MadSci Network: Chemistry
Query:

Re: How much energy can you obtain by detonating 1 Kg of TNT?

Date: Tue Jul 18 12:30:24 2000
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 963909938.Ch
Message:

How much energy can you obtain by detonating 1 Kg of TNT?
The NIST Chemistry WebBook gives several references for the heat of combustion ("DHc") of 2,4,6-trinitrotoluene (TNT). The balanced equation for this reaction is

C7H5N3O6 + 21/4 O2 7 CO2 + 5/2 H2O + 3/2 N2

The heat of combustion of TNT is about 3400 kJ/mol (810 kcal/mol), or about 15 kJ (just under 4 kcal) per gram. This value is a good indicator of the energy available from the detonation of TNT.

TNT requires a substantial amount of oxygen (42% oxygen by weight, or about gram oxygen per gram of TNT) for complete combustion. It is commonly mixed with other explosives in various proportions depending on the application. Presumably TNT is mixed with compounds which have extra oxygen, for example ammonium nitrate or nitroglycerine. The energy released by detonation of such mixtures will vary with the exact composition.

The Ordnance Shop is a good basic source for information on explosives with military uses. More detailed chemical information can be found at the TNT link given above.

Incidentally, there is another, brief answer on our site.

Dan Berger
Bluffton College
http://cs.bluffton.edu/~berger



Admin note:

Jason Goodman adds the following comments:

You answered 15 MJ/kg, based on complete combustion of TNT. However, the questioner asked about energy of *detonation*, which I expect is substantially less, since atmospheric oxygen is unavailable to complete the combustion after the expansion of hot gases following detonation.

In fact, computations of nuclear weapons yields, asteroid strikes, and the like (megatons, kilotons, etc.) use a value of about 4 MJ/kg. You have: kg tnt
You want: kilojoules
* 4612.0705
/ 0.00021682236

- from the sci.space FAQ (http://www.ksc.nasa.gov/facts/faq04.html)
1 megaton (MT) TNT = about 4.2e15 J or the energy equivalent of about .05 kg (50 g) of matter. Ref: J.R Williams, "The Energy Level of Things", Air Force Special Weapons Center (ARDC), Kirtland Air Force Base, New Mexico, 1963.

- from John Christie's answer in the previous Madsci question you mentioned:
I have lifted a value of 4 * 10^9 joule [for detonation of 1 ton TNT]

All of these results are consistent with 4.3 MJ/kg, with 10% uncertainty.

Admin Note: Please note, the value Dr. Berger quotes above is based on the heat of combustion, not the heat of detonation and there is a large difference between them. -- RJS.


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