azomage
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TNT Equivalents of Pyrotechnic Compositions
Note: I do not intend to make flash powder!
I was wondering about the energy released by various compositions, relative to TNT. I thought this would be fun to calculate but something seems very
amiss with my conclusion.
Let's say our two comps of interest are black powder and flash powder. I'll be using 2 sig figs for all molar masses and enthalpies.
Black Powder
2KNO3 + S + 3C → K2S + N2 + 3CO2
Enthalpy of Formation of Products: 1 mol K2S * -410 kJ/mol K2S + 1 mol N2 * 0 kJ/mol N2 + 3 mol
CO2 * -390 kJ/mol CO2 = -1580 kJ
Enthalpy of Formation of Reactants: 2 mol KNO3 * -490 kJ/mol KNO3 + 1 mol S * 0 kJ/mol S + 3 mol C * 0 kJ/mol C = -980 kJ
Change of Enthalpy: -1580 kJ - -980 kJ = -600 kJ
Molar Mass of Black Powder: 2 mol KNO3 * 100 g/mol KNO3 + 1 mol S * 32 g/mol S + 3 mol C * 12 g/mol C = 270 g/mol
So this means perfect combustion of black powder releases 600 kJ per 270 g black powder = 2.2 kJ/g black powder.
Flash Powder
3KClO4 + 8Al → 3KCl + 4Al2O3
Enthalpy of Formation of Products: 3 mol KCl * -440 kJ/mol KCl + 4 mol Al2O3 * -1700 kJ/mol Al2O3 = -8100
kJ
Enthalpy of Formation of Reactants: 3 mol KClO4 * -430 kJ/mol KClO4 + 8 mol Al * 0 kJ/mol Al = -1300 kJ
Change of Enthalpy: -8100 kJ - -1300 kJ = -6800 kJ
Molar Mass of Flash Powder: 3 mol KClO4 * 140 g/mol KClO4 + 8 mol Al * 27 g/mol Al = 640 g/mol
Perfect combustion of flash powder releases 6800 kJ / 640 g flash powder = 11 kJ/g flash powder.
Wikipedia says TNT releases 4.2 kJ/g TNT. Gasoline releases 45 kJ/g.
So...I'm not sure if I can believe this (nor do I want to verify experimentally!)...flash powder, gram for gram, is actually about 2.6 times stronger
than TNT?! And that TNT is only about twice as strong as black powder?
Does this seem right? If not, where did I go wrong? I'm also curious if there are tables that list energy released by various pyrotechnic
composition other than BP/FP.
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pantone159
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I don't know if your specific values are correct, but they would not surprise me. The power of TNT does not come so much from the amount of energy
released (though obviously there needs to be some), but in that it is delivered via a supersonic shock wave, which is very damaging to structures. It
is the supersonic shock, not just the energy, that makes TNT a high explosive.
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azomage
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Yeah, energy/time (power), brisance, and detonation velocity are the real deals I suppose. Wikipedia's table of explosive detonation velocities shows TNT as having a faster detonation velocity than flash powder.
[Edited on 14-11-2024 by azomage]
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Laboratory of Liptakov
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In your calculations you are missing the time in which the energy was released. In the case of gasoline you are even missing the second component, the
oxidizer (O2).
For example:
Flash powder 11 kJ / 0.1 s = 110 destruction units
TNT 4.2 kJ / 0.01 s = 420 destruction units
The decisive unit (not the only one) for the effectiveness of an explosive is the gas pressure, usually given in gigapascals (GPa). Or kilopascals ETN
= 300 kPa. TNT = 171.8 kPa. Semte x = 220 kPa...
Development of primarily - secondary substances CHP (2015) Lithex (2022) Brightelite (2023) Nitrocelite and KC primer (2024)
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Microtek
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The exact numbers vary based on the conditions since the reaction equations are not always the same (energetics yield different reaction products
depending on the density of the charge, for instance). The magnitudes of your calculated numbers are correct, however.
As others have already explained, energy is not all there is to consider.
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azomage
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Quote: Originally posted by Laboratory of Liptakov  | In your calculations you are missing the time in which the energy was released. In the case of gasoline you are even missing the second component, the
oxidizer (O2).
For example:
Flash powder 11 kJ / 0.1 s = 110 destruction units
TNT 4.2 kJ / 0.01 s = 420 destruction units
The decisive unit (not the only one) for the effectiveness of an explosive is the gas pressure, usually given in gigapascals (GPa). Or kilopascals ETN
= 300 kPa. TNT = 171.8 kPa. Semte x = 220 kPa... |
True. I like how you rewrote kilowatts as destruction units 
I also noticed that the black powder reaction releases 4 moles of gaseous products (nitrogen and carbon dioxide) while the flash powder reaction
releases 0 moles of gaseous products. So whatever gas pressure flash powder is contributing is only from the heating of local air? I guess this is
one reason why black powder is a good propellant.
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Etanol
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KCl is gas at explosive temperature of flash powder
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Laboratory of Liptakov
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Calculating mixtures on paper (in software) is interesting, but many, if not most, powerful (or popular) substances are created by chance. Similar to
Dynamite or Viagra. The performance of an energetic material is determined by a series of experiments. Not from series of calculations...
Development of primarily - secondary substances CHP (2015) Lithex (2022) Brightelite (2023) Nitrocelite and KC primer (2024)
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