section_path: Properties/Combustion/Reactants
section_name: Reactants
paragraph_text: Reactants
  
  H is unreactive compared to diatomic elements such as halogens or oxygen. The thermodynamic basis of this low reactivity is the very strong H–H bond, with a bond dissociation energy of 435.7 kJ/mol. The kinetic basis of the low reactivity is the nonpolar nature of H and its weak polarizability. It spontaneously reacts with chlorine and fluorine to form hydrogen chloride and hydrogen fluoride, respectively. The reactivity of H is strongly affected by the presence of metal catalysts. Thus, while mixtures of H with O or air combust readily when heated to at least 500°C by a spark or flame, they do not react at room temperature in the absence of a catalyst.