Ch.8 #51

[Clara_Nguyen_1D]

For the question 8.51, I understand that you need to use Hess's Law to find the enthalpy of the reaction, but why when we are asked to find the enthalpy density is the answer positive and not negative? How do we know when to make the answer positive or negative?

[Ara Yazaryan 1E]

In this case, energy is released when TNT is consumed. Thus it would be positive since the surrounding environment is getting an increase in energy. If the reaction was endothermic (e.g. bonds were formed) then the reaction energy would be - since energy is used up.

Hope that helps!