823 replies to this topic

[digger]

OK Then

Then equation that you have shown is a molar equation not a weight based equation (you have neglected the molecular weights)

So the easy way to do it would be:

3(KCl04) + 8Al = 4(2Al3O) + 3(KCL)
3*138.6 + 8*27 = 4*102 + 3*74.6
415.8 + 216 = 408 + 223.8
Now add the heats of formation
415.*-742 + 216*0 = 408*-4000 + 223.8*-1397
-308523.6 + 0 = -1632000 + -312648.6
Therefore Heat released for the stoichiometric reaction shown is -1636125 cal or per gram we need to divide by 631.8g to give 2589.6 cal/g or in new money 10842 J/g

This is a pretty significant amount of energy

Now if you work out the heat capacity of the products you can calculate what temperature the reaction will achieve.

[digger]

Here is a quick calc. There is a problem with as the cP is measured at room temp in practice the cP is temperature dependent. It is possible to model the predicted cP at the reaction temp, but I don't have the time to mess around with this. So here is a simple calc to give an idea of the reaction temp.

Al2O3 = 24.200 J/mol.K or 0.2373 J/g.K
KCL = 52 J/mol.K or 0.6971 J/g.K

Cp of rxn product of 1g of flash = (408/631.8)*0.2373 + (223.8/631.8)*0.6971 = 0.4002 J/g.K

Q=m.cP.(Trxn-Tamb)

Trxn= (Q/m.cP)+Tamb

Trxn=(10842/.4002)+20= 27111C

As you can see this is a massive temperature. In practice it will be lower than this as the heat capacity increases as the temperature rises. Also it is safe to assume that there will be a change of state of the reaction products to also consider, as potassium chloride sublimes at 1500C and aluminium oxide boils at 3000C.

So the temp will be significantly lower than that from the equation above. So maybe you could modify the equation to take into consideration the change of state energies.

So it can be said that the reaction products of this flash are gaseous initially. OK the products will cool to solids, however I would suggest that the negative explosive terminology needs carefully thought as the explosive thrust IS created by gas phase products.

[CCH Concepts]

im impressed. im guessing calculating actual temp would be best done with some modeling software like matlab, but i never really did get my head round matlab, they always had the postgrads teaching it and they could never be arsed.

the change in temp would be interesting to see as a graph. im assuming there would be a rapid increase and then small platos as the changes in state with the reaction slowing as the temp gets higher.

[digger]

im impressed. im guessing calculating actual temp would be best done with some modeling software like matlab, but i never really did get my head round matlab, they always had the postgrads teaching it and they could never be arsed.

the change in temp would be interesting to see as a graph. im assuming there would be a rapid increase and then small platos as the changes in state with the reaction slowing as the temp gets higher.

No need for Matlab. It is a simple calculation. You just need to find the physical data. In the absence of experimental data, the properties can be estimated using well defined physical data calculation equations. These can probably be found in Perry's Chemical Engineers Handbook.

[digger]

P.S. The actual reaction kinetics would be complex, as the equation shows only the final results and not the mechanism of the reaction. For example I guess the perchlorate decomposes releasing oxygen that then reacts with the aluminium. So the there would be at least two rates of reaction required, which of these is the rate determining step? How does the temperature change the rates? does it in fact change the rate determining step?

So I don't think that it could be modeled mathematically to solve for rate and temperature without a great deal of lab work

[CCH Concepts]

i would imagine it would make an interesting tool if someone had the expertise and patience to make it. but im not sure if it would be as useful as it would be interesting.

although knowing the potential enegery of a energtic material could be a useful tool before experimenting. would save the comment "that was alot more than i was expecting".

would be very interesting if someone made a java applet that simply required the reaction and the constants and it worked out the rest.

[MDH]

It seems to me CCH like what you were asking for earlier was the most powerful flash powder. If the perchlorate flash reaction occurs faster because perchlorate decomposes at a much lower temperature, it is releasing energy in less time than the magnesium sulfate flash, which would make it more powerful. Are you simply looking for what reaction produces the most energy overall, disregarding the speed or behavior of the reaction?

[Mumbles]

There is a chemistry program called ORCA that could probably easily do the calculation. I unfortunately do not know how to use it just yet. Maybe ask me in a year.

[CCH Concepts]

It seems to me CCH like what you were asking for earlier was the most powerful flash powder. If the perchlorate flash reaction occurs faster because perchlorate decomposes at a much lower temperature, it is releasing energy in less time than the magnesium sulfate flash, which would make it more powerful. Are you simply looking for what reaction produces the most energy overall, disregarding the speed or behavior of the reaction?

i was thinking about that actually. like the example of candle wax and vasiline being high energy but low or no burn rate. but they are key parts of chedite a HE. burn rate is very important consideration.

[cooperman435]

Im becoming more and more concerned (as are the users who keep messaging me) about the constant borderline HE conversations and perpetual flash obsession CCH.

Can you explain why you have such an interest in the noisy and dangerous side of pyro mate as its becoming a worry?

[CCH Concepts]

I have no interest in HE. I was simply stating it as an example that I am aware that rate of burn has a large impact on the power of any flash salute. That was simply the subject at hand, if you were to look through the posts I have made, at the start they were all about making a bang.

The majority of my posts are now about the chemistry, tooling, and physics. In this case I am simply interested in the maths and chemistry which would go into predicting a reaction, with flash being a simple two part formulae to start with.

i think your suspicion comes more from my naivety when I first joined the forum rather than my current contributions.

Edited by CCH Concepts, 27 April 2010 - 02:35 PM.

[Vic]

I don't know why there is this endless preoccupation with flash. It plays only a small part in fireworks albeit an important one a times, but nothing like the gratuitous attention it gets on the forum.
I am not anti flash if used wisely it's just people seem to be overly attracted by it I suppose it's to do with the power and the dangers associated at the end of the day it is a blunt instrument.
And with regards to HE, any posts about it or bordering on the subject in any way should be deleted

[CCH Concepts]

please don't get me wrong, I'm not preoccupied by flash this was just the subject at hand. as soon as i started to understand how to produce colors and other effects my attention shifted very quickly.
its takes no time at all to make some reasonable flash, but that not the same for the rest of pyro. but if you go a bit back the main question i have posed is that of negative explosives ad how effective they are because of there reported stability and improved safety. not just someone liking to make a bang.

as he HE i will say again i have no interest in it simply because i like the site and don't want to get banned and i value my liberty.

[digger]

I agree with "why the preoccupation".

I guess that most don't fully understand the term Negative explosive. As proved in the previous post the solid products are gaseous during the reaction.

So I don't understand how you think that a "Negative" explosive is any safer. Most forms of flash are considered thus.

Where did you get the statement that these are any safer.

Edited by digger, 27 April 2010 - 09:48 PM.

[CCH Concepts]

i posted the link the the artical, eairlier in the thread.

http://www.jpyro.com/wp/?p=321

"The magnesium sulfate/magnesium mixture is far safer on handling than the ordinary aluminum mixture. It was proved by an iron ball dropping test and a fire propagation test."

and yes my understanding of a negitive explosive isnt great hense me posting about it. i thought the difference between this and a standard flash would be to do with the fuel metal also being the salt metal, but this was more of a gauess than any scientific theory.

Edited by CCH Concepts, 28 April 2010 - 06:31 AM.