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: H₂ + O  >>>>>> H₂O

2H2O  >>>>>> 2H2 + O2	+576 KJ
O2  >>>>> 2O	+495 KJ

Answer 1

Since no phases are given, I am assuming all are in the gas phase.

H₂ + O ==> H₂O

eq.1: 2H₂O ==> 2H₂ + O₂ ... ∆H = 576 kJ

eq.2: O₂ ==> 2O ... ∆H = 495 kJ

reverse eq.1 and divide by 2 ... H₂ + 1/2 O₂ ==> H₂O ... ∆H = -288 kJ

reverse eq.2 and divide by 2 ... O ==> 1/2 O₂ ... ∆H = -247.5 kJ

add the two equations: H₂ + 1/2 O₂ + O ==> H₂O + 1/2 O₂ ... ∆H = -535.5 kJ

cancel 1/2 O2 on each side to end up with H₂ + O ==> H2O ... ∆H = -536 kJ