In AP Chemistry, an intermediate is a species that is produced in one elementary step of a reaction mechanism and consumed in a later step, so it never appears in the overall balanced equation or in a valid rate law.
An intermediate is a real chemical species (it actually exists, briefly) that gets made in one elementary step of a mechanism and then used up in a later step. Because it's both created and destroyed inside the mechanism, it cancels out when you add the steps together. That's why you never see it in the overall balanced equation.
Here's the catch that AP loves: experimentally determined rate laws can only contain species you can actually put in a flask, meaning reactants (and sometimes products or catalysts). Intermediates exist for too short a time to control or measure their concentration. So if the slow step of a mechanism involves an intermediate, you can't just write the rate law from that step's stoichiometry and call it done. You have to use an approximation, like the pre-equilibrium approach, to swap the intermediate's concentration for an expression written in terms of reactants. That substitution skill is exactly what Topics 5.4 and 5.9 are testing.
Intermediates live in Unit 5 (Kinetics), specifically Topics 5.4 (Elementary Reactions) and 5.9 (Steady-State / Pre-Equilibrium Approximation). Learning objective 5.4.A lets you write a rate law straight from an elementary step's stoichiometry, but that only works for single steps. Learning objective 5.9.A kicks in when the first step is NOT rate limiting. Per essential knowledge 5.9.A.1, you then need an approximation like pre-equilibrium to eliminate the intermediate from the rate law. In short, the entire point of Topic 5.9 is dealing with intermediates that sneak into the slow step. If you can spot an intermediate in a mechanism and substitute it out, you've mastered one of the most predictable question types in Unit 5.
Keep studying AP Chemistry Unit 5
Reaction Mechanism (Unit 5)
A mechanism is the step-by-step story of a reaction, and intermediates are the characters who appear mid-story and exit before the end. Checking that intermediates cancel when you sum the steps is how you verify a mechanism matches the overall equation.
Rate-Determining Step (Unit 5)
If the slow step contains only original reactants, the rate law comes straight from it. If the slow step contains an intermediate, you can't stop there. That single check decides whether a problem is a Topic 5.4 question or a Topic 5.9 question.
Catalyst (Unit 5)
Catalysts and intermediates are mirror images in a mechanism. A catalyst is consumed first and regenerated later, while an intermediate is generated first and consumed later. Both cancel out of the overall equation, so the order of appearance is how you tell them apart.
Transition State (Unit 5)
On an energy profile diagram, transition states are the peaks and intermediates are the valleys between peaks. A transition state is a fleeting arrangement of atoms mid-collision, while an intermediate is an actual species with a (short) lifetime.
Intermediates show up constantly in multiple-step mechanism questions. The classic MCQ gives you a mechanism with a fast equilibrium first step and a slow second step, then asks for the overall rate law. Practice problems like the NO₂ + CO mechanism (fast: NO₂ + NO₂ ⇌ NO + NO₃, slow: NO₃ + CO → NO₂ + CO₂) are the template. NO₃ is the intermediate, so you set the forward and reverse rates of step 1 equal, solve for [NO₃], and substitute into the slow step's rate law. The same pattern appears in N₂O₅ decomposition mechanisms. On the free-response side, the 2019 exam used NO₂ decomposition in a kinetics FRQ, and mechanism questions routinely ask you to identify the intermediate by name or formula, justify why it can't appear in the rate law, or distinguish it from a catalyst. Expect to do three things: spot it, explain it, and substitute it out.
Both intermediates and catalysts appear in mechanism steps but cancel out of the overall equation, so they look similar on paper. The difference is timing. A catalyst is consumed in an early step and regenerated in a later step (it's there at the start and the end). An intermediate is produced in an early step and consumed in a later step (it's absent at the start and the end). Quick test: find the species' first appearance. On the reactant side first means catalyst; on the product side first means intermediate.
An intermediate is produced in one elementary step and consumed in a later one, so it cancels out and never appears in the overall balanced equation.
A valid overall rate law can never contain an intermediate, because you can't measure or control an intermediate's concentration experimentally.
If the slow (rate-determining) step contains an intermediate, use the pre-equilibrium approximation to replace the intermediate's concentration with an expression in terms of reactants.
To tell an intermediate from a catalyst, check where the species first appears: products side first means intermediate, reactants side first means catalyst.
On an energy diagram, an intermediate sits in a valley between two activation energy peaks, while a transition state sits at a peak.
An intermediate is a chemical species that forms in one step of a reaction mechanism and gets consumed in a later step. It cancels out when the steps are added, so it appears in neither the overall equation nor the rate law.
No, not in the final answer. If an intermediate shows up in the slow step's rate law, AP expects you to use the pre-equilibrium approximation (per LO 5.9.A) to substitute it out and express the rate law in terms of reactants only.
An intermediate is made first and used up later; a catalyst is used up first and regenerated later. For example, in the NO₂ + CO mechanism, NO₃ is an intermediate because step 1 produces it and step 2 consumes it.
No. A transition state is the unstable, highest-energy arrangement at the peak of an energy diagram and exists only for an instant. An intermediate is a real species that sits in an energy valley between two peaks and can, in principle, be detected.
Look for a species that appears as a product in one step and a reactant in a later step, and that's missing from the overall equation. In the N₂O₅ decomposition mechanism, NO₃ fits that pattern, so it's the intermediate.
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