Overall reaction $\ce{H_2 + Br_2 -> 2HBr}$ Detailed reactions $\ce{M + Br_2 -> 2Br + M\tag{chain initiation}}$ There are two ways to initiate this reaction - thermally or photochemically (using light of short wavelength. Explosion can happen only at high temperatures $\ce{Br + H_2 -> HBr + H\tag{chain propagating}}$ this reaction is endothermic and slow at low temperatures $\ce{H + Br_2 -> HBr + Br\tag{chain propagating}}$ $\ce{H + HBr -> H_2 + Br\tag{chain propagating}}$ $\ce{M + 2Br -> Br_2 + M\tag{chain terminating}}$