Final answer:
The rate law for an overall reaction is based on the rate-determining step of the reaction mechanism and can be matched with experimental data to confirm the reaction's kinetics. It involves understanding the molecularity of elementary reactions and cannot be derived solely from the balanced chemical equation without experimental input.
Step-by-step explanation:
The rate law for an overall reaction is usually determined by the rate-determining step of the reaction mechanism. For elementary reactions, writing the rate law is straightforward since it corresponds to the molecularity of the reaction. For a unimolecular reaction, the rate law would be rate = k[A], indicating first-order dependency on reactant A. For a bimolecular reaction, the rate law is generally rate = k[A][B], showing second-order behavior, as it depends on the concentrations of both reactants A and B. However, deriving the overall rate law from the balanced chemical equation is not possible without experimental data or knowledge of the mechanism. In a case where the mechanism is known, such as the reaction 2NO(g) + Cl₂ (g) → 2NOCl(g), each elementary step has its own rate law:
- k₁ [NO][Cl₂] for the forward reaction of step 1
- k₁⁻ [NOCl₂] for the reverse reaction of step 1
- k₂ [NOCl₂][NO] for step 2
The overall rate law can be derived by adding up the elementary steps and identifying the rate-determining step, often confirmed by matching the experimentally derived rate law. If the experimental rate law is rate = k[NO]²[H₂], and it matches the rate law for the rate-determining step in the mechanism, it indicates consistency between the mechanism and observed kinetics.