Spontaneous exothermic reaction

Demonstration

In this demonstration experiment, a mixture of glycerol (propane-1,2,3-triol) and potassium manganate(VII) crystals bursts into flame, giving off clouds of steam, after a short time lag.

Lesson organisation

This reaction can be used as a fun demonstration to show a spontaneous reaction, or as an example of the redox reaction between a fuel and a powerful oxidising agent.

The time lag illustrates the speeding up of an initially slow exothermic reaction as the energy given out raises the temperature of the mixture.

Apparatus	Chemicals
Eye protection The teacher requires: Goggles or a face shield Safety screens Clean metal lid from a tin can or jar (Note 1) Heat resistant mat	The quantities of chemicals given are for one demonstration: Potassium manganate(Vll) (OXIDISING, HARMFUL, DANGEROUS FOR THE ENVIRONMENT), 2-3 g in the form of fine crystals Glycerol (propane-1,2,3-triol), about 1 cm3 in a test-tube Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection (teacher and students). The teacher/demonstrator should wear goggles or a face shield and use safety screens. 

Potassium manganate(VII), KMnO₄(s) (OXIDISING, HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard. Fine crystals of potassium manganate(VII) work much better than larger ones. Use a pestle to grind large crystals in a clean mortar, if necessary.

Glycerol (propane-1,2,3-triol), CH₂OHCH(OH)CH₂OH(l) - see CLEAPSS Hazcard. Old samples of glycerol are sometimes ineffective, possibly because of absorbed water from the air. It is sometimes better if the glycerol is gently warmed just before use.

1 If the lid from a jar has a plastic lining, this should be scraped off. Alternatively, use a small foil cake case which has been cleaned and dried.

IMPORTANT NOTE: Sometimes, the mixture fails to ignite. Do not discard the mixture into a rubbish container as this could still ignite some hours later. If it appears not to react, the mixture must be poured into plenty of water and then poured down the drain.

Procedure

a Put 2-3 g of potassium manganate(VII) in a small pile on the tin lid standing on the heatproof mat. Make a small hollow in the centre of the pile.

b Pour about 1 cm³ of glycerol into the hollow in the pile of potassium manganate(VII).

After about 20 seconds (but beware – it can be much longer), the mixture starts to give off steam. The glycerol in the mixture then ignites, burning with a bright, pinkish (lilac) flame for a few seconds more, leaving a dark brown or black residue.

Teaching notes

Eye protection and safety screens are essential. Small particles of potassium manganate(VII) may fly out.

A white background is useful. The reaction is even more spectacular in a darkened room.

Point out that the pink (lilac) colour of the flame is characteristic of potassium salts.

Redox chemistry
At advanced level, the redox nature of the reaction can be explored. Do this by allowing the residue to cool down and then dissolving it in water. This produces a green solution suggesting the presence of a Mn(Vl) species, as well as a brown solid, manganese(lV) oxide. This confirms the reduction of the manganate(Vll) ion; the glycerol has been oxidised to water (hence the steam) and carbon dioxide.

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated September 2016