# Need Help to calculate [DELTA]n , ( Not a lazy student looking for answers)

Discussion in 'Chemistry' started by Jennings, Mar 30, 2014.

1. ### JenningsRegistered Member

Messages:
35
First of all , please don't assume that I am just a lazy student looking for online answers (if you did not do that then forgive me) , but my chemistry teacher is abysmal and takes no time to explain the purposes of process . Many a time the only way i can understand how to complete a problem is with help from people on websites such as this . Any help would be greatly appreciated!

The topic is the change in internal energy for systems by chemical reactions using the equation
[DELTA]U = [DELTA]Hrxn + w
"The change in internal energy is the change in heat of rxn + the work"
This is the equation we learned in class . We were also taught that work = -[DELTA]nRT
whether this is correct i dont know .. but this is what we were taught in class . So to calculate the change in internal energy we must subtract [DELTA]nRT from [DELTA]Hrxn , where R = the gas constant (8.314x10-3) and T will also be constant at 298K , atleast for this exam. (Im also assuming that atm will be constant at 1 , but im not sure if this even applies to this problem & was not even mentioned by my teacher)
[DELTA]n would be the change in moles , as described by my teacher .

Example Problem:
N2(g) + 3H2(g) > 2NH3(g)
--using hess law i am able to calculate [DELTA]Hrxn = -91.8kJ , now i plug this into my eqtn:
[DELTA]U= -91.8kJ - [DELTA]nRT
Now for [DELTA]nRT
i think that [ mols of product (2) - mols of reactants (3+1=4) ] i can find [DELTA]n .. so
[DELTA]nRT= (2-4)(8.314x10-3)(298)
[DELTA]nRT= -4.95kJ
so
[DELTA]U = -98.1kJ + 4.95kJ
[DELTA]U = -93.15KJ

I dont know if this is the correct method , any help would be greatly appreciated !