On Tossing Cigarettes at Gasoline for #ChemMovieCarnival

Direct Link to the Twitter Feed - #RealTimeChemFor those of you that don’t know, next week is #RealTimeChem week.  It’s kind of a big deal for chemists.  It’s a time for all of the chemists across the globe to sit down, show each other what we’re doing (within reason), and share our passion.  Hopefully, even folks who aren’t scientists take notice of the tag and get to see some cool things.  Now, I don’t get to do much chemistry outside of my kitchen (look – the Maillard Reaction is a very complex piece of business…), but I do love to share my passion for chemistry.

To kick off the event, blogger See Arr Oh (obviously not their real name) called for chemistry bloggers to take their favorite chemistry moments from movies and write up a post.  Originally, I called dibs on cold-fusion and the movie The Saint.  Then, I started looking into the subject and immediately realized I was in WAY over my head.  Can’t let my ignorance show, now can I?

I kept thinking and thinking.  I only watch movies with big explosions and guns (or romantic comedies).  So I was watching every movie in my collection and eventually got to Zoolander.

In this movie there is a scene where the young model is hanging with his model buddies and things get out of hand at the gas station.  Brint sparks his zippo and, since everything is covered in gasoline, everything goes up in flames.  This actually happens (not the gasoline fights, the explosions).

So, I thought: cigarettes and gasoline.  That’s a great combination if I ever heard of one.

Before I get too much further, I feel it necessary to say this.  I am a scientist.  I am highly trained: I studied explosives and combustion in school and earned my degree in Engineering.  I am certified in fire safety, chemical hazards safety, explosives safety.  Do not, do not, do not attempt to repeat any of my experiments.  I am literally throwing fire at things that catch fire.  Don’t do it.

When we ignite gasoline, the reaction is about as follows:

5075 kJ per mole (a standard unit in chemistry, not an animal) is a good deal of heat.  To put that in perspective, just 1 mole of gasoline (about 1/3 cup or 85 mL or 115 g) is roughly equivalent to 5100 matches being lit at the same time (1 match gives off ~1kJ of heat).  Now, all reactions require some amount of start-up energy.  Even adding vinegar to baking soda requires a little (the heat released from dissolving baking soda into the water in the vinegar and then the mixing is enough).  How much energy does the combustion of gasoline require?  Not much – just a slight spark or an open flame will cause the gasoline to ignite in open air.

It is important, however, to note that liquid gasoline is not flammable while gaseous gasoline is.  As gasoline evaporates and mixes with the oxygen in the air, even the slightest spark will ignite the mixture provided the fuel-to-air-ratio is within proper limits.  For petrol, the lower limit of fuel-in-air (by volume) is about 1.4% and the upper limit is about 6.9%.  This means, relatively, that you don’t need much, but the window is VERY small.  To give an example, Hydrogen is explosive from 4% to 76%!  The other piece of important information is the auto ignition temperature.  For gasoline, the ignition temperature is about 540K (about 265°C – you could likely ignite it in your oven).

With all of this information, it would seem that a puddle of gasoline on the ground would be set up to drop a cigarette and cause a cool movie explosion.  But, we haven’t even talked about the cigarette yet.  How hot is the cherry on a cigarette?

burn profile

This is a burn profile for a ‘standard’ cigarette being smoked via square-wave profile by a machine.
Source

According to this, the cigarette can reach 900+°C on the sides, but quickly (4 seconds) returns to a really low temperature smoulder.  So you’d think that even the coldest region – at 350°C, would be able to ignite gasoline!  Nope.

Here are the details of the unpictured experiment.  I am not a ‘smoker’ despite living in the heart of tobacco country (tobacco is grown in all 120 KY counties), but…science.  So I bought a pack of cigarettes and set about taking a drag and tossing the cigarette at a puddle of gasoline.  I tried different “puddle media,” e.g. asphalt, concrete, grass, dirt, a pie pan.  I tried more gasoline, less gasoline.  Nothing would make these puddles of gasoline light!  Because these pictures were lame, I chose not to use them.  Sorry.

So why will cigarettes not ignite gasoline?

Although petrol vapour will auto-ignite (burst into flame without a pilot flame or spark) between 480 and 550 °C, experiments investigating hot surface ignition revealed that the temperature of a hot surface needed to reach 980–1130 °C before ignition of petrol vapour occurred. Indeed, contrary to what might be expected, it is not simply the temperature of the surface that is the most crucial factor in determining whether ignition might take place; one of the most important factors to consider is the energy transfer between the hot surface and the petrol vapour, and in this respect the residence time and surface area play equally important roles.  Source

Turns out that the cigarette just simply isn’t hot enough and the ashes get in the way.  But I think the best part of this paper is that not only did they attempt to use cigarettes, but cannabis, too.

In her hand?  A joint.  In the plastic tub?  Cigarettes and gasoline.

In her hand? A joint. In the plastic tub? Cigarettes and gasoline.  This paper was awesome.

One last thing: I’ve started a new job (yay!) so I have been super swamped. I’ll also be pushing out SEVERAL new posts over the next week for #RealTimeChem week. AND I should be a featured post for an established joint (details later). Sorry I’ve made everyone wait so long! I’ll get better! Promise.

-N.Tesla

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