Overview
This lecture covers potential energy diagrams for chemical reactions, including how to interpret exothermic and endothermic profiles, effects of catalysts, multi-step reactions, and key energy calculations.
Potential Energy Diagrams: Basics
- Potential energy diagrams plot potential energy (y-axis) against reaction coordinate (x-axis) from reactants to products.
- The highest point on the diagram is the transition state (activated complex).
Exothermic vs. Endothermic Reactions
- Exothermic reaction: products have less energy than reactants; ΔH (enthalpy change) is negative; heat is released.
- Endothermic reaction: products have more energy than reactants; ΔH is positive; heat is absorbed.
- ΔH is calculated as energy of products minus energy of reactants.
Activation Energy and Catalysts
- Activation energy (Ea) is the energy needed to reach the transition state from reactants.
- Forward activation energy: difference between transition state and reactants.
- Reverse activation energy: difference between transition state and products.
- Catalysts lower the activation energy, increasing reaction rate, but do not change ΔH.
Multi-Step (Reaction Mechanism) Diagrams
- Multi-step reactions have multiple transition states and intermediates.
- The step with the highest activation energy is the rate-determining (slowest) step.
- Each step’s enthalpy change depends on the energy difference between its starting and ending points.
Energy Calculations (Sample)
- Forward Ea = Energy of transition state - Energy of reactants.
- Reverse Ea = Energy of transition state - Energy of products.
- ΔH = Energy of products - Energy of reactants OR Forward Ea - Reverse Ea.
Drawing Multi-Step Diagrams
- For a three-step reaction: label each transition state (TS1, TS2, etc.) and intermediate.
- The rate-determining step has the highest transition state.
- To be overall endothermic, products are higher than reactants.
Key Terms & Definitions
- Potential Energy Diagram — Graph showing energy changes during a reaction.
- Transition State (Activated Complex) — Highest energy point during a reaction step.
- Activation Energy (Ea) — Minimum energy needed for a reaction to occur.
- Catalyst — A substance that lowers activation energy, speeding up a reaction.
- Enthalpy Change (ΔH) — Heat absorbed or released, given by products minus reactants.
Action Items / Next Steps
- Practice drawing potential energy diagrams for both one-step and multi-step reactions.
- Review definitions and equations for energy calculations from the diagram.