R2.2.7 Energy profiles and transition states (Higher Level Only) Notes

Energy Profiles: Visualizing Activation Energy and Reaction Steps

Understanding Energy Profiles

1. As discussed earlier, an energy profile is a graphical representation of the energy changes that occur as a chemical reaction progresses.
2. It provides valuable insights into the reaction mechanism by illustrating the energy of reactants, products, and intermediate stages.

Key Features of Energy Profiles

1. Reactants and Products:
   1. The starting point on the graph represents the energy of the reactants.
   2. The endpoint represents the energy of the products.
   3. For exothermic reactions, the products are lower in energy than the reactants ($\Delta H< 0$, energy is released).
   4. For endothermic reactions, the products are higher in energy than the reactants ($\Delta H >0$, energy is absorbed).
2. Transition State:
   1. The transition state is the highest energy point along the reaction pathway.
   2. It represents a fleeting, unstable arrangement of atoms where bonds are partially broken and formed.
   3. This is shown as a peak on the energy profile.
3. Activation Energy ($E_a$):
   1. The activation energy is the energy required to reach the transition state from the reactants.
   2. It is represented as the energy difference between the reactants and the peak of the graph.
4. Rate-Determining Step:
   1. For reactions that occur in multiple steps, the rate-determining step is the slowest step, characterized by the highest activation energy.
   2. 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:

1. It features one transition state (a single peak).