MadSci Network: Chemistry |
What a great question, Sarah. The subject of combustion is quite complex, so, unfortunately, I can't give you a simple answer. When a metal reacts with oxygen, heat is generated and the metal is converted to the metal oxide. The flame temperature is the temperature reached when a substance combusts in air with no heat loss. Not many flame temperatures are found on the Internet, but at the site: http://www.cheater.com/ homework/Homework/Schoolsucks/Uploads/Random/44507017.htm I found the flame temperature of magnesium to be about 2,800 C or 3,100 K, which happens to be an exceptionally high temperature. When a material is heated, it give off radiation; the higher the temperature, the more the radiation. The equation which governs the radiation of black bodies is M=sT^4 where M is the The excitance (M) of the black body, the power emitted per unit area, s is the Stefan-Boltzmann constant and T is the temperature. Thus, we could obtain a twofold increase in radiation by merely increasing the flame temperature from, say 2000 K to 2400 K. So magnesium oxide which results from the combustion of magnesium and oxygen is very, very hot, and therefore, gives off a great deal of radiation. The radiation given off when an object is heated is referred to as "black body" radiation. The equation above must be modified to recognize that most materials aren't perfectly black, so they are considered "grey body" radiators. At these high temperatures, however, the magnesium oxide is giving off a great deal of radiation. The other issue involved with black body radiation has to do with the color of the light emitted. A piece of steel heated to 1,000 C glows a bright red color; almost no blue light is emitted. As the steel melts at about 1,500 C, it gives off a very bright yellow-white light. The magnesium oxide at 2,800 C gives off light which is both much brighter do to the increase in black body temperature, and contains much more blue light than does the steel in my example. At http://cc.oulu.fi/ ~kempmp/colours.html ,Petri Pihko has provided an excellent description of both black body radiation, as well as the type of radiation produced by atomic and molecular emitters. It is interesting to note that barium, calcium, and strontium are all involved in the characteristic emission of specific colors as molecular emitters. When combined with chlorine, they give off green, orange, and red, respectively. Although they all belong to the same periodic group (they are all alkaline earth elements) and react with oxygen, as the molecules become heavier as we progress down the periodic chart, the energy released per unit mass becomes much less, therefore, the flame temperature lower and the light emitted is not so bright. When various oxides are heated to high temperature, the give off quite a bit of light. In your great grandparent's day, oil lamps heated strips of calcium oxide to give off a relatively large about of light. The calcium oxide, often called "lime" lights were popular along the front edge of a stage. So your question involved not only chemistry, but quite a bit of physics also. Keep up the good work.
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