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Question 8.51: positive vs. negative enthalpy

Posted: Mon Feb 08, 2016 5:43 pm
by Heerali Patel 3A
Question 8.51:
The enthalpy of formation of trinitrotoluene (TNT) is -67KJ/mol, and the density of TNT is 1.65 g/cm^3. In principle, it could be used as a rocket fuel, with the gases resulting from its decomposition streaming out of the rocket to give the required thrust. In practice, of course, it would be extremely dangerous as a fuel because it is sensitive to shock. Explore its potential as a rocket fuel by calculating its enthalpy density (enthalpy released per liter) for the reaction 4C7H5N3O6(s) + 21O3(g) -> 28CO2(g) + 10H2O(g) + 6N2(g).

The answer from the solutions manual is +23.9x10^3 KJ/L. My question is why the sign is positive because the enthalpy of reaction that I get is -13168.48KJ?

Re: Question 8.51: positive vs. negative enthalpy

Posted: Tue Feb 09, 2016 12:12 am
by Chem_Mod
Because it asked for the enthalpy released which literally means heat released (exothermic). So the negative sign is not necessary.