Reactivity of Dihydrogen

Dihydrogen is not very reactive under ambient conditions, but the lack of reactivity is kinetic (rather than thermodynamic) in origin, and arises from the strength of the H-H bond (Table 1.1). The branching-chain reaction of H₂ and O₂ is initiated by sparking and the resulting explosion (or ‘pop’ on a small scale) is well known in the qualitative test for H₂. Part of the reaction scheme is given (in a simplified form)  efficient branching results in a rapid, explosive reaction, and is the reason why it is effective in rocket fuels.

Halogens react with H₂  with the ease ofreaction decreasing down group 17. Even at low temperatures, F₂ reacts explosively with H₂ in a radical chain reaction. In the light-induced reaction of Cl₂ and H₂, the initiation step is the homolytic cleavage of the Cl^_Cl bond to give Cl^- radicals which react with H₂ to give H^- and HCl in one of a series of steps in the radical chain; HCl can be formed in either a propagation or a termination step.

Reactions of H₂ with Br₂ or I₂ occur only at higher temperatures and also involve the initial fission of the X₂ molecule. For Br₂ (but not for I₂) the mechanism is a radical chain .

Table 1.1 Selected physical properties of H₂