## 8.51

RenuChepuru1L
Posts: 58
Joined: Thu Jul 27, 2017 3:00 am

### 8.51

can someone explain why you only use the enthalpies of formation for two of the products in the reaction not all the products? and also just like the whole question, help

Thu Uyen Tran 1B
Posts: 31
Joined: Fri Sep 29, 2017 7:05 am

### Re: 8.51

The nitrogen gas can be included with the other products but it would not change the answer because the standard enthalpy formation of an element in its most stable form is equal to zero. Is there a specific part in the problem that you are having trouble with, or are you confused of how to approach the problem?

RenuChepuru1L
Posts: 58
Joined: Thu Jul 27, 2017 3:00 am

### Re: 8.51

thank you! my only other question is what are the step by steps for calculating enthalpy density, what would you do to solve this problem?

Diane Bui 2J
Posts: 61
Joined: Sat Jul 22, 2017 3:00 am

### Re: 8.51

In this problem, enthalpy density is defined as enthalpy released per liter. So, using the standard enthalpy of formations of the reactants and products, you find the enthalpy of the reaction by subtracting the enthalpy of the reactants from the enthalpy of the products. You would then divide that number by 4 moles since the enthalpy of that reaction is for 4 moles of TNT (as seen in the balanced equation). To find enthalpy density, you take the enthalpy per mole of TNT and multiply it by the molar mass of TNT and multiply that value by the density of TNT (given in the problem as 1.65 g/mL). In doing so, the units will cancel out to result in kJ/mL. A simple conversion of 1000mL=1L will result in the units of kJ/L, which is what enthalpy density is defined as in the problem.

DamianW
Posts: 35
Joined: Fri Sep 29, 2017 7:06 am

### Re: 8.51

How do you know when something is in its "most stable form"?

Tiffany 1B
Posts: 32
Joined: Fri Sep 29, 2017 7:05 am

### Re: 8.51

DamianW,
Something being in its "most stable form" refers to it being in its 'most pure/most stable' state, which I like to think of as how you would normally find it; an example of this would be oxygen which we know is all around us in the form of O2. In this particular problem, we know that N2 is in its most stable form because it's one of the diatomic elements we learned about in chem 14A and that Dr. Lavelle reviewed with us in week 1. Appendix 2A also has this information listed. Hope that helps.

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