use standard heast of formation

Question

Using standard heats of formation, calculate the standard enthalpy change for the following reaction.

2H2(g) + O2(g)2H2O(g)

ANSWER: kJ

Answer 1

You want to add the std heats of formation of the products and subtract from that, the sum of the std heats of formation of the reactants.  ∑∆H^o_products - ∑∆H^o_reactants

∆H^o for H₂ and O₂ are both zero

Look up the ∆H^o for H₂O(g) and multiply it by 2 to get your answer.  Be sure to use the ∆H^o for H₂O(g) and not H₂O(l)