Weird Science

Spontaneous combustion

Polish ver­sion is here

A Highly Exo­ther­mic Reac­tion

An exo­ther­mic reac­tion is a che­mi­cal reac­tion that results in a net rele­ase of heat into its sur­ro­un­dings (the oppo­site of an endo­ther­mic reac­tion, which you can read about here). This means that as the reac­tion pro­gres­ses, ther­mal energy is emit­ted. Of course, this isn’t some kind of che­mi­cal per­pe­tual motion machi­ne­—the heat pro­du­ced comes from the energy rele­a­sed by che­mi­cal bonds. When the total energy of che­mi­cal bonds in the reac­tion pro­ducts is lower than in the reac­tants, the dif­fe­rence must be rele­a­sed into the envi­ron­ment, in accor­dance with the law of con­se­rva­tion of energy (energy can­not be cre­a­ted or destroy­e­d—only trans­for­med from one form to ano­ther). In this case, the energy is rele­a­sed as heat.

There are many exam­ples of exo­ther­mic reac­tions. One par­ti­cu­larly stri­king exam­ple is spon­ta­ne­ous com­bu­stion. This phe­no­me­non occurs when an exo­ther­mic reac­tion gene­ra­tes eno­ugh heat to ignite the reac­tants without an exter­nal energy source. As a result, the reac­tion mixture igni­tes on its own, without the need for an exter­nal flame or spark.

What Do We Need?

We will need the fol­lo­wing mate­rials:

Ilustracja:

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Both rea­gents can be pur­cha­sed at a phar­macy. Some­ti­mes you may need to check mul­ti­ple loca­tions, but they are com­monly ava­i­la­ble. Potas­sium per­man­ga­nate is widely used as a water disin­fec­tant, while gly­ce­rin has a variety of appli­ca­tions. Once we have both ingre­dients, we can pro­ceed with the expe­ri­ment.

War­ning: This reac­tion invo­lves an open flame, and hot dro­plets of the reac­tion mixture may splat­ter. Extreme cau­tion must be exer­ci­sed. Due to the rele­ase of gases, this expe­ri­ment sho­uld only be con­duc­ted out­do­ors or in a well-ven­ti­la­ted fume hood. Addi­tio­nally, be care­ful with potas­sium per­man­ga­na­te­—it is a strong dye that can leave dif­fi­cult-to-remove pur­ple sta­ins on damp skin. The author assu­mes no respon­si­bi­lity for any poten­tial damage or inju­ries. You pro­ceed at your own risk!

Pre­pa­ra­tion

We need to pre­pare a fire-resi­stant sur­face. A good option is an asbe­stos plate (be care­fu­l—this mate­rial is car­ci­no­ge­nic!) or a cera­mic tile. I use a lefto­ver cera­mic tile from a bath­room reno­va­tion. On the tile, place a sheet of paper. Then, cre­ate a small mound of potas­sium per­man­ga­nate on top of the paper. It’s best to make a small inden­ta­tion at the peak of the mound. Care­fully drop a few drops of gly­ce­rin into the inden­ta­tion, then quic­kly step back to a safe distance. After seve­ral seconds, the mixture will ignite spon­ta­ne­o­u­sly. The entire pro­cess can be seen in the video below:

Why Does This Hap­pen?

The reac­tion can be repre­sen­ted by the fol­lo­wing equ­a­tion:

Ilustracja

Potas­sium per­man­ga­nate is a strong oxi­di­zer. Ini­tially, it oxi­di­zes gly­ce­ri­n—a tri­hy­droxy alco­ho­l—at a rela­ti­vely slow rate. As this pro­cess occurs, heat is rele­a­sed, gra­du­ally incre­a­sing the tem­pe­ra­ture of the reac­tion mixture. Since this reac­tion acce­le­ra­tes at higher tem­pe­ra­tu­res, a rapid chain reac­tion takes place, ulti­ma­tely lea­ding to igni­tion. The gly­ce­rin is pri­ma­rily oxi­di­zed into car­bon dio­xide and water.

Enjoy!

Fur­ther rea­dings:

Marek Ples

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