[Frederick95]

The thermite reaction is quite a common reaction and is quite spectacular. Iron (III) oxide and metallic aluminum produce iron and aluminum oxide in a few seconds. Find DH for this reaction given the following information:

2Al(s) + 3/2 O2(g) --> AlO3(g) DH = -1670 kJ/mol
2Fe(s) + 3/2 O2(g) -->Fe2O3   DH = -822 kJ/mol

My work:

-1670 kJ/mol + -822 kJ/mol
=2492 kJ/mol / 2
=1246 kJ/mol

[AWK]

_AlO3(g)

[Frederick95]

Sorry it is Al2O3(g)

[DrCMS]

See my reply about you water question. 

For this question start by writing down the thermite reaction. 
Compare that to the given reactions.

[AWK]

Sorry it is Al2O3(g)

Still wrong. Al2O3 is a solid

[Frederick95]

Thermite reaction

Iron (III) oxide and metallic aluminum produce iron and aluminum oxide

Fe2O3 + Al --> Fe + Al2O3

[The_Student]

My work:

-1670 kJ/mol + -822 kJ/mol
=2492 kJ/mol / 2
=1246 kJ/mol

why is the answer a positive value when there are two negative numbers adding? did you mean -1246 kJ/mol?
why did you divide 2492 kJ/mol by two (just curious)?

[Padfoot]

Frederick95 your 2nd eqn must be reversed.  The 2 eqns must add to give the thermite reaction.
http://en.wikipedia.org/wiki/Hess's_law