Carbide Cannon

October 4, 2012

Ms. Wilson’s chemistry class is looking at basic chemical reactions, and today they got to fire an acetylene cannon. When calcium carbide (CaC2) reacts with water (H2O) they produce acetylene (C2H2), which is quite explosive.

CaC2 + 2 H2O → C2H2 + Ca(OH)2

Acetylene is so flammable, because its carbons are held together by a triple bond: when the triple bond breaks it releases a lot of energy (about 839 kJ per mole).

Table 1: Bond strengths of simple hydrocarbons with carbon to carbon bonds

Name Chemical Formula Diagram Carbon to Carbon Bond Strength (kJ/mol)
Acetylene C2H2 839
Ethene C2H4 611
Ethane C2H6 347

The explosion is a result of the combustion of the acetylene:

2C2H2 + 5O2 –> 2H2O + 4CO2

And this whole process — carbide plus water to give acetylene, which is then burned — was used by miners in the early 20th century to make headlamps (among other types of lamps).

A carbide lamp (image via Wikipedia).

The cannon itself is a simple device, made of a 50cm tube of 2-3 inch diameter PVC (sorry about the mixed units), with a screw cap at one end. The carbide grains (about 0.5 g) are placed on the inside of the cap, which is then screwed on to the bottom of the tube. A few drops of water are then added through a small hole in the PVC using a plastic dropper — you can listen for the sizzling to tell if the carbide decomposition reaction is happening. Finally a flame is applied to the same hole as the water. The sock, by the way, is just lightly tucked in near the top of the PVC tube, about 5 cm in.

The explosion was loud, and Ms. Wilson’s sock traveled about 10 meters. It was suitably impressive. I think the student who was the most impressed was the one who had weighed out the calcium carbide, becaues 0.5 grams is really only four or five grains.

Ms. Wilson demonstrates the carbide sock cannon.

Citing this post: Urbano, L., 2012. Carbide Cannon, Retrieved April 21st, 2018, from Montessori Muddle: .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

Phosphorus: What is it good for?

September 13, 2011

So other than digging in Morocco, where do we get more phosphorus? Here’s a hint: the symbol for phosphorus on the periodic table… is “P.”

— Horwich (2011): The end of phosphorus on APM’s Marketplace.

Marketplace’s Jeff Horwich has an excellent article on the uses of the element phosphorus, where it comes from, why it’s getting scarce, and where we might get more.

The answers to these questions are:

  • It’s a key element in DNA, so the major use is fertilizer,
  • most of it comes from Morocco these days,
  • since Morocco supplies about 85% of the world supply, they’re developing a bit of a monopoly and the price is going up,
  • the main alternative sources are manure and urine that have lots of phosphorous. In fact, burning sewage leaves behind a phosphorous rich ash.

Marketplace tells the story in much more detail.

Citing this post: Urbano, L., 2011. Phosphorus: What is it good for?, Retrieved April 21st, 2018, from Montessori Muddle: .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

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