Overview
This lecture covers potential energy diagrams in chemical reactions, focusing on transition states, activation energy, and the role of kinetic energy distribution among molecules in reaction rates.
Potential Energy Diagrams & Transition States
- Exothermic and endothermic reactions both have a transition state representing the highest potential energy point during the reaction.
- The transition state is unstable due to high potential energy.
- High potential energy in atoms, ions, or molecules indicates high instability.
- Every reaction passes through a highly unstable transition state, explaining the existence of activation energy.
Example: Molecular Rearrangement & Stability
- A CH₃ group bonded to a nitrogen triple bonded to carbon is less stable in its initial form.
- Rearrangement occurs so that carbon achieves four bonds (greater stability) and nitrogen holds the lone pair (due to its electronegativity).
- During this process, the bond breaks, creating a transition state where the carbon has only six valence electrons and is very unstable.
- The instability at the transition state results from not satisfying the octet rule, leading to high potential energy.
Activation Energy & Kinetic Energy
- The activation energy is the energy barrier reactants must overcome to form products.
- Not all molecules at a given temperature have the same kinetic energy; they follow a distribution called the Boltzmann distribution.
- Most molecules have moderate kinetic energy, while some have very low or high kinetic energy.
- Only molecules with kinetic energy greater than or equal to the activation energy can react.
- If the activation energy is high, fewer molecules are able to react, slowing the reaction rate.
- Lower activation energy means more molecules can react, increasing the rate.
Key Terms & Definitions
- Transition State — The point of highest potential energy during a reaction, associated with maximum instability.
- Activation Energy — The minimum energy required for reactants to transform into products.
- Thermal Energy Distribution — The spread of kinetic energies among molecules at a given temperature, often described by the Boltzmann distribution.
- Boltzmann Distribution — A graph showing the number of molecules at each possible kinetic energy in a sample.
Action Items / Next Steps
- Review potential energy diagrams and identify transition states in example reactions.
- Study the Boltzmann distribution and its role in reaction rates.