Ammonium dichromate volcano
In this demonstration experiment, a small conical heap of orange ammonium dichromate(VI) is ignited and starts to decompose exothermically. The reaction resembles a volcanic eruption, producing sparks, a large volume of green chromium(lll) oxide 'ash', steam and nitrogen gas.
This demonstration experiment 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 2 minutes, but more time is needed for discussion and explanation before and after the demonstration.
The quantities of chemicals given below are for one demonstration:
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) (Note 3)
Silica gel granules
Refer to Health & Safety and Technical notes section below for additional information.
The teacher will require:
Access to a fume cupboard for Method A
Heat resistant mat
Large metal tray (Method A)
Tongs (Method A)
Watch glass (Method A)
Wooden spills (Method A) (Note 2)
Conical flask (1M) (Method B)
Glass or mineral wool (Method B)
Health & Safety and Technical notes
Work in a fume cupboard. Wear eye protection and avoid skin contact with ammonium(VI) dichromate. Consider wearing gloves. Do not be tempted to mix other chemicals with ammonium dichromate(VI).
Ammonium dichromate(VI), (NH4)2Cr2O7(s), (VERY TOXIC, EXPLOSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard. Disposal: the product of the reaction, the green chromium(III) oxide, Cr2O3(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, C2H5OH(l), (HIGHLY FLAMMABLE or, if IDA, HIGHLY FLAMMABLE and HARMFUL) - see CLEAPSS Hazcard.
Cobalt chloride paper - see CLEAPSS Hazcard.
1 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.
2 Soak 3 cm lengths of wooden spill in ethanol and leave overnight. Use a small weighing bottle with a lid for this.
3 Cobalt chloride paper can be bought from laboratory suppliers. To make your own see Standard techniques Preparing and using cobalt chloride indicator papers.
a 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.
b Soak about a 3 cm length of a wooden spill in ethanol. Stick this into the top of the pile so that about 2 cm protrudes, to act as a wick.
c 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, 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.
d 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.
a Carefully pour about 3 g of ammonium dichromate(VI) into a 1M conical flask so that it forms a small heap in the centre. 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.
b 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.
c Once the reaction has started, remove the flame and place the flask on a heatproof mat in full view of the class.
d 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.
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 that 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 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.
Health & Safety checked June 2007
Journal of Chemical Education - Ammonium dichromate volano
A video of nine quick fire exo and endothermic reactions from Teachers' TV which can be viewed through the TES website - Banging Chemistry: Fast and Furious
Page last updated on 03 January 2012