Catalytic Cycles: Tracing Steps Around a Closed Loop · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Walking around a catalytic loop

Science · Chemistry · ages 21-22
Name ______________________   Date ____________

  1. A proposed iridium cycle starts with Ir(I), 16 electrons, and ends with Ir(I), 16 electrons. What do these endpoint data show?

    Start: Ir(I), 16 electronsEnd: Ir(I), 16 electrons
    • The final metal has a higher oxidation state than the start
    • The starting oxidation state and electron count are restored
    • The endpoint data alone prove every intermediate is correctly balanced

  2. What is an elementary step?

    • One molecular event within a mechanism
    • The sum of all reactions in a mechanism
    • The time taken for one catalyst turnover

  3. Every catalytic network has exactly one slow step that alone determines its rate under all conditions.

    Circle one:   True   False

  4. For formal ionic counting, neutral L3Pd(II)Cl2 has three neutral two-electron L ligands and two anionic two-electron chloride ligands. Pd is in group 10. What is its electron count?

    • 14 electrons
    • 16 electrons
    • 18 electrons

  5. When adding proposed reaction steps, how must a species appearing on both sides be handled?

    • Cancel equal amounts and retain any excess on either side
    • Cancel its entire amount whenever it occurs anywhere on both sides
    • Retain its full amount on both sides, even when the amounts match

  6. Experiments show oxidative addition dominates turnover in a stated regime. Which change is most directly aimed at raising the rate, if other steps and catalyst stability remain unchanged?

    • Lowering the rate of oxidative addition
    • Lowering its activation free-energy barrier
    • Lowering the amount of active catalyst present

  7. Two runs use the same reactants, solvent, temperature, concentrations and catalyst loading, with only the ligand set changed. Different major products form. What does this controlled comparison support?

    • The solvent must have changed despite the stated controls
    • The rate-controlling step must no longer affect turnover
    • The ligand set affects the relative product pathways

  8. A proposed coupling scheme joins the bound Ar and R groups into ArR and regenerates Pd(0), but labels this transformation transmetalation. Which correction follows the taught cycle?

    • Call the bond-forming transformation reductive elimination
    • Keep transmetalation because it transfers both groups into the product
    • Call it oxidative addition because forming ArR raises the metal oxidation state
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Answer key

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

Walking around a catalytic loop W1-mt_FmsXXdFRNC-s1

  1. The starting oxidation state and electron count are restored · Both endpoints specify Ir(I) with 16 electrons. These properties match, but matching endpoints do not prove that every intermediate is correct.
  2. One molecular event within a mechanism · An elementary step represents one molecular event. A complete mechanism can contain several such events.
  3. False · Rate control can be shared and can change with concentrations, temperature or catalyst populations.
  4. 18 electrons · Pd(II) contributes 8 d electrons, three L ligands contribute 6 and two chloride ligands contribute 4: 18 total.
  5. Cancel equal amounts and retain any excess on either side · Only equal stoichiometric amounts cancel. Any excess remains in the net equation.
  6. Lowering its activation free-energy barrier · Lowering the barrier targets the step currently dominating turnover and can raise the rate. Another step may then become rate-controlling and limit the gain.
  7. The ligand set affects the relative product pathways · With the listed conditions fixed, the changed product ratio supports an effect of ligand choice on selectivity. It does not identify the exact affected step.
  8. Call the bond-forming transformation reductive elimination · Reductive elimination joins the two bound carbon groups and regenerates Pd(0). Transmetalation replaces a metal-bound group using another reagent.
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