A chemist studies the reaction between nitrogen monoxide and hydrogen gas. The overall equation for the reaction is: 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g). The rate equation for this reaction is: rate = k[NO]²[H₂]. Explain what the rate equation tells us about the mechanism of this reaction, and explain why the overall stoichiometric equation cannot be used on its own to determine the rate equation.

Edexcel A-Level Chemistry (9CH0) — 16.1 Rate equations and mechanisms · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The reaction between nitrogen monoxide and hydrogen is thought to proceed via a two-step mechanism. In the first step, two molecules of NO combine with one molecule of H₂. In the second step, an intermediate reacts with a further molecule of H₂.

Model answer (5 marks)

The rate equation shows the reaction is second order with respect to NO and first order with respect to H₂, giving an overall third‑order rate law (2+1=3). This indicates that the slow, rate‑determining step must involve two NO molecules and one H₂ molecule, as only these species appear in the rate expression. The stoichiometric equation, however, only represents the overall change of reactants to products and does not reveal the individual elementary steps or which species are involved in the slow step; therefore it cannot be used to deduce the rate law.

Examiner tips

  • State the orders with respect to each reactant and the overall order; link the rate law to the slow step; explain why the overall equation is insufficient; keep wording concise and use correct terminology.
  • Use the exact wording from the mark scheme: "second order with respect to NO", "first order with respect to H₂", "overall third order", "rate‑determining step must involve two NO and one H₂", "rate equation cannot be derived from the stoichiometric equation".
  • Include the reasoning that the rate law reflects the slow step, not the overall stoichiometry.
  • Avoid extraneous explanation or repetition of the question.

Mark scheme (5 marks)

  1. The rate equation shows the reaction is second order with respect to NO and first order with respect to H₂
  2. The overall order of the reaction is third order (2 + 1 = 3)
  3. The rate-determining (slowest) step must involve two molecules of NO and one molecule of H₂
  4. The rate equation can only be determined experimentally, not from the overall equation
  5. The stoichiometric equation represents the overall change and not the individual steps, so it gives no information about the mechanism or which species are involved in the slow step

Key terms in this question

rate equation · mechanism

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