculated on the board, emphasizing that the fulminate is less stable since it has higher formal charges.
  2. Apply the flame to the underside of the cyanate compound, it smokes, but not much else happens.
  3. Apply the flame to the underside of the fulminate compound, it explodes within a few seconds.

Details

The cyanate ion has several resonance structures, whose relevance to its properties are predicted by the formal charges of the atoms.

cyanate

The fulminate ion also has several resonance structures:

fulm3

Note that the formal charges of these structures are generally higher than cyanate, which predicts that they are less favorable molecular configurations.  This leads to the facile decomposition of the fulminate ion into CO2(g), N2(g), and some silver salts.

Safety

explosion

fire

noise

projectile

toxic

Special Safety Notes

  • This is surprisingly loud and sharp; warn the audience before heating the fulminate
  • Wear goggles
  • Apply the flame at the end of a long stick, not directly with a short lighter
fulm1
The two compounds set up on the ring stands.
fulm2
Applying flame to the cyanate from the underside.