Core Concept

Energy diagrams represent the energy changes that occur during a chemical reaction. They help us visualize the difference in energy between reactants and products, the activation energy required for the reaction, and whether a reaction is exothermic or endothermic.

Key Components of an Energy Diagram

1. Reactants:

   • Represent the starting substances in a reaction.

   • Located on the left side of the energy diagram.

2. Products:

   • Represent the substances formed in a reaction.

   • Located on the right side of the energy diagram.

3. Activation Energy ($E_a$​):

   • The minimum energy required for the reactants to transform into products.

   • Represented as the energy "hump" between the reactants and the peak of the diagram.

   • This energy is needed to break existing bonds and start the reaction.

4. Transition State:

   • The highest-energy point on the energy diagram, located at the peak of the activation energy hump.

   • Represents an unstable arrangement of atoms where bonds are partially broken and new bonds are partially formed.

5. Enthalpy Change (ΔH):

   • The difference in energy between the products and reactants.

   • ΔH < 0 : Indicates an exothermic reaction (energy is released).

   • ΔH > 0 : Indicates an endothermic reaction (energy is absorbed).

Types of Energy Diagrams

1. Exothermic Reactions:

   • Energy is released during the reaction.

   • Diagram Characteristics:

     • Products have lower energy than reactants.

     • ΔH is negative.

   • Diagram Shape:

     • The line starts at a higher energy level for reactants, rises to the transition state, and then drops to a lower energy level for products.

2. Endothermic Reactions:

   • Energy is absorbed during the reaction.

   • Diagram Characteristics:

     • Products have higher energy than reactants.

     • ΔH\Delta HΔH is positive.

     • Example: Photosynthesis, where plants absorb energy from sunlight.

   • Diagram Shape:

     • The line starts at a lower energy level for reactants, rises to the transition state, and ends at a higher energy level for products.

Analyzing Energy Diagrams

1. Identify ΔH:

   • Measure the difference in energy between reactants and products.

   • If products are lower than reactants, the reaction is exothermic; if higher, the reaction is endothermic.

2. Determine Activation Energy ($E_a$​):

   • Measure the energy from the reactants to the peak (transition state) of the diagram.

3. Understand the Effect of a Catalyst:

   • In the presence of a catalyst, the peak (activation energy) is lower, which speeds up the reaction by allowing it to proceed with less energy input. Catalysts do not change ΔH or the energies of the reactants and products.