The Rate-Determining Step and Reaction Mechanisms · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

The slow step sets the pace

Science · Chemistry · ages 17-18
Name ______________________   Date ____________

  1. For NO2 + CO, the rate is k[NO2]2 and CO is order zero. How many NO2 particles are involved in or before the slow step?

    • One
    • Zero
    • Two

  2. Which step sets the overall speed of a multi-step reaction?

    • The fastest step
    • The slowest step
    • The middle step

  3. A catalyst is remade at the end of the reaction, so it is not used up.

    Circle one:   True   False

  4. A mechanism predicts rate = k[A], but the measured rate is k[A][B]. What should you do?

    • Keep the mechanism and ignore the data
    • Reject the mechanism
    • Adjust the data to fit the mechanism

  5. Why does a reactant from a step after the slow step miss out of the rate equation?

    • Its step is too fast to affect the measured rate
    • It is not present in the reaction mixture
    • It reacts before the slow step happens

  6. A reaction is first order in X and zero order in Y. What does that mean for the slow step?

    • X is in the slow step and Y joins later
    • Y is in the slow step and X joins later
    • Both X and Y meet in the slow step

  7. A catalyst changes the measured orders of a reaction. What does that tell you?

    • It opened a new route with a different slow step
    • It only sped up the old slow step
    • It raised the barrier of the old route

  8. Two rival mechanisms predict the same measured rate equation. A student says both must be true. What is the flaw?

    • The rate equation must have been measured wrongly
    • Both mechanisms are proven true at once
    • A fit only keeps a mechanism alive, so kinetics alone cannot pick the winner
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Answer key

For grown-ups. Fold this page away before handing over the rest.

The slow step sets the pace W1-mt_PM1Vo2CkVD-s1

  1. Two · Order two in NO2 means two NO2 particles meet in or before the slow step.
  2. The slowest step · The slowest step caps the speed. Faster steps cannot push past it.
  3. True · The catalyst takes part and is then regenerated. The sentence is true.
  4. Reject the mechanism · A predicted equation that disagrees with the data kills the mechanism.
  5. Its step is too fast to affect the measured rate · Steps after the slow one finish too quickly to hold anything up.
  6. X is in the slow step and Y joins later · Order one puts X in or before the slow step, while order zero pushes Y to a later step.
  7. It opened a new route with a different slow step · New orders mean a new slow step, which means a new route. The old one was replaced.
  8. A fit only keeps a mechanism alive, so kinetics alone cannot pick the winner · Matching the data is necessary but not proof, since different routes can give the same equation.
Worksheet · LightMySky