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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Wienerbroed on June 26, 2021, 12:20:52 PM

Title: Need help calculating Internal energy
Post by: Wienerbroed on June 26, 2021, 12:20:52 PM
I need help solving a question about finding the change in internal energy.
The question is:

When it explodes, nitroglycerin splits into nitrogen gas, carbon dioxide, oxygen and water vapor. The enthalpy of formation is -365,0 kJ/mol for nitroglycerin, -393,5 kJ/mol for carbon dioxide and -241,8 kJ/mol for water vapor.
Calculate the enthalpy of reaction (ΔH) as well as the change in internal energy (ΔU) when nitroglycerin completely decomposes with a constant pressure of one bar. Assume an ideal gas is formed, that performs volume change work.


I've been able to calculate the ΔH, but I do not know how to calculate the ΔU. I know that I'm supposed to use
ΔU = q + w and PV = nRT, but I don't know how.
Title: Re: Need help calculating Internal energy
Post by: Borek on June 26, 2021, 01:18:10 PM
Reaction equation would probably help.
Title: Re: Need help calculating Internal energy
Post by: Orcio_Dojek on June 26, 2021, 02:07:55 PM
@Wienerbroed

Did you read about enthalpy definition ?

ΔH = ΔU + pΔV

Here pressure is constant so the second term (pΔV) relies only on the gaseous products (water is also included as a gas).
Title: Re: Need help calculating Internal energy
Post by: Wienerbroed on June 26, 2021, 05:05:39 PM
@Orcio_Dojek

Yes, I am aware of that, however I do not know how to get a measurement for the change in volume.
Title: Re: Need help calculating Internal energy
Post by: Borek on June 27, 2021, 03:10:17 AM
Yes, I am aware of that, however I do not know how to get a measurement for the change in volume.

And that's where the reaction equation comes to the rescue.

How many moles of gas before? After? Can you use this information to calculate ΔV?
Title: Re: Need help calculating Internal energy
Post by: Wienerbroed on June 27, 2021, 07:23:27 AM
@Borek

We don't know the amount of moles for any given substance. I only know what each of the substances moles are relative to each other thanks to the reaction formula:

4C3H9N3O9  :rarrow: 6N2 + 12CO2 + 10H2O + O2

I assume the amount of moles of gas before the reaction is 0, since all products were part of the nitroglycerin in liquid form?
Can I use the enthalpy of formation to somehow calculate each products amount of moles?
Title: Re: Need help calculating Internal energy
Post by: Orcio_Dojek on June 27, 2021, 08:00:29 AM
Quote
Can I use the enthalpy of formation to somehow calculate each products amount of moles?
No, as enthalpy of formation relates on formation of compound from pure elements.
Title: Re: Need help calculating Internal energy
Post by: Wienerbroed on June 27, 2021, 08:32:48 AM
@Orcio_Dojek

Any idea to how I can find the amount of moles in that case?
Title: Re: Need help calculating Internal energy
Post by: Orcio_Dojek on June 27, 2021, 08:35:33 AM
I see that there are 29 moles of gas for every 4 moles of nitro-glycerine.
Title: Re: Need help calculating Internal energy
Post by: Borek on June 27, 2021, 10:10:38 AM
We don't know the amount of moles for any given substance.

I've been able to calculate the ΔH

These two statements contradict each other - unless you did calculations per 1 mole of nitroglycerine. If so, do the same for ΔU.
Title: Re: Need help calculating Internal energy
Post by: Wienerbroed on June 27, 2021, 10:25:59 AM

These two statements contradict each other - unless you did calculations per 1 mole of nitroglycerine. If so, do the same for ΔU.


Don't I need the concentration of each substance? To find ΔU, i need to find ΔV, and to do that I should need the concentration (since V = n/c). How should I find the concentration in that case?
@Borek
Title: Re: Need help calculating Internal energy
Post by: Orcio_Dojek on June 27, 2021, 11:27:58 AM
Quote
Don't I need the concentration of each substance? To find ΔU, i need to find ΔV, and to do that I should need the concentration (since V = n/c). How should I find the concentration in that case?
Equation tells us that N2 constitutes about 21 % of products (quantitative), CO2 about 41 % and so on...

 :-\
Title: Re: Need help calculating Internal energy
Post by: Wienerbroed on June 27, 2021, 11:54:30 AM
How could we find the concentration of nitroglycerin?
Title: Re: Need help calculating Internal energy
Post by: Orcio_Dojek on June 27, 2021, 12:12:38 PM
Hard to say, it can be 100 or 99 % pure or - mixed with something. But - if not stated otherwise - it can be assumed that it is 100 % pure.
Title: Re: Need help calculating Internal energy
Post by: Borek on June 27, 2021, 12:14:17 PM
Show how you did calculations of ΔH.

You don't need concentration of anything here.
Title: Re: Need help calculating Internal energy
Post by: Wienerbroed on June 27, 2021, 01:28:47 PM
@Borek

I calculated the enthalpy of reaction by adding the enthalpy of formation for all the products together and subtracting the enthalpy of formation for nitroglycerin.
Title: Re: Need help calculating Internal energy
Post by: Borek on June 27, 2021, 02:44:39 PM
I calculated the enthalpy of reaction by adding the enthalpy of formation for all the products together and subtracting the enthalpy of formation for nitroglycerin.

Show the numbers.