1. The problem statement, all variables and given/known data Given that the bond energy of hydrogen-hydrogen bonds is 436 KJ/mol, that of hydrogen-oxygen bonds is 464 KJ/mol, and those in oxygen molecules 496 KJ/mol, what is the approximate heat of reaction for 2H2 + O2→2H2O 2. Relevant equations Hproducts-Hreactants 3. The attempt at a solution So, I understand that heats of reaction involving hydrogen gas and oxygen gas to liquid water is exothermic, so my answer must be negative. My book gives and answer of -488 KJ/mol but I keep ending up with -440 KJ/mol: 2(464) - [2(436) + 496] = -440 KJ/mol. Calculation error? I went over the numbers twice. Thanks!