Chris The Great
Hazard to Others
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Registered: 29-10-2004
Location: Canada
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Huh? Bond energy calculations.
Well, because heat of formations and the like aren't available for everything, and I want to calculate combustion/explosion energies, I figure I
can do it by bond energy. However, I've run into the problem of my calculations being off by very large amounts when doing some calculations for
methyl nitrate, to see how it works etc.
For referrence, methyl nitrate explodes with 1612kcal/kg of energy, with the following decomposition:
2CH3NO3 --> 2CO2 + 2H2O + H2 + N2
The chart for the bond energies that I used is at http://www.cem.msu.edu/~reusch/OrgPage/bndenrgy.htm
The bond energy in the 2 CH3NO3 molecules is:
(6 x 99 C-H) + (2 x 85.5 C-O) + (2 x 55 N-O) + (4 x 143 N=O) = 1447kcal
The bond energy of the explosion products is:
(4 x 192 C=O) + (4 x 111 H-O) + (1 x 104 H-H) + (1 x 226 N3N) = 1542kcal
The obtain the total energy, we substract:
1447kcal - 1542kcal = -95kcal
Indicating we have an exothermic reaction releasing 95kcal of energy for every 154 grams of methyl nitrate.
But, that doesn't add up with methyl nitrates 1612kcal/kg energy release. If we convert the 95kcal/154g into kcal/kg, we only get 617kcal/kg.
So, something, somewhere, is wrong 
Also, it gives large bond energies for a C to C triple bond, and a C to N triple bond, which should be enodthermic and release energy on breaking,
which is why hydrogen cyanide and acetylene can explode on their own if detonated. I added up the energies of the detonation products for both of
these and they always give me an endothermic detonation reaction, which it is not.
Can anyone see what is wrong here?
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Marvin
National Hazard
Posts: 995
Registered: 13-10-2002
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Well, for a start,
You are adding up numbers based on average bond energies to get 2 big numbers. Yoiu then subtract to produce a much smaller number which you find has
a large error. This only implies a small error in the big numbers though, maybe as little as a few percent in this case.
The intrinsic method is inaccurate, try a different one, say based on Hess' law cycle. You may also find that the rigid equation of production
isnt perfectly true, and the products actually contain significantly less CO2 and hydrogen, more water and some carbon monoxide.
Bonds are never endothermic, they form because its a lower energy state. Like electrons 'falling' into ions to form neutral atoms, atoms
fall partway into the potential well of other atoms to form molecules. The electrons dont see 'bonds' as such.
For the reason acetylene, metal acetylides and such can explode, dont think of the breaking of the bond, this cannot release energy, think of the
formation of C-C and Metal-Metal bonds as being the energy drivers. Think of the difference between bond energy, baed on atoms coming together, and
heat of formation, based on elements in the natural state being rearanged.
I hate Calories and I'm not about to actually check the math, sorry 
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