Energy Profiles: Visualizing Activation Energy and Reaction Steps
Understanding Energy Profiles
- As discussed earlier, an energy profile is a graphical representation of the energy changes that occur as a chemical reaction progresses.
- It provides valuable insights into the reaction mechanism by illustrating the energy of reactants, products, and intermediate stages.
Note
- The horizontal axis represents the reaction coordinate, which tracks the progress of the reaction from reactants to products.
- The vertical axis represents the potential energy of the system.
Key Features of Energy Profiles
- Reactants and Products:
- The starting point on the graph represents the energy of the reactants.
- The endpoint represents the energy of the products.
- For exothermic reactions, the products are lower in energy than the reactants ($\Delta H< 0$, energy is released).
- For endothermic reactions, the products are higher in energy than the reactants ($\Delta H >0$, energy is absorbed).
- Transition State:
- The transition state is the highest energy point along the reaction pathway.
- It represents a fleeting, unstable arrangement of atoms where bonds are partially broken and formed.
- This is shown as a peak on the energy profile.
- Activation Energy ($E_a$):
- The activation energy is the energy required to reach the transition state from the reactants.
- It is represented as the energy difference between the reactants and the peak of the graph.
- Rate-Determining Step:
- For reactions that occur in multiple steps, the rate-determining step is the slowest step, characterized by the highest activation energy.
- This step acts as a bottleneck, limiting the overall reaction rate.
Single-Step Reactions
In a single-step reaction, the energy profile is relatively simple:
- It features one transition state (a single peak).
- There are no intermediates—only reactants and products.
Example
- Consider the reaction:
$$A + B \rightarrow C + D$$ - If this reaction is exothermic ($\Delta H< 0$), the energy profile will show:
- Reactants at a higher energy level than products.
- A single peak representing the activation energy.
Example
Combustion of Hydrogen
The reaction:
$$2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$$
is a single-step exothermic reaction. Its energy profile would show:
- A single peak, representing the activation energy.
- Products ($H_2O$) at a lower energy level than reactants ($H_2$ and $O_2$).
In this reaction:
- The activation energy ($E_a$) corresponds to the energy needed to break the $H-H$ and $O-O$ bonds.
- The transition state represents the point where new bonds between hydrogen and oxygen are partially formed.
Multistep Reactions
- Many reactions proceed through multiple steps, each with its own transition state and intermediate.
- The energy profile for such a reaction is more complex, featuring:
- Multiple peaks, each corresponding to a transition state.
- Intermediates, which are more stable than the transition states but less stable than the reactants or products.
Identifying the Rate-Determining Step
- In multistep reactions, the rate-determining step is the step with the highest activation energy.
- This step determines the overall reaction rate.
Example
Reaction of Nitrogen Dioxide with Carbon Monoxide
- The reaction:
$$NO_2(g) + CO(g) \rightarrow NO(g) + CO_2(g)$$
proceeds in two steps:- $NO_2 + NO_2 \rightarrow NO_3 + NO$ (slow, rate-determining step).
- $NO_3 + CO \rightarrow NO_2 + CO_2$ (fast step).
- The energy profile for this reaction shows:
- Two peaks (two transition states).
- An intermediate ($NO_3$) between the peaks.
- The first peak is higher than the second, confirming that the first step is the rate-determining step.
Tip
The number of peaks in an energy profile corresponds to the number of transition states, which equals the number of elementary steps in the reaction mechanism.
Interpreting Energy Profiles in Context
Energy profiles are not just theoretical: they are practical tools for analyzing reaction mechanisms and optimizing reaction conditions. For instance:
- Catalysts:
- A catalyst lowers the activation energy by providing an alternative reaction pathway.
- On an energy profile, this appears as a lower peak, while the overall energy change ($\Delta H$) remains unchanged.
- Temperature Effects:
- Increasing the temperature gives reactant molecules more kinetic energy, increasing their ability to overcome the activation energy barrier.
Common Mistake
- Students often confuse intermediates with transition states.
- Remember, intermediates are relatively stable species that can sometimes be isolated, while transition states are fleeting and cannot be directly observed.
