1. The problem statement, all variables and given/known data http://i4.minus.com/j7HwKoL8yhl96.JPG [Broken] 2. Relevant equations The enthalpy is equal to the heat of the products minus the heat of the reactants. 3. The attempt at a solution We are not supposed to use a bond enthalpy table to ascertain the enthalpy of the reactions. I know that B is clearly exothermic, as is C. B is a phase change and in the process energy is released from the gas into the environment as it transitions into the lower-energy liquid. C is a combustion reaction and that's obviously exothermic. D is clearly endothermic. Heat must be added to change water from a solid to a liquid. However, what about A? It's a composition or synthesis reaction. Forming bonds also releases energy. However, bonds have to be broken before the water can be formed. I'm not sure how I'm supposed to tell whether A is exothermic/endothermic from just looking at it. Is there a way?