[KyleH]

Alright so I have to finish up an Enthalpy reaction.  We did experiments in class and constructed chemical equations.  The question is: find the heat of combustion of Magnesium.

So all I need to know to actually finish that question is what it actually means....
I'm trying to set up an equation that shows the beginning of the reaction and the end.

What i have is Mg + O_2g = MgO[/s][/s]

Okay, and here are my other equations

MgCl₂ + H₂O ---> MgO + 2HCl        H = 0.4184 kJ
2HCl + Mg     ------------> H₂ + MgCl₂ H =-1.338 kJ
H_2[/sub] + 1/2O₂ -------> H₂O         H = -285.8 kJ


My experimental heat of combustion per mol is -286.720
This is horribly wrong, with a percent error of more than 50%.  The real enthalpy value is -601.6 kJ

Does anyone see what I could've done wrong?  I used Hess's law, which states the steps of individual reactions adds up to the total reaction.  Maybe my equatoins aren't balanced and i don't realize it?