A two-step profile has step-1 barrier 60\ kJ mol^-1 and step-2 barrier 95\ kJ mol^-1. Identify the rate-determining step. [1 point]

United StatesChemistrySyllabus dot point

How does a potential-energy diagram for a multistep reaction show intermediates and identify the rate-determining step?

Topic 5.10 Multistep Reaction Energy Profile: interpret an energy diagram with more than one peak to identify intermediates, the activation energy of each step, and the rate-determining step.

A focused answer to AP Chemistry Topic 5.10, covering multistep potential-energy diagrams, identifying intermediates in the valleys, the activation energy of each step, and locating the rate-determining step from the highest barrier, with full worked examples.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-04

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section

What this topic is asking
Reading a multistep profile
Activation energy of each step
Finding the rate-determining step
Try this

What this topic is asking

The College Board (Topic 5.10) wants you to read a multistep potential-energy diagram (more than one peak) to identify intermediates, the activation energy of each step, and the rate-determining step. This extends the single-step profile of Topic 5.6 to the realistic case of a mechanism with several elementary steps, each with its own barrier.

Reading a multistep profile

A two-step mechanism therefore shows two humps with a dip between them. The dip is the intermediate; it is lower in energy than the surrounding transition states but is still a genuine, if transient, species. The reactants and products are the levels at the far left and far right.

Activation energy of each step

So to find a step's activation energy you locate where that step begins (reactants for the first step, the preceding intermediate for later steps) and subtract that energy from the step's peak. Reading each barrier from its correct starting level is the most common error to avoid.

Finding the rate-determining step

The rate-determining step is the one with the highest activation energy, the tallest barrier measured from its own starting level. It is the slowest step and the bottleneck for the overall rate, consistent with Topic 5.8. On a clearly drawn profile the rate-determining step is usually the one whose peak rises highest above the level just before it, though you should always compare the barrier heights rather than just the absolute peak heights.

Analyzing a two-step profile

A profile has reactants at $20\ \text{kJ mol}^{-1}$ , first peak at $100$ , intermediate at $60$ , second peak at $90$ , products at $30$ (all $\text{kJ mol}^{-1}$ ).

step 1 Activation energy of step 1

From reactants to first peak: $100 - 20 = 80\ \text{kJ mol}^{-1}$ .

step 2 Activation energy of step 2

From the intermediate valley to the second peak: $90 - 60 = 30\ \text{kJ mol}^{-1}$ .

step 3 Identify the rate-determining step

Step 1 has the higher barrier ( $80 > 30$ ), so step 1 is rate-determining.

step 4 Overall enthalpy

$\Delta H = 30 - 20 = +10\ \text{kJ mol}^{-1}$ ; the reaction is endothermic, regardless of the barrier heights along the way.

Try this

Q1. A two-step profile has step-1 barrier $60\ \text{kJ mol}^{-1}$ and step-2 barrier $95\ \text{kJ mol}^{-1}$ . Identify the rate-determining step. [1 point]

Cue. Step 2, because it has the higher activation energy.

Q2. Explain how a multistep profile lets you count the number of elementary steps and intermediates. [2 points]

Cue. The number of peaks equals the number of elementary steps; the number of valleys between peaks equals the number of intermediates.

Exam-style practice questions

Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AP 2023 (style)4 marksSection II (long FRQ, part). A two-step reaction has a profile with energies (in

\text{kJ mol}^{-1}

): reactants

40

, first peak

130

, intermediate (valley)

70

, second peak

110

, products

50

. (a) Calculate the activation energy of each step. (b) Identify the rate-determining step and justify. (c) Identify the location of the intermediate. (d) State whether the overall reaction is endothermic or exothermic.

Show worked answer →

A 4-point quantitative FRQ on a multistep profile.

(a) Activation energies (1 point): step 1 $= 130 - 40 = 90\ \text{kJ mol}^{-1}$ ; step 2 $= 110 - 70 = 40\ \text{kJ mol}^{-1}$ .
(b) Rate-determining step (1 point): step 1 has the higher activation energy ( $90 > 40$ ), so it is the rate-determining (slow) step; the taller barrier is the bottleneck.
(c) Intermediate (1 point): the intermediate sits in the valley between the two peaks, at $70\ \text{kJ mol}^{-1}$ .
(d) Overall (1 point): products ( $50$ ) are higher than reactants ( $40$ ), so $\Delta H = +10\ \text{kJ mol}^{-1}$ and the reaction is endothermic.

Markers reward both activation energies, identifying step 1 as rate-determining from the higher barrier, locating the intermediate in the valley, and the overall endothermic conclusion.

AP 2021 (style)1 marksSection I (multiple choice). On a two-step energy profile, a reaction intermediate is found at (A) the first peak (B) the valley between the two peaks (C) the second peak (D) the products. Justify your choice.

Show worked answer →

A 1-point conceptual MCQ. The answer is (B).

An intermediate is a real, if short-lived, species that sits in the energy valley between two transition states (the peaks). Peaks are transition states, not intermediates. The trap is confusing the intermediate (a valley) with the transition states (the peaks).

Related dot points

Sources & how we know this

AP Chemistry Course and Exam Description — College Board (2020)