Sunday, December 5, 2010

Understanding Fireworks Through Thermodynamics and Chemistry

For centuries, fireworks have produced intense feelings of excitement, joy, and wonder. But what chemical and thermodynamic properties actually describe the sites and sounds of this process? In this post I will describe the basic chemical processes occurring within the combustion of fireworks as well as the ensuing visual displays.

First, in order to understand the underlying principles, you must understand the basic structure of a firework. Located within the firework are 4 primary components: an oxidizing agent, a reducing agent, a coloring agent, and a binding agent. The oxidizing agent supplies the oxygen necessary for combustion, the reducing agent burns the oxygen released by the oxidizer, the coloring agent absorbs the energy and reradiates photons of a specific wavelength, and the binders maintain the intermolecular structure and organization of these agents.

What processes are responsible for the firework’s explosive power? The answer lies within the chemistry of the materials. In order for combustion to occur, there must be oxygen. Oxygen is supplied to the firework by way of an oxidizer that may come in the form of a nitrate, chlorate, or perchlorate. Potassium nitrate (KNO3), the active oxidizer found in black powder, is able to supply two of its three oxygen atoms per molecule as defined by the balanced oxidation equation for potassium nitrate (1). Potassium perchlorate (KClO4) on the other hand, is able to supply all 4 oxygen atoms because the chlorine atom can bond an additional oxygen atom, thereby making it a superior oxidizer (2).

(1): 4 KNO3 → 2 K2O + 2 N2 + 5 O2

(2): 4 KClO4 → 4 KCl + 8 O2

Next, the reducing agents rapidly bond with the free oxygen molecules to create stable compounds. These stable compounds are found after combustion has occurred and act as a benchmark for understanding the total release of energy. All molecular bonds have a quantity of energy contained within them, known as their bond energy. When a chemical reaction occurs that results in molecules with a different bond energies being created, the difference in bond energies describes the amount of total energy released. Therefore, it is this exothermic release of energy that we recognize as the firework’s explosion. This process of oxidation and reduction can be demonstrated by the balanced chemical equation for the combustion of black power:

2 KNO3 + S + 3 C → K2S + N2 + 3 CO2.

How then, can different colors be created through processes like these? Once again the answer lies within the type of materials used. All materials absorb radiation and then reradiate in back out to achieve an equilibrium. When we imagine the atomic structure of an atom, it contains protons, neutrons, and electrons. The electrons circle around the nucleus (composed of the neutrons and protons) at different ranges known as orbitals.

When an atom is stable, its electrons circle around the nucleus in their “ground state.” This means that the electrons flow around the nucleus in the lowest possible stable orbital associated with that particular atom. If however, the atom is unstable, then it is in an “excited state” and the electrons circle around the nucleus at higher orbitals. The emission of light occurs when an atom’s electrons decrease orbital levels from an excited state to a less energetic or ground state.

When different elements decrease orbital states, they release different wavelengths of light, thus displaying the ROYGBV spectrum of color. Red hues for example, which have a wavelength of approximately 652nm, are produced by the decrease of electron states of strontium or lithium compounds, whereas sodium compounds, which have a wavelength of approximately 615nm, produce yellows, and coppers compounds produce blues.

The molecular and thermodynmic properties of fireworks is extremely complex and influenced by numerous contextual factors. The oxidation and subsequent reduction of the right chemicals can result in not only large concussive explosions, but also dazzling displays of light. By manipulating these fundamental principles of material science you can create a truly awe-inspiring chemical performance.

For continued inquiry please explore, “The Chemistry of Fireworks” by Michael S. Russel, found online at:

http://books.google.com/books?id=yxRyOf8jFeQC&lpg=PA27&ots=COHVulrQ-3&dq=black%20powder%20%2B%20atmospheric%20chemistry&pg=PA26#v=onepage&q&f=false

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