8.51 energy "released"

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Grace Ramey 2K
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8.51 energy "released"

Postby Grace Ramey 2K » Wed Jan 17, 2018 9:25 am

In the solutions manual for this problem, the enthalpy of the reaction is -13168 kJ/mol, which correpsonds to 4 mol of TNT. So I understand why you would divide this number by 4 to get the enthalpy per one mole of TNT, but I don't understand why the solutions manual makes that answer positive (+3292 kJ/mol). It refers to this number as the energy "released". Does that affect its +/- sign?

8.51
The enthalpy of formation of trinitrotoluene (TNT) is -67 kJ·mol-1, 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).

Thank you for the help!

Austin Ho 1E
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Re: 8.51 energy "released"

Postby Austin Ho 1E » Wed Jan 17, 2018 10:23 am

I'm not 100% sure either, but I think in this case since we're trying to find the enthalpy per density, we want a positive amount of enthalpy. To be honest I was a bit confused too but perhaps enthalpy per density can only be positive.

Clara Rehmann 1K
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Re: 8.51 energy "released"

Postby Clara Rehmann 1K » Wed Jan 17, 2018 12:36 pm

I assumed that the value was positive because we were looking at the energy from the perspective of the surroundings rather than the perspective of the system. Technically the TNT is losing energy by releasing it, but when observing that process from the outside, a positive amount of energy is being released into the surroundings.

Sirajbir Sodhi 2K
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Re: 8.51 energy "released"

Postby Sirajbir Sodhi 2K » Wed Jan 17, 2018 1:36 pm

The answer is positive because it refers to "energy released." The reaction releases +3292 kJ/mol, but the enthalpy of the reaction per mole of TNT is -13618 kJ/mol. In this case, the sign of enthalpy just has to do with the way the question is worded.

Renee Delamater 2H
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Re: 8.51 energy "released"

Postby Renee Delamater 2H » Wed Jan 17, 2018 9:28 pm

I agree; technically the energy is released so it makes sense that it would be a negative number (because exothermic reactions are negative) but since it is in relation to the outside receiving the energy, it is gaining a positive amount of energy.


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