Read a Mechanism's Energy Profile Before Naming Its Steps
Mechanism questions reward identifying each species' role — catalyst, intermediate, or product — from the energy profile before answering anything about rate.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Catalyst vs intermediate vs product on an energy diagram
A catalyst appears at the start and end of the mechanism unchanged, lowering the activation energy of every step it participates in; an intermediate is formed in one step and consumed in a later one, appearing as a local minimum on the energy profile, never in the overall balanced equation.
Catalysts lower activation energy, and the effect can depend on temperature
The lowering in activation energy a catalyst provides is generally not identical at two different temperatures — comparing a catalysed and uncatalysed reaction at T₁ versus T₂ needs the Arrhenius equation applied at each temperature separately, not a single flat 'catalyst effect' assumed to be temperature-independent.
Collision theory and the steric factor
A steric factor different from 1 signals that the simple collision-theory rate (based purely on collision frequency and energy) doesn't match the experimental rate — this is usually attributed to molecular orientation requirements or structural complexity of the reactant, not to activation energy itself being different.
OlympIQ exam lens
- ✓A catalyst is unchanged at the start and end of a mechanism; an intermediate is formed then consumed partway through.
- ✓A catalyst's effect on activation energy and rate is not automatically the same at every temperature — check with the Arrhenius equation.
- ✓A steric factor away from 1 reflects orientation/structural effects, not a change in activation energy.
Don't call a species that appears mid-mechanism and later disappears a catalyst — that's the definition of an intermediate; a true catalyst is present unchanged before the first step and after the last.
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