Expt:Spontaneous exothermic reaction

From Learn Chemistry Wiki

Introduction

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.

Apparatus and chemicals

• Eye protection
• Safety screens

The teacher requires:

• Clean metal lid from a tin can or jar (see note 1)
• Heat resistant mat
• Access to (quantities of chemicals given are for one demonstration):

• Potassium manganate(Vll) (Oxidising agent, Harmful, Dangerous to the environment), 2-3 g in the form of fine crystals (see note 2)
• Glycerol (propane-1,2,3-triol) (Low hazard), about 1 cm³ in a test-tube (see note 3)

Technical notes

Potassium manganate(VII) (Oxidising agent, Harmful, Dangerous for the environment) Refer to CLEAPSS® Hazcard 81

Glycerol (propane-1,2,3-triol) (Low hazard) Refer to CLEAPSS® Hazcard 37

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.
2. 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.
3. Old samples of glycerol are sometimes ineffective, possibly because of absorbed water from the air.

Procedure

Health & Safety

Wear eye protection throughout and use safety screens.

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).

It is sometimes better if the glycerol is warmed just before use. 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.

Reference

This experiment has been reproduced from Practical Chemistry: http://practicalchemistry.org/experiments/spontaneous-exothermic-reaction,48,EX.html

Useful resource

This website has a movie showing the experiment 'Oxidation of glycerine by potassium permanganate': http://www.jce.divched.org/JCESoft/CCA/CCA1/R1MAIN/CD1R1870.HTM#1920