Calculate the enthalpy change using Hess Law

Question

using the given chemical reaction below with the net chemical reaction, calculate the enthalpy change.

Net Reaction: 4NH₃ + 3O₂  >>>>>>  2N₂ + 6H₂O

N2 + 3H2 >>>>>> 2NH3	-92 KJ
H2O >>>>> H2 + ½ O2	+286 KJ

Answer 1

4NH₃ + 3O₂ ==> 2N₂ + 6H₂O ... TARGET EQUATION

Given:

eq.1: N₂ + 3H₂ ==> 2NH₃ ... ∆H = -92 kJ

eq.2: H₂O ==> H₂ + 1/2 O₂ ... ∆H = +286 kJ

Procedure:

reverse eq.1 and multiply by 2 ... 4NH₃ ==> 2N₂ + 6H₂ ... ∆H = +184 kJ

reverse eq.2 and multiply by 6 ... 3O₂ + 6H₂ ==> 6H₂O ... ∆H = -1716 kJ

add the equations to get ...4NH₃ + 3O₂ + 6H₂ ==> 2N₂ + 6H₂ + 6H₂O ... ∆H = -1532 kJ

cancell the 6H2 on each side to end up with ... 4NH₃ + 3O₂ ==> 2N₂ + 6H₂O ... ∆H = -1532 kJ