Thermochemical Diagrams and Energy

Exothermic and Endothermic Reactions

Introduction

A discussion of thermochemical diagrams, including the identification of reactants, products, activation energy, transition state (or activated complex), and enthalpy changes for both exothermic and endothermic reactions.

Exothermic Reactions

  • Reactants: Located on the left side of the diagram, representing the starting point of the reaction.

  • Products: Located on the right side of the diagram, representing the end result of the reaction.

  • Activation Energy: The energy required to initiate the reaction, represented by the distance from the reactants' energy level to the highest point on the curve.

  • Transition State/Activated Complex: The state at the very top of the curve, representing the point of highest energy where the reaction is in transition.

    • This is where there's way more energy available to do things.
    • It is essential for a reaction to occur, as it represents overcoming the energy barrier.
    • Represents the reaction in between its two potential forms.

  • Enthalpy Change (\Delta H): The change in energy between the reactants and products.

    • In exothermic reactions, energy is lost, so \Delta H is negative (\Delta H < 0).

Endothermic Reactions

  • Reactants: Starting energy level on the left.
  • Products: Ending energy level on the right.
  • Activation Energy (E_a): The energy required to start the reaction; it is significantly larger in endothermic reactions compared to exothermic ones.
    • Symbolized as capital E with a little a after it (E_a).
  • Activated Complex/Transition State: The highest energy point in the reaction.
  • Enthalpy Change (\Delta H): The change in energy from reactants to products.
    • In endothermic reactions, energy is gained, so \Delta H is positive (\Delta H > 0).

Specific Example: Formation of Water

  • The formation of water is a highly exothermic reaction.

    • H2 + O2 \rightarrow H_2O

  • Hydrogen is used in combustion engines because it releases a lot of energy and can be recreated from water.

  • Energy in Chemical Bonds: Energy is stored within chemical bonds.

  • Breaking Bonds: Energy must be added to break the bonds of the reactants.

    • After breaking bonds, atoms have higher potential energy, placing them at the top (transition state).

  • Reconfiguring and Forming New Bonds: As atoms reconfigure, they release energy, forming new compounds at a more stable energy state.

  • Bonds break, so they all move through our activation energy up to our transition state or activated complex, and then new bonds form.

  • For the formation of water, the enthalpy change (\Delta H) is large and negative.

    • \Delta H = -486 per mole.

  • Reversibility: The reaction is potentially reversible.

    • Water can be broken apart through electrolysis to regain hydrogen and oxygen.
    • H2O \rightarrow H2 + O_2
    • In the reverse reaction, the enthalpy change (\Delta H) is positive.
      • \Delta H = +486 per mole.