Ammonium dichromate volcano

In this demonstration experiment, students witness a spectacular exothermic decomposition reaction, as a small conical heap of orange ammonium dichromate(VI) is ignited. The reaction resembles a volcanic eruption, producing sparks, a large volume of green chromium(lll) oxide ‘ash’, steam and nitrogen gas.

The demonstration can be used to show chemical change, since the products are dramatically different from the starting material. The reaction is a striking example of an exothermic decomposition reaction. The energy given out heats up the products, and steam and sparks are also produced.

The demonstration can be used to stimulate interest at public presentations, such as Open Days, so long as a suitable fume cupboard is available or method B is used.

The demonstration itself lasts about two minutes, but more time is needed for discussion and explanation before and after the demonstration.

Equipment

Apparatus

For both methods

• Eye protection
• Bunsen burner
• Heat resistant mat
• Large spatula

For method A

• Access to a fume cupboard (see note 8 below)
• Large metal tray
• Tongs
• Watch glass
• Wooden splints (see note 9 below)

For method B

• Conical flask, 1000 cm³
• Glass or mineral wool

Chemicals

• Ammonium dichromate(Vl) (VERY TOXIC, EXPLOSIVE, DANGEROUS FOR THE ENVIRONMENT), 3–10 g
• Ethanol (HIGHLY FLAMMABLE) or industrial denatured alcohol (IDA) (HIGHLY FLAMMABLE, HARMFUL)
• Cobalt chloride paper (TOXIC) (see note 7 below)
• Silica gel granules

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Consider wearing gloves, and avoid skin contact with ammonium(VI) dichromate.
• Work in a fume cupboard.
• Ammonium dichromate(VI), (NH₄)₂ Cr₂ O₇ (s), (VERY TOXIC, EXPLOSIVE, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC007. Do NOT be tempted to mix other chemicals with the ammonium dichromate (VI). For disposal: the product of the reaction, the green chromium(III) oxide, Cr₂ O₃ (s), is LOW HAZARD but there may be traces of unreacted ammonium dichromate(VI) dust in the residue after the reaction. Wear gloves when transferring or sweeping up the residue into a plastic bag. Place it in the refuse.
• Ethanol, C₂ H₅ OH(l), (HIGHLY FLAMMABLE or, if IDA, HIGHLY FLAMMABLE and HARMFUL) – see CLEAPSS Hazcard HC040A.
• Cobalt chloride paper – see CLEAPSS Hazcard HC025. Cobalt chloride paper can be bought from laboratory suppliers. To make your own, see these standard techniques for preparing and using cobalt chloride indicator papers.
• Method A must be carried out in a fume cupboard, preferably a portable one giving all-round visibility. Method B is suitable for use in the absence of a fume cupboard. This is because the plug of glass or mineral wool prevents specks of solid escaping from the flask.
• Soak 3 cm lengths of wooden splits in ethanol and leave overnight. Use a small weighing bottle with a lid for this.

Procedure

Method A

1. In a fume cupboard, make a conical heap (no wider than 50 mm diameter) of about 10 g of ammonium dichromate(VI) on a heatproof mat. Place it on a metal tray to collect the large volume of chromium(III) oxide produced.
2. Soak about a 3 cm length of a wooden splint in ethanol. Stick this into the top of the pile so that about 2 cm protrudes, to act as a wick.

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. Chromium(III) oxide is a flaky green solid resembling dry tea leaves that has a considerably larger volume than the original compound. Some of this oxide shoots into the air during the reaction. Although chromium(III) oxide is low hazard, there may be some unchanged ammonium dichromate(VI) present. This may be inhaled, hence the need for carrying out this method in a fume cupboard.
4. The ‘volcano’ reaction increases in rate and continues for 30–45 seconds. Use tongs to hold a watch glass just above the erupting volcano for a few seconds. It becomes steamed up with water vapour from the decomposition reaction. Confirm that this is water with the blue cobalt chloride paper. It should turn pink.

Method B

1. Carefully pour about 3 g of ammonium dichromate(VI) into a 1000 cm³ conical flask so that it forms a small heap in the center. A folded cone of paper used as a funnel may be helpful for this. Place a loose plug of glass or mineral wool in the mouth of the flask to prevent loss of chromium(III) oxide powder during the reaction.
2. Start the reaction by heating the ammonium dichromate(VI) from underneath with the tip of blue Bunsen flame. The orange solid begins to give off sparks, and decomposes into a flaky green solid. This has a considerably larger volume than the original compound.

3. Once the reaction has started, remove the flame and place the flask on a heatproof mat in full view of the class.
4. As the rate of the ‘volcanic’ reaction increases and continues over a period of 30–45 seconds, the flask will steam up, and a little steam may escape through the wool plug. The presence of water on the inside of the flask can be confirmed using blue cobalt chloride paper. It will turn pink.

Teaching notes

The equation for this decomposition reaction is:

(NH₄)₂Cr₂O₇(s) → Cr₂O₃(s) + N₂(g) + 4H₂O(l)

At advanced level the redox nature of the reaction could be explored. The dichromate ions oxidise the ammonium ions to nitrogen and water. In the process, chromium is reduced from its +6 oxidation state in dichromate to its +3 oxidation state in the chromium trioxide.

To confirm this is a decomposition reaction and not combustion, the flask in method B could be flushed with nitrogen gas from a cylinder (if available) before the reaction is started. The reaction is unaffected.

Evidence for the formation of the invisible gas (nitrogen) could be obtained by replacing the wool plug with a loose sandwich of wool and silica gel granules to absorb any steam – see the diagram above. Weigh the flask before and after the reaction. A mass loss will indicate that a gas has been lost, although it is difficult to ensure that no steam escapes as well.

Further resources

A video of nine quick-fire exo and endothermic reactions from Teachers’ TV is available on the Tes website.