Re: Why does iron give bright orange sparks when burnt on a flame yet

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.