Fireworks
From Chempedia
Contents |
Fireworks
Introduction
Millions of people watch and use fireworks throughout the year. However, very few people understand the physical and chemical properties that must exist for such a complex combustion reaction to occur. The first type of fireworks was firecrackers; they were developed approximately two thousands years ago by the Han Dynasty to ward off evil spirits. It wasn’t until five hundred years later that modern day fireworks were developed. Today, due to chemical advances, we can control many aspects of fireworks, including the color and shape.
Chemistry
One type of everyday firework is sparklers; they consist of six main components: fuel, oxidizing agent, reducing agent, regulator, binder, and the coloring agent. The fuel used in fireworks is black powder; it is a blend of potassium nitrate, charcoal, and sulfur in a ratio of 75:15:1. The oxidizing and reducing agents are the two components of a sparkler that work hand-in-hand; the oxidizing agent produces oxygen needed for the mixture inside the firework to burn—nitrates and chlorates are often used.
XNO3 → XNO2 + ½O2
This chemical equation represents what happens to the nitrate. The reducing agent burns the oxygen provided by the oxidizing agent to produce hot gases, which include sulfur and charcoal. The chemical equations that best represent the gas production are:
S + O2 → SO2, and C + O2 → CO2
In order to better control the reaction rates, pyrotechnicians often use a regulating metal. The idea is that the greater the surface area of the metal present, the faster the reaction will occur (Collision Theory). Binders are used to hold the mixture of chemicals and black powder together; a common binder is dextrin, a type of starch. The last component added is the coloring agent, which is responsible for the bright visual effects.
Figure 1
Aerial fireworks also have six main components (figure 1), but only five pertain to the actual firework, while one is just for the launch. The five parts are the following: black powder, stars (specific elements for color), the main fuse, time-delay fuse, and the container in which all the previous parts are put in or attached to. The sixth part, not attached to the main firework is the launch tube in which the firework is placed in.
Structure
For a simple firework, black powder is placed in the center of the firework, whether it is spherical or cylindrical. The stars are placed around the powder which will ignite and blow the (burning) stars outward in a specific pattern/shape. There is a main fuse and time-delay fuse that are lit at the same time. The main fuse goes to the bottom where black powder is located, on the outside of the shell; when black powder is ignited in a small closed container, the resultant heat and gas will push vigorously until there is an explosion to launch the tube. The time-delay fuse does just what it sounds like it does, it burns slowly (by coarser grained black power) until the firework approximately reaches apogee at which time the black power (fine-grained) is ignited and bursts the stars outward for the magnificent display of the firework.
Colors and Shapes
Fireworks explode in different colors due to the burning of metal and salt compounds caused by the excitation of electrons. When metal and salt compounds are heated, their energy increases causing the electrons to move to a higher orbital. However, electrons cannot exist in this excited state for long periods of time, so they must fall back to their normal energy state. When this happens they release or emit photons of light. We see different colors of light due to the fact that different photons have different wavelengths of light. The shorter the wavelength the more purple in color, the longer the wavelength the more red in color. The light that is given off by fireworks falls into two different categories, incandescent and luminescent light. Incandescent light is light produced from heat, or reactive metals. These metals reach such high temperatures that they give off bursts of bright light to release the extra energy. Luminescent light is light produced by other sources which can occur at low and high temperatures. Luminescent light is affiliated with the electrons moving from the excited states back to their lower energy levels. Each color of light that you see in the sky as a firework explodes has to do with a specific element. An element releases a photon with a specific wavelength (of light) that is compacted into a dough-like feature, and then put in various amounts into a shell. The elements used in today’s fireworks with respect to color can be found in table 1 below.
| Color | Compound | Wavelength (nm) |
| Red | Strontium salts, Lithium salts, Lithium carbonate,Li2CO3 = red, Strontium carbonate, SrCO3 = bright red | 652 |
| Orange | Calcium salts, Calcium chloride, CaCl2 | 668 |
| Yellow | Sodium salts, Sodium chloride, NaCl | 610-621 |
| Green | Barium compounds + Chlorine producer, Barium chloride, BaCl2 | 589 |
| Blue | Copper compounds + Chlorine producer, Copper(I)chloride, CuCl | 505-535 |
| Purple | Mixture of Strontium (red) and Copper (blue) compounds | 420-460 |
| Silver | Burning Aluminum, Titanium, or Magnesium |
The shapes of fireworks are obtained in two ways: by placing the salts (colored stars) in a specific pattern within the firework shell or by the reaction time of the chemical components. For example, slow and controlled reactions produce fountains, and shapes such as palm trees and the raining image. Listed below is a table of the most common shapes that we see in fireworks. Table 2 states how this shape is made and what is required inside of the shell to make it happen.
| Palm | Contains large comets or charges in the shape of a solid cylinder that travel outward, explode and then curve downward like the limbs of a palm tree. |
| Round Shell | Explodes in a spherical shape, usually of colored stars. |
| Ring Shell | Explodes to produce a symmetrical ring of stars |
| Willow | Contains stars (high charcoal composition makes them long-burning) that fall in the shape of willow branches and may even stay visible until they hit the ground. |
| Roundel | Bursts into a circle of maroon shells that explode in sequence |
| Chrysanthemum | Bursts into a spherical pattern of stars that leave a visible trail, with an effect somewhat suggestive of the flower |
| Pistil | Like a chrysanthemum shell, but has a core that is a different color from the outer stars |
| Maroon Shell | Makes a loud bang |
| Serpentine | Bursts to send small tubes of incendiaries skittering outward in random paths, which may culminate in exploding stars |
Conclusion
As one can see there are a lot of chemical properties that go into creating fireworks and giving them their distinct shapes and colors. Without simple chemical reactions such as a redox reaction and combustion reactions, the mechanics of the firework could not function.
Works Cited
D. Linn Coursen, "Pyrotechnics," in AccessScience@McGraw-Hill, http://www.accessscience.com/, DOI 10.1036/1097-8542.560200, last modified: July 12th, 2002. Accessed on September 23rd 2005 (includes figure 1)
Brain, Marshall. "How Fireworks Work," in How Stuff Works. [http://people.howstuffworks.com/fireworks.htm last modified 2005 Accessed on September 24th 2005 (includes Table 2)
Fireworks by Grucci, "Anatomy of a firework" in NOVA online. [http://www.pbs.org/wgbh/nova/fireworks/anatomy.html last modified January 2002. Accessed September 24th 2005
Table 1 found at: http://scifun.chem.wisc.edu/CHEMWEEK/fireworks/fireworks.htm
Authors
Researched and written by: Josh Miller, Cassandra O'Neal, Steve Pinta, Stephanie Puetz
