How do i calculate the theoretical delta H for a reaction?

To calculate ##Delta H## for a reaction, you must substract of formation of products to enthalpy of formation of reagents.

Let us see it with an example:

##"CH"_4 ("g") + 2 "O"_2 ("g") rightarrow "CO"_2 ("g") + 2 "H"_2 "O" ("g")##

We must know enthalpy of formation of all and :

• ##Delta H_"form" ("CH"_4) = -17.9 " kJ/mol"##, from the reaction: ##"C" + 2 "H"_2 rightarrow "CH"_4##

• ##Delta H_"form" ("H"_2"O") = -241.82 " kJ/mol"##, from the reaction: ##"H"_2 + 1/2 "O"_2 rightarrow "H"_2 "O"##

• ##Delta H_"form" ("CO"_2) = -393.5 " kJ/mol"##, from the reaction: ##"C" + "O"_2 rightarrow "CO"_2##

• ##Delta H_"form" ("O"_2) = 0 " kJ/mol"##, like every element at its natural state.

So, now:

##Delta H_"reaction" = ## ##= sum n cdot Delta H_"form" ("products") - sum m cdot Delta H_"form" ("reagents") =## ##= [(-393.5) + 2 cdot (-241.82)] - [(-17.9) + 2 cdot 0] = -859.24 " kJ/mol"##

where ##m, n## represent the coefficients in the reaction.

Note: albeit there are other methods to obtain enthalpy (such as Hess's law, for example), I think this is the one which better adjusts to what you are looking for.