Build an elementary-step sum and test its rate prediction

Try: a balanced step is not the whole route

The original symbolic target is A₂ + 2 B → 2 AB. Letters label fictitious conserved atom types, not real chemical elements. The default route starts with a slow A₂ → 2 A event, followed by fast A + B → AB. Using each once leaves one A in the net products. Predict and repair the second count before inspecting the complete sum.

Two repetitions of A + B → AB mean two separate two-particle events. They do not create one four-particle collision or change that elementary step’s concentration exponents. Atom totals are conserved even in an incomplete sum; the residual species ledger tests the additional target-ratio requirement.

Understand what an elementary premise permits

Molecularity counts the reactant entities in one elementary event. A stipulated unimolecular A₂ event has rate k[A₂]; an elementary A₂ + B event has rate k[A₂][B]. Those statements use the explicit elementary premise. Reading the overall target as k[A₂][B]² would invent a simultaneous three-particle event that the supplied mechanisms do not contain. Three-body elementary events are rare; an overall equation does not establish one.

The first event is assumed rate limiting and later completion rapid, with negligible intermediate accumulation. Under that approximation, define r as A₂ disappearance for one primitive target cycle. AB formation is approximately 2r. Writing two whole target cycles changes the written sum, not this reference definition or the elementary collision law. Initial transient behavior and accumulating intermediates are not simulated.

Experiment: balance and rate can disagree

Repair the unimolecular route using step counts 1 and 2. It now reproduces the target, yet doubling B does not change its predicted slow-event rate. The original public rate packet does double when B doubles, so this route’s stated kinetic model conflicts with the packet.

The B-assisted route uses A₂ + B → AB + A, then A + B → AB. Counts 1 and 1 reproduce the target. Its baseline rate 0.004 mol L⁻¹ s⁻¹ at A₂=0.100 and B=0.200 mol/L gives k=0.200 L mol⁻¹ s⁻¹. Doubling either reactant doubles the predicted rate. The separate free-C change does not affect that law. Later rows test the baseline-calibrated constant rather than refitting it.

Inspect cancellation roles

The third route consumes C in A₂ + C → AC + A, restores C in AC + B → AB + C, then consumes A in A + B → AB. With one of each event, A and AC are intermediates: produced before being consumed. C is supplied, consumed and regenerated. All cancel, but their sequence roles differ. C is only a proposed carrier here; regeneration alone does not demonstrate real acceleration or lower barriers.

The third route’s stipulated slow event predicts dependence on free C and none on B, conflicting with this public packet. Free C is not automatically total carrier concentration when bound forms exist. No binding equilibrium or carrier inventory approximation is silently substituted.

Apply and continue

Capture a route that has the correct overall ratio but fails a controlled rate comparison. Explain which observation contradicts which premise. Then apply the unfamiliar elementary-step question: change only one reactant concentration rather than multiplying every concentration by the same factor. A matching candidate remains a hypothesis; other routes can share its net equation and rate law.

OpenStax Chemistry 2e §12.6 supplies the mechanism conventions. Revisit controlled initial-rate comparisons for the evidence design and HS catalyst/equilibrium reasoning for the distinction between faster approach and changed equilibrium. Continue with the separately authored pre-equilibrium, collision-energy/orientation and multistep energy-profile activities. Real catalyst investigations and independent learner acceptance remain separate evidence requirements.

Use an intermediate observation as an additional constraint

A net equation and even a matching rate law can fit different pathways. In the hypothetical carrier route, AC is a bound intermediate; the simpler B-assisted route has no AC species. A validated, appropriately timed AC-specific signal would support an AC-containing route over that particular AC-free alternative. It would not prove the complete mechanism or rule out an unmodeled side pathway.

Changed supplied evidence: A fictional detector records a transient AC signal above a calibrated blank during product formation, then the signal falls. Another record reports no signal but supplies no detection limit. Which constrains the candidate routes more directly?