AP Chemistry 5.10 Multistep Reaction Energy Profiles Study Notes - New Syllabus Effective fall 2024
AP Chemistry 5.10 Multistep Reaction Energy Profiles Study Notes.- New syllabus
AP Chemistry 5.10 Multistep Reaction Energy Profiles Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
Represent the activation energy and overall energy change in a multistep reaction with a reaction energy profile.
Key Concepts:
- Reaction Energy Profile for a Multistep Reaction
Reaction Energy Profile for a Multistep Reaction
A reaction energy profile (or potential energy diagram) shows how the energy of a system changes as reactants are converted into products. In a multistep reaction, the profile contains multiple energy peaks — each corresponding to an elementary step in the mechanism. Each peak represents a transition state, and the valleys between them represent intermediates.
- Each elementary step has its own activation energy— the energy required to reach the transition state for that step.
- The step with the highest activation energyis the rate-limiting step.
- The overall energy change (\( \mathrm{\Delta E_{rxn}} \)) is the difference between the energy of the products and reactants.
- Intermediates appear as valleys between two peaks — lower in energy than transition states but higher than stable reactants or products.
Diagram Representation:
- Each peak → transition state (highest energy point of a step)
- Each valley → intermediate (short-lived species formed between steps)
- Vertical distance from reactants to transition state → activation energy
- Vertical distance from reactants to products → overall energy change (\( \mathrm{\Delta E_{rxn}} \))
A multistep reaction involves multiple transition states and intermediates. The largest energy barrier (highest peak) corresponds to the rate-limiting step, and the overall energy change determines whether the reaction is exothermic or endothermic.
Example
The following mechanism occurs in two steps:
Step 1: \( \mathrm{A + B \rightarrow C} \) (slow)
Step 2: \( \mathrm{C \rightarrow D} \) (fast)
Sketch or describe the reaction energy profile and identify:
- The activation energy for each step
- The intermediate
- The overall energy change
▶️ Answer / Explanation
Step 1: Because Step 1 is slow, it has the larger activation energy . On the energy profile, this appears as the taller first peak.
Step 2: The second peak is smaller, representing a faster step with a lower activation energy.
Intermediate: The valley between the two peaks represents the intermediate (C).
Overall Energy Change: The difference between the energies of reactants and products gives \( \mathrm{\Delta E_{rxn}} \). If products are at a lower energy level, the reaction is exothermic.
Result:
- Two peaks = two transition states
- One valley = one intermediate
- First peak (larger) = rate-limiting step –
- \( \mathrm{\Delta E_{rxn}} \) = energy difference between reactants and products
Example
A reaction mechanism consists of three steps:
Step 1: \( \mathrm{X + Y \rightarrow Z} \) (moderate speed)
Step 2: \( \mathrm{Z \rightarrow P} \) (slow)
Step 3: \( \mathrm{P \rightarrow Q} \) (fast)
Which step determines the overall rate, and how will this appear on the energy profile?
▶️ Answer / Explanation
Step 1: The first peak corresponds to formation of the intermediate \( \mathrm{Z} \).
Step 2: This is the rate-limiting step and has the highest activation energy . Therefore, it appears as the tallest peak on the energy profile.
Step 3: The last step is fast, so it has the lowest energy barrier — a small final peak.
Overall Energy Change: The energy difference between reactants and final products represents \( \mathrm{\Delta E_{rxn}} \). If products are lower in energy, the reaction is exothermic; if higher, endothermic.
Result:
- The tallest peak → Step 2 → rate-determining step
- Valleys → intermediates
- Reactant-to-product difference → overall \( \mathrm{\Delta E_{rxn}} \)