Posted on February 8, 2012 by Mr. Bigler
Today I did one of my favorite demos for my classes. The demo is convincing visual evidence that forming intermolecular bonds releases energy.
When I explain to my students that breaking bonds requires energy, most of them believe it. Evidence might include the fact that you have to heat ice to melt it, and you have to heat water to boil it. However, many students have trouble grasping the concept that, because law of conservation of energy must hold, this means that when water freezes, the process must give off the same amount of heat as it took to melt the ice. My students accept this on the surface, but most of them have trouble really believing that ice gives off heat when it freezes.
The demo gives very convincing evidence that freezing a liquid gives off a substantial amount of energy. I learned about it at ChemEd 2009 from Liz Velikonja, a chemistry teacher from Brooklyn, NY. It starts with melting and beginning to boil paraffin (wax) in a test tube clamped to a ring stand. I used a Bunsen burner:
After this step, the test tube contains liquid paraffin, and there is a substantial amount of flammable paraffin vapor outside the mouth of the test tube.
At this point:
- I put a beaker of ice water over the end of the test tube, which causes the liquid paraffin in the test tube to solidify.
- Forming the intermolecular bonds of the solid releases a large amount of thermal energy.
- Because the heat is produced in the test tube, the only direction it can go is out the mouth of the test tube.
Heat Escaping from the Test Tube
When the heat reaches the flammable paraffin vapor outside the mouth of the test tube, it ignites spectacularly:
Note the shower of condensed paraffin raining down from the fireball.
After the paraffin has burned, condensed paraffin vapor lingers in the air, and the lab bench is covered in a layer of wax:
(Thanks to Dina Robles, one of my current students and an LEHS yearbook photographer, for taking the pictures!)
