So change in H. And from 1 mol, where whenever 1 mol of H reacts with Another Mole of H to form 1 Mol of H 2104, energy will Be Released Into The environment. And this is known as an exothermic reaction. In other words, exothermic reaction is A reaction in which the energy of products is lower than the energy of reactants, and the energy is released into the environment. Now, let's look at the same Exact diagram. But now we're going to work backwards. So we basically want to begin with a diatomic H two molecule, and we want to somehow get to this H two.