The Decomposition of Ammonium Dichromate

What is Ammonium Dichromate?

Ammonium dichromate is an orange coloured crystalline substance.  It is a salt formed by reacting the base ammonium hydroxide with dichromic acid. It is because dichromic acid is a powerful oxidizing agent and the oxidation state of nitrogen atoms in ammonia is – 3, that ammonium dichromate is an unstable and reactive compound, that decomposes easily. This is because the salt anion has an atom in a high oxidation state which is capable to oxidize the nitrogen atoms from the ammonium cation, and vice versa. In the case of ammonium dichromate, ammonium nitrogen is oxidized to elemental nitrogen, while the chromium atom is being reduced to trivalent state.

What is the science behind the experiment?

As you can see from the reaction enthalpy below, the reaction is exothermic, which means that energy in the form of heat is released in the reaction. Although this is energetically possible, you will need to initiate the decomposition of ammonium dichromate by adding some external energy. This energy needs to be enough to overcome the activation energy barrier, after which the decomposition reaction continues spontaneously.

ammonium dichromate enthalpy equation

The volcano effect can be realized if we analyze the products of the reaction. Namely, elemental nitrogen and water produced in the reaction are both in gaseous state, while chromium (III) oxide is a green solid. The liberation of nitrogen and water vapor agitate the incandescent particles of chromium (III) oxide

The chromium (III) oxide that is formed in this reaction is very voluminous, so the volume is much greater at the end of the reaction than in the beginning, which gives the effect of a volcanic eruption.

How to conduct the experiment

What you will need

Chemicals

Apparatus

  • Large Container
  • Heatproof mat
  • Wooden splint
  • Fume Cupboard
  • Tongs
  • Watchglass
  • Blue cobalt chloride paper

Step 1

On a heatproof mat make a small conical heap (less than 50mm wide) of about 10 g of ammonium dichromate (VI). Place it on a larger metal tray to collect the large volume of chromium (III) oxide produced.  The whole apparatus should them be placed inside a fume cupboard.

 

Step 2

Soak a 3 cm length of a wooden splint in ethanol and put this into the top of the pile so that about 2 cm protrudes. This will act as a wick. See below for a diagram of how the experiment should look.

ammonium-dichromate-setup

Step 3

Light the wick. As the wick burns down into the ammonium dichromate (VI), the orange solid begins to give off sparks and decompose into chromium(III) oxide, giving it the appearance of a volcano erupting.

Step 4

The ‘volcano’ reaction increases in rate and continues for 30 – 45 seconds. You can use tongs to hold a watch glass just above the erupting volcano for a few seconds. It will become steamed up with water vapour from the decomposition reaction.  You can confirm that this with blue cobalt chloride paper that will turn pink.

As all the ammonium dichromate may not react during the experiment it is highly recommended that you conduct the experiment in a fume cupboard, as ammonium dichromate is extremely toxic and may be carcinogenic.

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