Question: is h2 to 2h exothermic?
Answer: Endothermic Reaction. You can predict if a reaction is endothermic or exothermic by looking at the energies of the reactants and products (but this way is really hard with more complicated reactions). For example if you're given H2 = 2H, it would probably be endothermic since H2 is lower in energy than 2 H.