Energy Diagrams
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Additional Worked Out Examples/ Practice
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Relating Properties to Composition: Predicting classification based on descriptive properties
Topic Summary & Highlights
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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.
Practice Tips
The energy levels at the start and end of the diagram represent reactants and products, indicating if the reaction is exothermic or endothermic.
The transition state at the curve's peak is the highest energy, unstable intermediate where bonds are partially broken and formed.
Exothermic reactions release energy with products lower in energy than reactants, while endothermic reactions absorb energy with higher-energy products.
Catalysts lower the activation energy by providing an alternative pathway, reducing the peak's height in the diagram.
The x-axis shows the reaction progress, and the y-axis represents potential energy throughout the reaction.
Higher activation energy correlates with slower reaction rates, as fewer molecules have enough energy to overcome the barrier.
Types of Energy Diagrams
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.
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.
Key Components of an Energy Diagram
Reactants:
Represent the starting substances in a reaction.
Located on the left side of the energy diagram.
Products:
Represent the substances formed in a reaction.
Located on the right side of the energy diagram.
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.
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.
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).
Analyzing Energy Diagrams
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.
Determine Activation Energy ($E_a$):
Measure the energy from the reactants to the peak (transition state) of the diagram.
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.