flash powder

[salter]

I'm currently doing a project for my calculus class involving doing calculations with flash powder.

It has been a while since my last and only chemistry course and my teacher was anything but competent... I can't recall how to find out how much energy is released from an exothermic reaction or how to find the activation energy or anything of the sort. Any help would be appreciated. I'm using [ce]5KNO3 + 3S + 2Al[/ce] flash powder.

[CaptainPanic]

Step 1:

Write down the reaction equation. This means that you must find out the reaction products.

 

Step 2:

Find the "heat of formation" of the components (heat of formation of the elements (S and Al) is zero, look up the rest).

 

Step 3:

Calculate the heat of reaction.

 

Alternative step 2:

google on "heat of reaction" for "flash powder", and hope for the best.

[UC]

You're not going to be able to determine much useful about that reaction at all. First of all, it's not a stoichiometric composition. There is an excess of nitrate there which could form numerous products as a result. Secondly, pyrotechnics tend to have very hard to predict products. That reaction will make sulfur dioxide and potassium oxide as some of it's products. These *might* form potassium sulfite, but they may also not due to the high temperature and dispersion of the mix. The dispersion of the mix will probably also prevent the reactions from finishing. 30% of it may go flying. It depends how the mixture is contained when ignited and the particle size of the reactants. The simplest stoichiometric mix with 3:2 ratio of sulfur to aluminum would be:

 

[ce] 10Al + 15S + 18KNO3 -> 5Al2O3 + 15SO2 + 9K2O + 9N2 [/ce]

 

or it may be

 

[ce] 10Al + 15S + 18KNO3 -> 5Al2O3 + 6SO2 + 9K2SO3 + 9N2 [/ce]

 

or

 

[ce] 10Al + 15S + 18KNO3 -> 5Al2O3 + 3K2SO3 + 6K2S2O5 + 9N2 [/ce]

 

Or some combination of these. There are also things I have not considered, like the possible formation of SO3 or sulfates. Even then, the reaction is not balanced.

[John Cuthber]

If this is for a calculus class then it hardly matters what value you use as long as it's not absolutely stupid.

 

It will,I think, be dominated by the heat of combustion of Al.

 

As long as you say you are making that aproximation then the maths is still valid and you should get full marks for it.

[salter]

Yeah we are going to try to do some predictions, but they dont need to actually match the results for us to get credit. I have some other ideas that are more complicated (and would actually tie in calculus) but this is the one piece I didn't remember.

 

I'm feeling stupid about my ratios that I gave because those are by weight, not number of particles.