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Multistep energy profiles

A multistep reaction's energy profile is a mountain range, not a single hill: one peak per step, a valley for each intermediate. The tallest peak is the bottleneck, and the start-to-finish drop is still the overall ΔH.

§1

Peaks, valleys, and the tallest barrier.

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A multistep reaction energy profile shows one barrier (transition state) for each elementary step — so a two-step reaction has two peaks. Between the peaks sits a valley: an intermediate, a real species formed along the way.

The highest barrier corresponds to the slowest, rate-determining step. That tallest peak is the bottleneck that limits the overall rate.

The overall ΔH is still just the difference between the starting reactants and the final products — read from the two ends of the profile, ignoring the peaks and valleys in between.

UNIT 5 TOPIC 5.10 • MULTISTEP REACTION ENERGY PROFILE MULTISTEP REACTION ENERGY PROFILE potential energy reaction coordinate TS1 TS2 TS3 I1 I2 reactants products largest Ea Peaks = transition states TS sit at the tops of the barriers — points of maximum potential energy. Valleys = intermediates Intermediates (I) sit in energy wells between the steps; real species. Rate-determining step The step with the LARGEST activation- energy barrier is the slowest and sets the overall rate. Here that is step 2 (the climb from I1 up to TS2). CED ANCHOR A reaction mechanism is a sequence of elementary steps. The slowest step — the one with the highest Ea — is rate- determining and controls the rate law. AP Chemistry · Unit 5 · Kinetics
Fig. 5.10.1 A multistep reaction has one barrier per step. Each peak is a transition state; each valley between peaks is an intermediate. The highest barrier corresponds to the slowest, rate-determining step, and the overall ΔH is still reactants minus products.
§2

Reading a multistep profile.

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Identify peaks, valleys, the tallest barrier, and the endpoints.

  1. Count the peaks. One transition state per elementary step; the number of peaks equals the number of steps.
  2. Find the intermediates. The valleys between peaks are intermediates — real species, unlike the fleeting transition states.
  3. Locate the tallest barrier. The highest peak is the rate-determining step, the overall bottleneck.
  4. Read the overall ΔH. Compare the first reactant energy to the final product energy; the in-between features do not change ΔH.
§3

The pieces you'll meet.

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Peaks are transition states; valleys are intermediates.

peaks
Peaks
Transition states — one per elementary step.
valleys
Valleys
Intermediates — real species between steps.
tallest
Tallest barrier
The rate-determining step; the overall bottleneck.
intermediate
Intermediate
A valley species; more stable than a transition state.
ΔH
Overall ΔH
Reactants to final products, ignoring in-between features.
steps
Step count
Number of peaks equals the number of steps.
§4

Worked example: a two-step profile.

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Profile. Reactants → peak 1 (tall) → valley (intermediate) → peak 2 (shorter) → products, with products below reactants.

Steps. Two peaks mean two elementary steps; the valley between is an intermediate.

Rate-determining step. Peak 1 is taller, so the first step has the higher barrier and is rate-determining.

Overall ΔH. Products sit below reactants, so ΔH < 0 (exothermic) — read from the two ends, independent of the two peaks and the valley.

§5

Mistakes that cost real points.

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Pitfall · 01

"The valleys in a multistep profile are transition states."

The valleys are intermediates — real, if short-lived, species that sit at energy minima between steps. The peaks are the transition states. Confusing valleys (intermediates) with peaks (transition states) inverts the picture.

Fix. Read peaks as transition states and valleys as intermediates. Intermediates are minima; transition states are maxima.

Pitfall · 02

"The rate-determining step is the one with the lowest barrier."

The rate-determining step has the highest barrier (tallest peak), not the lowest — it is the hardest step to get over and thus the bottleneck. Picking the lowest peak reverses the logic.

Fix. Identify the rate-determining step as the tallest barrier on the profile.

Pitfall · 03

"The overall ΔH is the height of the biggest barrier."

Overall ΔH is the difference between the initial reactants and the final products, not any barrier height. A profile can have a huge barrier but a small ΔH, or vice versa; the two are unrelated features.

Fix. Read overall ΔH from the two endpoints of the profile; ignore the peaks and valleys in between.

§6

Skill Check.

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