This article offers a way to motivate students to understand chemical reactions. Even if they are not keen to know why a glow stick glows in the dark, they will surely be eager to find out how fireflies or jellyfish produce light, or to discover how blood is detected at crime scenes. The article can serve either as an introduction to chemical reactions or to give attractive examples of redox reactions and also to illustrate the levels of energy in the shell of an atom.
Luminescence: The production of light, usually at low temperatures, for example by chemical reactions or electrical energy. Incandescence, in contrast, is light generated by high temperatures.
The best example of chemi-luminescence may be the chemical reaction used to make glow sticks up, which is the result of cracking a small vial of one chemical into a chemical it will react with. Chemi-luminescence is also found in some fungi and earth worms. It is most common in the oceans, where many organisms from fish to worms living at great depths, have glowing organs. Chemists have exploited these -emitting reactions as markers in a large number of laboratory and clinical tests.
Chemiluminescence: A type of luminescence in which the electrons are excited by a chemical reaction, for example the reaction of luminol described in .
When you snap a glow stick and it begins to glow, the light produced is an example of chemiluminescence (see ). Glow sticks comprise a plastic tube containing a mixture including diphenyl oxalate and a dye (which gives the glow stick its colour). Inside the plastic tube is a smaller glass tube containing hydrogen peroxide. When the outer plastic tube is bent, the inner glass tube snaps, releasing the hydrogen peroxide and starting a chemical reaction that produces light (see ). The colour of light that a glow stick produces is determined by the dye used (see ).
It involves the fuel (TCPO) plus the oxidant (H2O2)reacting to produce a proposed intermediate, in this example shown as adioxetane; although, this reaction probably produces many intermediates,and others, such as hydroperoxyoxalate, have been proposed (Milofsky andBirks, 1991; Choksi et al., 1990).
One of the suggested reaction sequences in the reaction of peroxyoxalates,of which bis(2,4,6-trichlorophenyl)oxlate (TCPO) is the most prominentexample, follows.
One of the drawbacks of using luminol is that the reaction can be catalysed by other chemicals that may be present at the crime scene, for example, copper-containing alloys, some cleaning fluids such as bleach, and even horseradish. Clever criminals can clean up the blood with bleach, which destroys the evidence of the blood, but bleaching the carpet may alert people to the crime sooner. Urine also contains small amounts of blood, which can be enough to catalyse the reaction of luminol. Once luminol has been applied to the area, it may prevent other tests from being performed there. However, despite these drawbacks, luminol is still used by forensic scientists as a tool to solve crime.