The enthalpy is equal to the heat of the products minus the heat of the reactants.
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?
Last edited by a moderator: