J.R. S. answered 08/27/18
Ph.D. University Professor with 10+ years Tutoring Experience
You need to know either the ∆Hf for each of the reactants and products, or the ∆H of the reaction.
If you have ∆Hf reactants and products, then ∆Hrx = ∑n∆Hproducts - ∑n∆Hreactants
If you have ∆Hrx, then compute ∆H for 0.40 mole by multiplying by 0.40/2 or by 0.2