If the rate-determining step is unimolecular, what is the apparent order of the reaction?

• A. First-order
• B. Zero-order
• C. Second-order
• D. Third-order

Answer: (A)

When the slowest step in a reaction mechanism governs the overall rate and that step involves only one molecule transforming, the rate can only depend linearly on how much of that molecule is present. In other words, the rate law takes the form rate = k [A], so the reaction is first-order with respect to that species. If the rate-determining step is unimolecular and sets the pace of the whole process, the apparent overall order of the reaction is one. Choosing more molecules in the slow step would be needed for higher apparent orders (two for second-order, three for third-order), while zero-order would mean the rate doesn’t change with concentration.