Peaks (Maxima): Each peak represents a Transition State (or activated complex). This is a high-energy, unstable arrangement of atoms where old bonds are breaking and new bonds are forming simultaneously.
Troughs (Minima): The valleys between peaks represent Reaction Intermediates. These are distinct chemical species that are produced in one elementary step and consumed in a subsequent one.
Activation Energy (): The energy required for an elementary step to proceed. It is measured as the vertical distance from the energy level of the reactants (or intermediate) to the top of the succeeding peak.
Enthalpy Change (): The net energy difference between the final products and the initial reactants. If products are lower in energy, the reaction is exothermic; if higher, it is endothermic.
It is vital to distinguish between the physical nature of intermediates and transition states to understand reaction kinetics.
| Feature | Reaction Intermediate | Transition State |
|---|---|---|
| Stability | Relatively stable; exists for a finite time | Highly unstable; fleeting existence |
| Isolation | Can sometimes be isolated or detected | Cannot be isolated; theoretical state |
| Profile Location | Trough (Local Minimum) | Peak (Local Maximum) |
| Structure | Complete molecules or ions | Partial bonds in flux |
Count the Steps: Always count the number of peaks to determine the number of elementary steps in the mechanism. The number of intermediates will always be one less than the number of steps ().
Calculate Carefully: When asked for the activation energy of the second step, measure from the intermediate trough to the second peak, NOT from the initial reactant line.
Check the Sign of : Ensure you compare the very beginning (reactants) to the very end (products). Intermediates do not affect the overall calculation.
Catalyst Effects: If a catalyst is added, look for a new profile with lower peaks. A catalyst provides an alternative pathway with a lower for the rate-limiting step.
The 'Highest Peak' Trap: Students often assume the highest peak on the y-axis is the rate-limiting step. However, if a step starts from a very low-energy intermediate, it might have a larger than a step that reaches a higher absolute energy but starts from a high-energy intermediate.
Intermediates vs. Catalysts: Remember that intermediates are produced then consumed (Trough), while catalysts are present at the start, consumed, and then regenerated at the end (not usually shown as a trough on a standard profile).
