Water sensitive flash powder

Everyone should know, that it is not wise to extinguish every fire with water. Special hazards need special care. We can demonstrate this with an interesting pyrotechnic mixture: Negative X.

This is a metal-catalyzed, strongly exothermic redox reaction which is initiated by nascent hydrogen (formed from ammonium ions of the acidic hydrolyzing salts and the finely divided zinc powder).

Equipment

Metal plate, large spatula, beaker (100 ml), protective glasses, squirt bottle or pipette (50 ml)

Chemicals

Ammonium chloride (Xi), ammonium nitrate (O) (previously dried in a desiccator over conc. Sulfuric acid (C) or phosphorus pentoxide (C)), zinc dust (F) (use fresh if possible)

How-to

Protective measures:

Wear protective goggles when preparing and initiating the experiment!

The reaction mixture is so sensitive to moisture that one must be careful to use only dry ammonium nitrate. This is especially important because this salt is hygroscopic. Otherwise, the reaction will start while the substances are being mixed. Work in the fume hood or outdoors.

Mix a tablespoon full of ammonium chloride and ammonium nitrate in a beaker and pour this mixture onto a fire-proof base.

Carefully add half the amount of zinc powder with a spoon and mix with the large spatula so that your hand is not over but next to the mixture.

Ignite by adding a drop of water from a 50 ml pipette or a long glass tube. The reaction starts so slowly that you have enough time to withdraw your hand with the pipette.

The experiment can also be started with a piece of ice instead of water.

What happened?

The vapors contain (note the smell and smoke formation) NO2 and NH3 as well as H2O and thus also solid NH4NO2.

Student-friendly explanation

Ammonium ions react acidic in aqueous solution (can be shown in an experiment). Zinc is a reactive metal that reacts with acids to generate hydrogen (can also be shown).

At first atomic hydrogen is produced (nascent hydrogen). This reacts with nitrate ions to form nitrite ions and the nitrogen oxides NO and NO2.

The released reaction energy ignites the mixture. Some reactions are:

Zn + 2 NH4+ —> Zn2+ + 2 H + 2 NH3

2 H + NO3 —> NO2 + H2O

NH4+ + NO2 —> N2 + 2 H2O

H + H+ + NO3 —> NO2 + H2O

(and so on …)

The scientifically correct explanation

In an acidic environment, zinc transfers electrons directly to nitrate ions with the formation of NO, NO2 and NH3 (maybe also N2).

Oxidation: Zn —> Zn2+ + 2 e

Reduction: NO3 + 2 H+ + e—> NO2 + H2O (and further reductions)

In addition, zinc also transfers electrons to water or to hydrogen ions or ammonium ions, with hydrogen and ammonia being formed.

Reductions: 2 H+ + 2 e —> 2 H —> H2

2 NH4+ + 2 e —> 2 NH3 + 2 H —> H2