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1 Atomic Structure and Properties 2 Compound Structure and Properties 3 Properties of Substances and Mixtures 4 Chemical Reactions 5 Kinetics 6 Thermochemistry 7 Equilibrium 8 Acids and Bases 9 Thermodynamics and Electrochemistry

6 Thermochemistry

6.1 Endothermic and Exothermic Processes 6.2 Energy Diagrams 6.3 Heat Transfer and Thermal Equilibrium 6.4 Heat Capacity and Calorimetry 6.5 Energy of Phase Changes 6.6 Introduction to Enthalpy of Reaction 6.7 Bond Enthalpies 6.8 Enthalpy of Formation 6.9 Hess’s Law

Energy Diagrams

Learning Objective 6.2.A Represent a chemical or physical transformation with an energy diagram.

Quick Notes

  • Energy diagrams illustrate how potential energy changes overall for a chemical or physical process.
  • They show whether a process is endothermic or exothermic by comparing reactant and product energy levels.
  • Key features:
    • Reactants on the left, products on the right.
    • ΔE = difference between product and reactant energy levels.
    • -ΔE = exothermic
    • +ΔE = endothermic

Full Notes

What Is an Energy Diagram?

An energy diagram is a graphical way of showing the overall energy change that occurs during a chemical reaction or physical process. The y-axis represents potential energy.

Note: Energy level diagrams are simpler than reaction profile diagrams (see 5.6. Energy level diagrams show only the relative energy of reactants and products, while reaction profile diagrams also show activation energy and the transition state(s).

AP Chemistry comparison of energy level diagrams and reaction profile diagrams

Exothermic Energy Diagram

AP Chemistry exothermic energy diagram with products lower in energy than reactants

ΔE < 0

Endothermic Energy Diagram

AP Chemistry endothermic energy diagram with products higher in energy than reactants

ΔE > 0

Physical Transformations

Energy diagrams also apply to phase changes and other physical transformations:

AP Chemistry diagram of ice melting as an endothermic process
AP Chemistry diagram of water freezing as an exothermic process

Matt’s Exam Tip

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Matt’s exam tip

Be clear on the difference between an energy level diagram and a reaction profile diagram:

  • An energy level diagram shows just the relative energy of reactants and products (ΔE only).
  • A reaction profile diagram shows how energy changes during the reaction, including activation energy (Ea) and the transition state.
AP Chemistry comparison of energy level diagrams and reaction profile diagrams

Summary

Energy diagrams are visual tools that represent whether a process is endothermic or exothermic. They show the energy difference (ΔE) between reactants and products and can also be extended to reaction profile diagrams that include activation energy. Understanding these diagrams allows us to connect energy changes with experimental observations, reaction spontaneity, and kinetics.