Potential Energy Changes and Catalysts

Aim

  • Understand how potential energy changes can predict the transition from reactants to products during chemical reactions.

Warm-up Questions

  1. What is the role of a catalyst in a chemical reaction?
  2. Explain how a catalyst may increase the rate of a chemical reaction.

Key Ideas on Catalysts

  • Catalysts speed up chemical reactions by lowering the activation energy required.
  • They do not get consumed in the reaction and can be reused.

Chemical Bonds and Potential Energy

  • Chemical bonds store energy known as Potential Energy:
    • This energy is available to perform work during reactions.
  • Potential Energy Diagrams visually represent changes in energy throughout reactions.

Collisions in Reactions

  • Kinetic Energy is converted to Potential Energy during collisions between reactants.
  • Reactants must collide effectively to react and generate products.

Activated Complex

  • Proper collision orientation is essential:
  • An Activated Complex is a temporary state formed after initial collisions. This complex can:
    • Either revert back to reactants.
    • Rearrange to create final products.

Reading Potential Energy Diagrams

  • Axes:
    • Vertical Axis: Change in Potential Energy.
    • Horizontal Axis: Reaction Coordinate.
  • Example:
    • For the reaction: A + B
      ightarrow C + D + ext{HEAT}
    1. Reactants/Potential Energy (A + B)
    2. Activated Complex/Potential Energy (Peak of the diagram)
    3. Products/Potential Energy (C + D)

Questions on Potential Energy

  • Why is the potential energy of products less than that of reactants? This indicates that a reaction releases energy (exothermic).

Understanding Energy Changes in Diagrams

  • Lines in Diagrams:
    • Line 4: Activation Energy to form the activated complex (difference between reactants and activated complex).
    • Line 5: Heat of Reaction (ΔH) (difference between reactants and products).
    • Line 6: Activation energy for the reverse reaction (difference between activated complex and products).

Influence of Catalysts on Potential Energy Diagrams

  • Dotted lines represent the effect of catalysts:
    • The catalyst lowers the activation energy, thereby increasing reaction rate without altering the overall energy change of the reaction.

Heats of Reaction Table

  • Various reactions show the change in enthalpy (ΔH) under standard conditions (101.3 kPa, 298 K).
  • Exothermic reactions have negative ΔH values, indicating energy release, while endothermic reactions have positive ΔH values.

Six Main Outcomes of Reactions

  1. Endothermic Reactions:

    • Products have more energy than reactants; thus, ext{ΔH} > 0.
    • More heat absorbed results in higher energy products.

  2. Exothermic Reactions:

    • Products are lower in energy than reactants; thus, ext{ΔH} < 0.
    • Energy is released when the products form.

Practice Questions

  1. For the reaction: ext{C}6 ext{H}4 ext{Cl}2(s) + ext{energy} ightarrow ext{C}6 ext{H}4 ext{Cl}2(g) , classify as endothermic or exothermic.
  2. Analyze a given energy diagram for reaction A + B --> C and assess whether it is endothermic or exothermic, supporting your answer with energy reasoning.

Exit Ticket

  • Summarize what you've learned about potential energy changes in reactions and the role of catalysts in influencing reaction rates.