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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.