Getting Started
Most chemical reactions do not occur in a single, simple collision. Instead, they proceed through a sequence of simpler, fundamental reactions, collectively known as a reaction mechanism. This chapter explores how we can visualize the energy changes at the molecular level for these complex, multistep processes using reaction energy profiles, which map the journey from reactants to products.
What You Should Be Able to Do
After completing this section, you will be able to:
Sketch a reaction energy profile for a given two- or three-step reaction mechanism.
Label the reactants, products, reaction intermediates, and transition states on an energy profile.
Identify and distinguish the activation energy for each elementary step.
Determine the overall enthalpy change (ΔH) for the reaction from its energy profile.
Locate the rate-determining step by analyzing the activation energies of the mechanism.
Key Concepts & Analysis
We will analyze multistep reactions through the lens of Process and Causation, tracking the transformation of reactants into products through a series of distinct steps, each with its own energy requirements and consequences for the overall reaction rate.
Inputs & Preconditions: Reactants and Initial Energy
The starting point for any reaction is the reactants. On a reaction energy profile, the potential energy of these initial substances establishes the baseline energy level. The reaction begins from this state, and all subsequent energy changes are measured relative to it. For a reaction to proceed, reactant molecules must collide with sufficient energy and in the correct orientation.
Key Steps / Mechanism: The Path from Reactants to Products
A reaction mechanism describes the sequence of elementary steps—individual molecular events—that lead from reactants to products. Most reactions involve one or more short-lived, high-energy species that are not present at the beginning or end.
Elementary Step: Each individual step in a mechanism. On an energy profile, each elementary step is represented by a single "hump."
Transition State (or Activated Complex): At the peak of each energy hump is a transition state. This is a fleeting, high-energy, and unstable arrangement of atoms as bonds are breaking and new bonds are forming. It is the maximum energy point during an elementary step and cannot be isolated.
Reaction Intermediate: In a multistep reaction, the product of one elementary step may be the reactant for the next. Such a species is called a reaction intermediate. Intermediates are located in the energy "valleys" between the peaks on an energy profile. They are more stable than transition states but are typically consumed before the final products are formed.
Consider a generic two-step reaction:
Step 1: A + B → I
Step 2: I + C → D
Here, A, B, and C are reactants, D is the final product, and I is a reaction intermediate. This mechanism would have two transition states and one intermediate, resulting in an energy profile with two humps.
Outputs & Effects: Products and Overall Energy Change
The final substances formed are the products. Their potential energy represents the final energy level on the profile. The overall enthalpy of reaction (ΔH) is the difference between the potential energy of the products and the potential energy of the reactants.
Exothermic Reaction: If the products are at a lower energy level than the reactants, ΔH is negative, and the reaction releases energy.
Endothermic Reaction: If the products are at a higher energy level than the reactants, ΔH is positive, and the reaction absorbs energy.
Controls & Limiting Factors: The Rate-Determining Step
In any multistep process, there is always one step that is slower than all the others. This is the rate-determining step (RDS), and it acts as a bottleneck, limiting the overall speed at which products can be formed.
Activation Energy (Ea): The energy required to get from the reactants (or intermediate) to the transition state for a given step. It is the height of the energy barrier for that step.
Identifying the RDS: The rate-determining step is the elementary step with the largest activation energy. On an energy profile, this corresponds to the tallest energy hump relative to its starting valley. The overall reaction cannot proceed faster than its slowest step.
Key Models & Representations
The reaction energy profile is the central model for visualizing the energetics of a multistep reaction. The table below deconstructs the key features of a typical two-step profile.
| Feature on Energy Profile | Description | What It Represents Chemically |
|---|---|---|
| Peaks (Maxima) | The highest energy points on the reaction coordinate. | Transition States: Unstable, momentary arrangements of atoms where bonds are breaking and forming. |
| Valleys (Minima) | The low-energy points between two peaks. | Reaction Intermediates: Species that are produced in one elementary step and consumed in a subsequent one. |
| Activation Energy (Ea) | The energy difference between a starting point (reactants or intermediate) and the peak of the subsequent energy barrier. | The minimum energy required for an elementary step to occur. The highest Ea identifies the rate-determining step. |
| Enthalpy Change (ΔH) | The net energy difference between the final products and the initial reactants. | The overall heat absorbed or released by the reaction. It is independent of the mechanism or activation energies. |