# Last thermchemistry question

1. Oct 20, 2007

### ace123

This is my last thermochemistry question.

The standard molar internal energy of formation of N2O5 (g) is 17.433 KJ/mol at 298 K. What is the standard molar enthalpy of formation of N2O5 (g) at the same temperature.

I just need hint, like what do i do with the 298K?

Last edited: Oct 20, 2007
2. Oct 20, 2007

### eli64

its delta H = delta U (E) + P delta V

molar internal is delta E and if you had data for P and delta V, you could get delta H (enthalpy). But what if this is done at CONSTANT volume, what happens to P delta V (your question doesn't give this condition but it also doesn't give P or delta V values - it might be leading you this way)? how can you find delta H under this constant volume condition?

3. Oct 20, 2007

### ace123

Well if volume is kept constant then the delta H will be just E?

4. Oct 20, 2007

### eli64

Yes, correct,

and NO, I'm not stalking you, just trying to help out where I can - solving chem problems is fun for me and you seem to have some fun problems :rofl:

5. Oct 20, 2007

### ace123

Only reason they are fun is because my professor gets them from some website and doesn't even check to see if they relate to what we have done in class. Which is why I'am forced to ask for help

Edit: So why even give the 298K. Just to tell us that it's not standard pressure?

6. Oct 20, 2007

### eli64

I think you mean standard state but no, the temp is not necessary to get the answer. However, temp is often specified as well in these thermochem questions but not part of the standard states (1atm and 1M). Might as well be though,

Your prof is trying to get through some material without lecturing about it in class (might be because he is behind in his schedule) and so is trying to get you to go look this up (or ask for help ... :) ) and read other parts of your textbook, there must be another part of the chapter on thermochem that deals with total energy of a reaction, which is what I should have suggested that you go do in the first place..... ;)

enough fun for me, I've got to go back and do my own work....

Last edited: Oct 20, 2007
7. Oct 20, 2007

### ace123

Well thanks for your help and I won't bother you again. Well at least for today. Unfortunately this really isn't in my textbook. Only part that deals with energy. Also my chem class is a lecture so is supposed to lecture about it :)