Hi, I am doing a practice test for my chemistry final and I do not understand one of the answer they gave for this thermochemical equation question. I do not understand how they got 400kJ.

Question

The question is: Butane has a deltaH of -2871 kJ/mol when it undergoes complete combustion into carbon dioxide and liquid water. Given the following deltaHf values:

Carbon dioxide = -393.5kJ/mol
Oxygen gas = 0kJ/mol
liquid water = -285.8 kJ/mol
a) write the balanced thermochemical equation for the complete combustion of 1 mol of butane.

The answer given is: C4H10 =6.5O2 ---> 4Co2 + 5H20 + 400kJ.

I just don't understand how they calculated the kJ to be 400. Any help is greatly appreciated.

Answer 1

You need the delta Hformation of C4H10.
Then
dHcomb = [dHproducts]-[dHreactants]
dHcomb = [(4*-393.5) + (5*-285.8)]-[(dHf C4H10)] = ?

Answer 2

Thank you, so when I did that I got -137kJ. so is their answer wrong or mine. They had 400kJ.