1. The problem statement, all variables and given/known data Internal Energy Change 2. Relevant equations N/A 3. The attempt at a solution If internal energy change equals enthalpy change, then I am looking for the reaction which has the enthalpy change closest to zero. I know it cannot be A, as the formation of water is highly exothermic. I know it cannot be C, as the vaporization of water is highly endothermic. This leaves B and D. I think it should be D, as it is a decomposition. I find that the breaking of bonds of halogens in B to seem endothermic.