* Energetic Light *Energetic Light *Why Do Things Why Do Things ““GlowGlow””??

Bioluminescence: The production of light by living organisms. Bioluminescence may result from the absorption of light (fluorescence or phosphorescence, e.g. in many deep-sea fish or in fireflies.)

Chemiluminescence: A type of luminescence in which the electrons are excited by a chemical reaction (e.g., the reaction of luminol, glow-sticks, or a forensic crime scene test)

How Do Electrons “Get Excited?”

• Orbital = area of high probability of finding an electron orbiting around the nucleus of an atom• Ground State = Lowest energy level for a stable atom that electrons reside (n=0)• Excited State = Higher energy level for an atom, when the electron has absorbed energy, and

has moved further away from the nucleus, but is still attracted to and “bound” by the nucleus of the atom (n=1,2,3…)

• Excitation occurs when: 1. an electron absorbs energy (e.g. from a photon of light) or 2. collision with a nearby atom or particle

As electrons move from a higher energy level to a lower energy level, they release the excess

energy in the form of a photon of light.

The color of visible light emitted depends on the amount (quanta) of energy emitted by the

electron as it moves from a higher energy level to a lower energy level.

Electromagnetic Spectrum

• Electrons do not stay in excited states for very long, and as soon as electrons return to their ground states, they emit a photon with the same energy as the one that was absorbed.

The energy of light determines its wavelength and visible color

Energy Wavelength ColorLow 680 nm red

600 nm orange550 nm yellow500 nm green425 nm blue

High 400 nm indigo

The energy of one quantum (photon) of light is inversely proportional to its wavelength, where E is the energy of one quantum of light, h is Planck's constant, λ is wavelength, and c is the speed of light.

E = hc / λ

Flame Tests

• When metal ions are heated over a flame, they emit light based on each element’s emission spectrum.

The Reaction of Luminol

• The luminol reaction is an oxidation-reduction reaction in which a photon of light is released from an excited molecule.

• Luminol is oxidized by the hydrogen peroxide, the potassium ferrocyanide solution is reduced, and aminophthalic acid, nitrogen gas, water and light are produced.

• The electrons of luminol are elevated to an excited state by the oxidation-reduction reaction.

• When the electrons return to the ground state, visible light is emitted.

• The light appears to glow blue, because the energy emitted is very high, which means the wavelength is short, and the frequency is high.

Luminol reaction – (Long version)

• In a basic (alkaline) solution, luminol exists in equilibrium with its anion, which bears a charge of -2. The anion can exist in two forms (or tautomers), with the two negative charges delocalised on either the oxygens (the enol-form) or on the nitrogens (the ketol-form).

• Molecular oxygen (O2) combines with the enol-form of the luminol anion, oxidizing it to a cyclic peroxide. The required oxygen is produced in a oxidation-reduction reaction involving hydrogen peroxide (H2O2), potassium hydroxide and potassium hexacyanoferrate(III) (K3[Fe(CN)6], potassium ferricyanide.

• The hexacyanoferrate(III) ion ([Fe(CN)6]3-) is reduced to the hexacyanoferrate(II) ion ([Fe(CN)6]4-, giving potassium ferrocyanide, K4[Fe(CN)6]), while the two oxygen atoms from the hydrogen peroxide are oxidized from oxidation state -1 to 0:

• The cyclic peroxide then decomposes to give 3-aminophthalate (3-amino-1,2-benzenedicarboxylic acid) in an excited state, along with a molecule of nitrogen (N2) – see. This decomposition reaction is favored because the cyclic peroxide molecule is highly unstable, and the reaction involves breaking some weak bonds. It is also favored because of the increase in entropy (disorder) due to the liberation of a gas molecule. When the excited 3-aminophthalate drops down to the ground state, a photon of blue light is released.

Citations

Carolina Biological Supply Company – Teacher Resourceshttp://www.carolina.com/category/teacher%20resovities/luminol%20the%20glowing%20reaction.do

NASA Goddard Space Center – Teacher Corner

http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/background-atoms.html

ACS – Journal of Chemical Education Digital On-Line Video Library

http://www.jce.divched.org/JCESoft/CCA/CCA3/MAIN/ILUMIN/PAGE1.HTM

Purdue University – Chemistry Education

http://chemed.chem.purdue.edu/demos/main_pages/5.9.html