UNIT: 5.6 Notes on Reaction Energy Profiles

Reaction Energy Profiles

• Overview: Unit 5.6 focuses on reaction energy profiles, their components, and how to construct them.

Key Concepts in Reaction Kinetics

• Collision Theory:
  • Reactions occur when reactive particles collide.
  • Two additional conditions are required for a reaction to happen:
  • Appropriate molecular orientation during the collision.
  • Sufficient energy during the collision, termed as activation energy.

Energy Profile Axes

• X-Axis:

  • Labeled as reaction progress or reaction coordinate.
  • Represents the progression of the reaction from reactants to products.

• Y-Axis:

  • Labeled as potential energy.
  • Displays the energy stored in the bonds of reactants and products.

Components of Reaction Energy Graphs

• Reactants:

  • Located on the left side of the graph, representing their potential energy.

• Products:

  • Found on the right side, usually at a lower energy than reactants in an exothermic reaction.

• Activation Energy (Ea):

  • Represented as E_a.
  • The energy needed to transition from reactants to products, visualized as a 'hill' on the graph.
  • Example Calculation:
    • If reactant energy = 2 kJ and peak (transition state) = 10 kJ, then:
      $E_a = 10 ext{ kJ} - 2 ext{ kJ} = 8 ext{ kJ}$.

• Transition State:

  • Also known as the activated complex.
  • It's a transient state where some bonds are broken, and some are formed, situated at the top of the energy hill.

Change in Energy (ΔE)

• Represented as ΔE or sometimes ΔH (for heat or enthalpy change).
  • Indicates the overall energy difference between reactants and products:
  • If products are lower in energy than reactants, it signifies an exothermic reaction (energy released).
  • Conversely, if products are higher, it signifies an endothermic reaction (energy absorbed).
    • Endothermic Curve Example:
    • Reactants at a lower energy than products in the energy profile.

Energy Curve Examples

• Exothermic Reaction:

  • Graph shows reactant energy higher than product energy.

• Endothermic Reaction:

  • Graph shows product energy higher than reactant energy.

Sample Question Analysis

• Given Reactions:

  • Reaction 1 (exothermic - lower product energy).
  • Reaction 2 (endothermic - higher product energy).

• Activation Energy Comparison:

  • Activation energy for reaction 1 is greater than reaction 2.

• Faster Reaction Prediction:

  • Reaction 2 will be faster due to its lower activation energy.
  • Claim-Evidence-Reasoning Approach:
  • Claim: Reaction 2 is faster.
  • Evidence: It has lower activation energy.
  • Reasoning: More collisions will have enough energy to form products.

Assignment and Review

• Practical Instruction:
  • Label a reaction energy graph indicating:
  • Potential energy
  • Reaction coordinate
  • Product energy
  • Activated complex energy
  • Activation energy
  • Change in enthalpy (ΔH)
  • Final Thoughts:
  • Discuss the activation energy of forward vs reverse reactions based on given energy diagrams, emphasizing that the forward reaction often has a greater activation energy in exothermic cases.