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Black powder is more of a reaction than a burn of fuel and oxidizer.

I did some experimenting with charcoal and saltpetre. While making a charcoal fire i threw in some saltpetre, usually saltpeter alone burns slowly with a molten residue left over, but when i added it to the fire it created a slow burning but intense and bright light that almost melted the steel basin the fire was in. So I did some investigating, firstly I took some 3mm charcaol chips and lit one, then I threw some saltpetre on, again a intense, bright flame appeared, while observing i found that the saltpetre creates a molten pool in which the charcoal 'dissolves', I tried this with sugar, to simulate a smoke mix, but nothing close happened. So it turns out that saltpetre burns sugar when mixed with it, but actually chemically 'reacts' with charcoal. I have a question to add to this, is there any other item in the world that reacts this vigourisly with saltpetre? And if you have some charcoal and saltptre to spare can you pls try this, its really amazing.

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mitchells (author) 6 years ago
http://www.youtube.com/watch?v=IO-qThQdUok My video of the intense thermal energy that this reaction can put out.
mitchells (author) 7 years ago
Thanks for the comments!
I do understand chemical equations and balancing, but what confuses me is when sulfur is added, How does the formula change? And how can zinc sulfur be a rocket fuel? I know that zinc is a low energy fuel, ive tried magnesium sulfur and it became a flash powder, but sulfur doesnt have any oxygen to feed the reaction, when this fuel burns in a motor it pushes gases out so that no oxygen can get in, so where does the oxygen come from?! Unless this is some whack elemental binding reaction, that exerts heat, maybe like the release of energy when atoms are split, instead when atoms join, possibility?
As for where the energy comes from, as always it comes from different amounts of energy to maintain a chemical bond. 

Basically, it takes energy to break the bonds in KNO3.  This is the activation energy and is the reason it takes heat to ignite something flamable.  What happens when this occurs is the Potassium, Oxygen, and Nitrogen atoms bond to whatever it can and they prefer to do so with whatever takes the least amount of energy to form a stable bond. 

After everything has bonded you have excess energy because all the new bonds took less energy then the old bonds leaving excess.  This is the heat you see after the reaction.
The reaction that you were running didn't contain any sulfur unless your charcoal had been contaminated. The equation with the sulfur was just easier to cope with. As for where the oxygen comes from, it all comes from the Salt Peter. Instead of reacting with Molecular Oxygen(O2) from the atmosphere, in this reaction the Salt Peter donates its Oxygen atoms to react. This is possible because Salt Peter has a loose bond with its oxygen and in the presence of the right elements is gladly breaks down to react. The sulfur is only used in Black Powder because it reacts with the Salt Peter at a lower temperature and releases heat. This allows black powder to combust at lower temperatures.

I don't know where you came to Zinc Sulfur as a rocket fuel from the comments but in a Zinc-Sulfur reaction you have
Zn(s) + S(s) --> ZnS(s)
In this reaction, the sulfur is being used as an Oxidizer(definition 2) since it's Valence is 6 and Zinc has a Valence of 2.  What this means is Sulfur will take 2 electrons to have a total of 8 in its outer shell(making it an oxidizer) while Zinc will give up 2 to have 0 in its outer shell.  This sort of thing means that those elements will bond readily and be stable.  I hope this helps.

First off, burning is a chemical reaction.  Saltpeter is just providing a solid form of reactable oxygen.  As you see below(if you are familiar with this type of notation) the Saltpeter is being stripped of its oxygen to react with the carbon. At the same time the Potassium is reacting with the sulfur to produce more energy.  This energy is actually used to force the Oxygen reaction reducing the reaction temperature.  Molecular Nitrogen is a byproduct.
2 KNO3(s) + 3 C(s) + S(s) -----> N2(g) + 3 CO2(g) + K2S(s)
What you are seeing is this reaction with a significant reduction in surface area for the reaction.  In gunpowder these chemicals are ground very fine and mixed together.  This means that all the fuel is in contact with oxidizer.  When the flash point is reached all the fuel reacts at once.  In your reaction the surface of your charcoal has to be burned to get to the next layer so all that explosive energy is released in a slower reaction even though this reaction releases more energy overall than normal burning.

Note that Mitchells did not include any sulfur in his experimentation, just potassium nitrate and carbon (plus whatever big hydrocarbons were left in the charcoal).  That means that the reaction is slightly different than for proper blac powder.
True but the only formula that I could find for Sulfur-less powder was horribly unbalanced.  I had to balance this one but it was more reasonable so I went with it.  I didn't worry about the difference because the example is essentially the same, you are just missing the sulfur used to lower the initiating temperature.
Ah, that makes sense; thanks!

I tried to work it out myself, but didn't have the patience to look up all the subreaction energies and barriers.  I'm a physicist, not a chemist!

Once concern I have is whether the sulfur is essential to binding up all the potassium.  Without it, I wonder whether you could end up with different reaction products, such as, oh, potassium cyanide...
This is the horribly unbalanced equation that I found originally:
4 KNO3 + C7H8O → 3 K2CO3 + 4 CO2 + 2 H2O + 3 N2

I believe the
C7H8O is the chemical make-up of charcoal.  I am trying to find an exact reference to this but right now it is just an assumption based on context. 

As you see, I didn't use this as it is terribly unbalanced and I could see some of those numbers ending up in the double digits and I just didn't want to do it.  The K2CO3 is Potassium Carbonate and is slightly alkaline but is harmless.  They use it in soap.

Well, proper wood charcoal is mostly carbon, because the volatiles are driven off by the pyrolysis.  "Charcoal" briquettes have all kinds of nasty chemicals in them as binders, lighting agents and whatnot.  According to Wikipedia, C7H8O is probably one of the cresols, an aromatic hydrocarbon which looks like phenol with a methyl group.

If we assume that the reactants and products are correct, then working out the stoichiometry is just linear algebra :-)  I started doing it by hand, but it got out of hand pretty quickly.  I just found an online balancer which gave me a good answer:  34 KNO3 + 5 C7H8O = 17 K2CO3 + 18 CO2 + 20 H2O + 17 N2

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