The mechanism proposed for the reaction of H₂(g) and I₂(g) to form HI(g) consists of a fast reversible first step involving I₂(g) and I(g), followed by a slow step. Propose a two-step mechanism for the reaction H₂(g) + I₂(g) → 2 HI(g), which is known to be first order in H₂ and first order in I₂.