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Stoichiometry and Stoichiometry Calculator


AirsoftSmokeScreen

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I've searched the forum but didn't found threads about this and maybe it will help others too.

So, I understand that Pyro Stoichiometry Calculators show you the exact amount of oxidizer and the fuel for the reaction to be complete and without residues.

But... the online calculators are limited, I mean you can only choose one type of oxidizer and one type of fuel.

As in pyro we almost always mix 3 or more chemicals how do we calculate the exact stoichiometric ratios where there are involved more that 2 reactants?

As an example:

The stoichiometric mass ration between KNO3 and Aluminum is:

 

 

Stoichiometric Mass Ratio

 

2.248 units of Potassium Nitrate to 1 unit of Aluminium.

 

0.445 units of Aluminium to 1 unit of Potassium Nitrate.

 

30.8% Aluminium. 69.2% Potassium Nitrate.

 

And between KNO3 and sulfur:

 

 

Stoichiometric Mass Ratio

 

2.523 units of Potassium Nitrate to 1 unit of Sulphur.

 

0.396 units of Sulphur to 1 unit of Potassium Nitrate.

 

28.4% Sulphur. 71.6% Potassium Nitrate.

 

 

 

Ok, but if we are to mix these three ingredients ( KNO3, Al and S) how do we calculate the exact Mass ratios?

 

I hope you understand what I'm talking here :)

Sorry for my bad English

 

Thanks

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So, I understand that Pyro Stoichiometry Calculators show you the exact amount of oxidizer and the fuel for the reaction to be complete and without residues.

 

We've got to iron out this word "exact" because it's not. I'd also describe it as hoping to control the residues. There are always end products, of various forms. I think you really just mean to say "burn cleanly"

 

It gets rather complicated. For example, you often get partial decomposition of Potassium nitrate in different situations, often with the oxide or hydroxide being produced, keeping some of the oxygen. You even get some Carbonate and Sulfate in small quantities in many compositions (carbonate and hydroxide being exempt from KNO3/S/Al of course.) Also, Sulfur can form sulfide with the Potassium, liberating that extra oxygen. It can also form Aluminium sulfide. Have you seen Sulfur/metal flash powder? The classic Zinc/Sulfur reaction can be quick, but other mixes can be outright devastating, like more traditional flash.

 

Even with two part formulas it can thus be hard to predict, as there is often a combination of reactions going off in different proportions with different end products, simultaneously. You can get partially oxidised fuel and partially reduced oxidizers remaining in small quantities despite the fact that according to simple theoretical reactions there shouldn't.

 

Stoichiometry is really a guide which can help you predict the right spot in pyrotechnics, and while I use it all the time for making formulas, its really just that, a guide. It's useful to know what the likely major reactios are going to be by calculating theoretical reactions, for example, knowing what the theoretical mixtures with organic components to know what would in theory give you all CO2 or CO, and you can decide on a mixture of the two gasses according to the use. Usually I aim for mostly monoxide, with enough theoretical Carbon dioxide to buffer you from left over carbon remaining to dirty the flame, if it's a colour.

 

I'd say the closest thing to what you are asking are the rocket fuel simulators that are out there (but not all free. You can tap in any number of chemicals (as long as they are on file, other widse you need to get the thermodynamic data and atomic weights and so on and add it yourself.

 

In those, you write in the percentages, and it gives you a simulation of the results. While it's not quite going to match reality, the really long list of end products demonstrates how complicated how complex it is. It's just a guideline too, but easier than pen and paper and more complete. You just alter your formula until you're getting a spread of products that look like what you're after, and you have some confidence that you have a good head start for when you go out to trial it.

Edited by Seymour
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I agree with seymour.

 

 

For many purpose the stoichiometry is not that important. Glitters for example do not run with stoichiometric mixtures, they´re made to give a reactive residue. Another important fact is, that most pyrotechnic reactions are solid/solid reactions. That means the particle size should be as fine as possible for good results. Big particles leave a greater residue than small particles, because the reaction does not go the way you expect in the theoretical calculation.

 

 

Another good example is KClO4/ Al Flashpowder. The stoichiometric formula would be around 66% KClO4 and 34% Al. The loudest formula based on this two components is (referred to shimizu) around 72% KClO4 and 28% Al. It´s because the reaction gives two mole oxygen:

 

 

8 KClO4 + 16 Al -> 8 KCl + 8 Al2O3 + 4 O2

 

 

That would be about 1/4 mole oxygen per 100g flashpwder. 1/4 mole are aproximately 6 L oxygen at room temperature. There results a (additional to normal flash) pressure of 60 bar in a salutshell with 100g flash and a volume of 0.1 L. If you scale up to say 1000K reaction temperature the pressure increases to a much higher level.

 

 

But to get back to your question:

 

You got two stoichiometric formulas and want them to mix up and still be stoichiometric. Its like you got a batch of both and mix them. The ratio of chemicals do not change by this operation, so every mix you make of this two batches is still stoichiometric. For example you mix 100g Al/KNO3 and 100g S/KNO3 you got

 

140.8 g KNO3

30.8 g Al

28.4 g S

 

Sum: 200g

 

Or put in percentage: 140.8 / 200 = 70.4% and so on.

 

 

But you can just use this way, if your reaction do not change.

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