1. The problem statement, all variables and given/known data Which of the following reactions is likely more exothermic? 1) Vaporized sodium metal reacts with diatomic chlorine gas to form one mole of solid sodium chloride. 2) Solid sodium metal reacts with diatomic chlorine gas to form one mole of solid sodium chloride. 2. Relevant equations Exothermic - bond improvement or formation. 3. The attempt at a solution I would think that the first reaction is more exothermic since we are going from gas phase sodium metal to solid sodium chloride. The second choice parallels the first choice except for the fact we're dealing with solid sodium metal (which is at its standard state and thus its enthalpy of formation is 0).