|MadSci Network: Chemistry|
Your question highlights one of the most difficult areas of the physical sciences; why didn't a particular reaction happen the way you expected or predicted. And often times the answer is simply "I don't know" or "I don't have enough information about how your experiment was conducted to understand either why your experiment didn't produce the expected results, or why you didn't observe the results correctly." An amazing number of inventions and discoveries are made by accident. Researchers investigating why a compressed cylinder of Freon was unexpectedly empty opened up the cylinder and found a fine powder inside. That fine powder turned out to be Teflon. Catalyzed polyester resin spilled on glass cloth Friday afternoon and not cleaned up until Monday was the basis for a class of composite materials simply referred to as "fiberglass." An adhesive that wasn't as sticky as desired became the basis for "Post-it Notes." So, in concluding that iron is more reactive than zinc, the questions remain "how did you conduct the experiment, what did you observe, what are your conclusions, and how did you reach those conclusions?" As far as the relative reactivity of zinc versus iron powder is concerned, one consideration is the heat of combustion. The following reference provides a short list of the amount of heat produced when various materials combine with oxygen: http://www.chemeng.ed.ac.uk/~emju49/SP2001/webpage/deh/deh2.html According to that reference, zinc produces 700 KJ/mole oxygen, while iron produces 530 KJ/mole oxygen. So, zinc produces a significant amount of energy when burned in air, and according to this information, appears somewhat more energetic than iron. A number of metal powders are used in pyrotechnics (fireworks). For example at: http://www.pyrouniverse.com/chem.htm we find that iron gives yellow, branching sparks, and zinc gives white sparks. Zinc will also directly combine with sulfur; zinc/sulfur is sometimes used as a propellant for amateur rockets. Another link: http://www.pyrouniverse.com/colors.htm explains some of the causes of color in fireworks. They indicate that aluminum and magnesium give white sparks because of the high amounts of energy released during combustion, compared with iron producing more orange sparks because of its lower energy. Therefore, based upon the color of sparks produced by different metal powders, zinc appears to produce higher reaction temperatures than iron. A company that sells zinc powder, JT Baker, provides some informative information in their material safety data sheet: http://www.jtbaker.com/msds/englishhtml/Z0858.htm Their MSDS sheet states: "Autoignition temperature: ca. 460C (ca. 860F) The listed autoignition temperature is for Zinc powder (layer); dust cloud is ca. 680C (1255F). Zinc powder is not pyrophoric but will burn in air at elevated temperatures. Bulk dust in damp state may heat spontaneously and ignite on exposure to air." Their MSDS sheet for iron can be found at: http://www.jtbaker.com/msds/englishhtml/I7495.htm The MSDS sheet for iron states: "Moderate fire hazard in form of dust when exposed to heat or flame. Can react with water to liberate flammable hydrogen gas. Minimum ignition temperature, iron dust cloud: 430C (805F). Ultrafine iron powder (ca. 5 microns) is pyrophoric and can ignite spontaneously in air." So, according to this information, the temperature for obtaining dust explosion is less for iron than for zinc, although it appears that you can have a dust explosion with either one. Although the heat of combustion indicates the amount of energy released by a particular reaction, the ease by which a reaction takes place is determined by the mechanism by which the reaction takes place. Some reactions are very energetic, for example the reaction between hydrogen and oxygen. However, this reaction doesn't take place very easily; the reaction has a high activation energy. An MSDS sheet for hydrogen can be found at: http://www.boc.com/gases/pdf/msds/G004.pdf The autoignition temperature for hydrogen is listed as 570C, higher that either iron or zinc. So, the reaction between oxygen and zinc appears to generate a significant amount of heat, there may be other things taking place. The vapor pressure of zinc is quite high compared with other materials, as shown at: http://heliumclub.usc.edu/Refs/Vapor.htm At 487 C, zinc has a vapor pressure of one torr; iron has to reach 1857 C to have the same vapor pressure. Zinc has a low melting point of 419 C. I suspect that during combustion, zinc will tend to melt and vaporize prior to reacting with oxygen in the air, adsorbing some heat in the process. Because of the low melting point, the zinc will tend to melt and agglomerate when heated in air, losing a lot of surface area, and thus appearing less reactive than it actually is. The surface of the solid iron particles, on the other hand may react directly with oxygen, maintaining a large amount of surface area and therefore appearing more reactive. The bottom line is that either iron or zinc are able to produce violent reactions under the correct (or possibly incorrect) conditions. The MSDS sheets for both metals list conditions to AVOID. To me, that sounds like good advise.
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