Let's say the standard size firecracker if there is a standard.
What about doing an experiment? Make a pendulum with a little cup on the end and set the firecracker in their and try to focus the explosion so it will rotate the pendulum ,
Then measure the change in height of the pendulum . mgh is the gravitational potential energy. Ballistic pendulum idea or do something with momentum conservation.
Nope. Too many losses, would introduce errors on the order of 50% or more.
Using standard enthalpy tables will get you quite close to the answer.
Perhaps the best experimental approach is to make the firecracker explode in a calorimeter. Some ideas are probably mentioned in this book: http://www.springerlink.com/content/x40850v6628r6270/
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