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Energy from burning two different substances

Note before you read this blog entry please be aware of a later blog entry 5th June 2010 which explains more about this subject.

Let us consider two fuels, one of which you might have in your house (or lunchbox) while the other I hope you do not have. By the way you can go to jail for the unauthorised possession of picric acid.

Picric acid (2,4,6-trinitrophenol) has a molecular formula of C6H4N3O7, so the molecular weight is 230.1 g mol-1. So one kilo is 4.346 moles. The heat of combustion of picric acid is 2.59497 MJ mol-1, so one kilo will give out 11.28 MJ when burnt.

A Cadbury crunchie has a mass of 40 grams, and it can provide 780 kJ of energy to a person eating it. So a kilo of crunchies would have 19500000 Joules (19.5 MJ). So weight for weight crunchie bars have almost twice as much energy in them than the classic world war one explosive for shells and grenades.

It is important to note that many explosives are oxygen deficient, so when they detonate the energy release is lower than the release which occurs when they are burnt in an excess of oxygen.

For picric acid I suspect that the explosion will form carbon monoxide and or soot. Here is an equation for a possible reaction which could occur when picric acid explodes.

2 C6H4N3O7 –> 3 N2 + 4 H2O + CO2 + 11 CO

In the same way in a human body the metabolism can change from aerobic to anaerobic. In the anerobic respiration the end product is lactic acid

Given the choice I would much rather eat a crunchie than picric acid, it is a little known fact that many explosives are very toxic. I know that nitroglycerine is a heart drug while some of the dinitrotoluenes formed during the synthesis of TNT are carcinogenic.

By the way some other energy values

Walkers Prawn Cocktail crisps, 21810 kJ per kilo

 Cadbury Picnic bar 19300 kJ per kilo

Fazel Dumle Snacks 190 kcal per 40 grams


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