Practice a College Board-style short-answer question on Kinetics. Write your response, then reveal the model answer to see exactly what earns each point.
Short Answer Question · Unit 5 · Reaction Mechanisms & Rate Law
The decomposition of ozone, 2O₃(g) → 3O₂(g), is proposed to occur through the following two-step mechanism:
Step
Elementary reaction
Relative speed
1
O₃ → O₂ + O
Slow
2
O₃ + O → 2O₂
Fast
The experimentally determined rate law for the overall reaction is rate = k[O₃].
A
Verify that the two-step mechanism sums to the overall balanced equation, and identify any intermediate species.
✓ Model answer (earns the point)
Adding the two steps: (O₃ → O₂ + O) + (O₃ + O → 2O₂) gives 2O₃ → 3O₂ after canceling the O that appears as a product in Step 1 and a reactant in Step 2 — matching the overall balanced equation. The species O (atomic oxygen) is the intermediate, since it is produced in Step 1 and fully consumed in Step 2, never appearing in the overall equation.
Why it scores: Shows the addition of the two steps with the canceling species explicitly identified AND names O as the intermediate with a reason (produced then consumed).
B
Explain why the experimentally determined rate law, rate = k[O₃], is consistent with Step 1 being the rate-determining step.
✓ Model answer (earns the point)
Since Step 1 (O₃ → O₂ + O) is the slow, rate-determining step, the predicted rate law comes directly from its molecularity: rate = k[O₃], since O₃ is the only reactant in that elementary step. This matches the experimentally observed rate law exactly, which supports the validity of this proposed mechanism.
Why it scores: Explicitly derives the rate law from the rate-determining step's molecularity AND confirms it matches the given experimental rate law, rather than just restating that they match.
C
Sketch (in words) how the reaction energy profile for this two-step mechanism would differ from a single-step reaction, and identify which peak on the multistep profile would be higher.
✓ Model answer (earns the point)
Unlike a single-step reaction's profile (one smooth hump from reactants to products), this two-step mechanism's energy profile would show two separate peaks (one for each elementary step) with a dip between them representing the intermediate, O, at a local energy minimum. Because Step 1 is the slow, rate-determining step, it has the higher activation energy — its peak would be taller than the peak for Step 2, reflecting that Step 1 requires more energy to overcome and is therefore the bottleneck of the overall reaction.
Why it scores: Describes the two-peaks-with-a-dip structure (intermediate at a local minimum) AND correctly identifies that the rate-determining step's peak (Step 1) is the taller one, with a stated reason.
How to score points on SAQs
Always show the mechanism summing to the overall equation. Cancel out the intermediate explicitly rather than just asserting the steps add up.
Name the intermediate and justify it. "O is the intermediate because it's made in Step 1 and consumed in Step 2" scores higher than just naming it.
Derive rate laws from the rate-determining step's molecularity, not the overall equation. Show this connection explicitly for full credit.
On energy profile questions, describe shape AND reasoning. State which step's peak is higher and explain why (it's the rate-determining/slowest step).
Keep it tight. 2–4 sentences per part is plenty. Long answers don't score higher; they just waste exam time.