[Davep]

I need to know the enthalpy of combustion (heat of combustion) when 1-Hydroxybenzotriazole is burnt stoichiometrically in air.  It turns out that HOBt is explosive when heated to 160⁰C.

I have been unable to find any enthalpy of combustion information for HOBt and have resorted to using bond enthalpy.  Is this okay?

I may have simplified the equation too much but I used:

4C₆H₅N₃O + 39(O₂ + 79/21N₂) => 10H₂0 + 24CO₂ + 12NO₂ + 146 5/7N₂

The HOBt comprises 16C-H, 12C-C, 12C=C, 8C-N, 4N-N, 4N-OH and 4N=N bond(s) and the oxygen has 39O=O bonds. The enthalpy for bonds broken I calculate to be 42666kJ/mol. (The minus sign signifies a single bond, the equals sign a double bond.)

The reactants have 20H-O, 48C=O and 24N=O bonds. The enthalpy for bonds formed totals 59780kJ/mol. I have ignored the Nitrogen as it cancels out on both sides of the equation.(I hope this is okay!)

Therefore, the enthalpy of combustion  for HOBt is -17114kJ/mol.

You may have already guessed that I'm not a chemist but an engineer and there is a chance that some of this compound could get into an incinerator.  I need to work out the allowable mass of HOBt that can be burnt at 0.11bar.

Any help in verifying/correcting my equation/calculation will be appreciated.  Current calculations indicate 17grams worth of the stuff.

[eugenedakin]

Hello Davep,

Your answer is definately along the right track.  The problem arises when we are talking about reactions that actually happen in the real world (contamination with iron, insufficient oxygen uptake in a chamber, contaminants, combustion efficiency,  etc...). 

When the reaction has insufficient oxygen, products such as carbon monoxide can be produced. 

One method of identifying the combustion products is to acquire a gas sample and have it analyzed.  This would help you identify the stoichiometry for your specific situation and allow you to reasonably determine the overall reaction.

You can use your suggested equation, but make sure that the wording in your proposal allows for various efficiencies/inefficiencies in combustion.

I hope this Helps,

Eugene