Final answer:
The rate law for the elementary reaction H₂(g) + I₂(g) → 2 HI(g) is expressed as rate = k[H₂][I₂], showing it is first order in both H₂ and I₂, and second order overall.
Step-by-step explanation:
To write the rate law for the given elementary reaction H₂(g) + I₂(g) → 2 HI(g), we must consider the reaction to be an elementary step in a reaction mechanism. The rate law for an elementary step is directly derived from the stoichiometry of the reactants in that step. For this reaction, the rate law will depend on the concentration of H₂ and I₂. Assuming that the rate constant for this reaction is represented as k, the rate law can be expressed as:
rate = k[H₂][I₂]
This rate law indicates that the reaction is first order in both H₂ and I₂, and second order overall. It's important to remember that the rate constant k has units that are determined by the overall order of the reaction. For a second-order reaction, the rate constant would have units of 1/(M·s), where M represents molarity (moles per liter).