Activation Energy

Understanding Kinetics: Effect of Temperature on Reaction Rate

Introduction

  • Discussion focuses on:
  • Effect of temperature on reaction rates
  • Concept of activation energy (Eₐ)
  • Experimental determination of Eₐ in the kinetics lab
  • Reference: Section 13.4 of Chang text

Molecular Collisions

  • Reactions initiated by collisions between molecules.
  • Not all collisions have sufficient energy to initiate a reaction due to the activation energy barrier.
  • Activation energy: minimum energy needed for a reaction to occur.
  • Significance: Without activation energy, reactions could occur spontaneously (e.g., combustion).

Reaction Profile Diagram

  • Visual representation of energy changes during a reaction.

  • Reactants to products: Changes in internal energy as a function of reaction progress.

  • Exothermic Reaction Example:

    • Reactants have higher energy than products → energy is released (ΔH < 0).
    • Adjust the axis to indicate the drop in energy.

  • Endothermic Reaction Example:

    • Reactants have lower energy than products → energy is absorbed (ΔH > 0).

  • Activation energy is depicted as a barrier that reactants must overcome to form products.

Key Concepts about Activation Energy

  • Energy investment required to break chemical bonds and form an activated complex during a reaction.
  • Activated complex: a transient state in which new bonds are forming and old bonds are breaking.
  • Understanding Activation Barrier:
  • Forward (exothermic) reactions have Eₐ that influences whether the reaction proceeds.
  • Reverse (endothermic) reactions have their own Eₐ, which is not equal but related to forward Eₐ.

Arrhenius Equation

  • Equation: K = A * e^(-Eₐ/RT)
    • K: rate constant
    • A: pre-exponential factor (collision frequency)
    • R: gas constant
    • T: temperature (in Kelvin)
    • Eₐ: activation energy
  • Higher temperatures increase average kinetic energy, allowing more molecules to surpass the activation energy barrier.
  • Impact of Eₐ and T on Rate Constant (K):
  • Increased Eₐ leads to decreased K, resulting in slower rates.
  • Increased T leads to increased K and faster reaction rates.

Graphical Analysis of the Arrhenius Equation

  • Taking the logarithm of both sides gives:
  • log(K) = log(A) - (Eₐ/R)(1/T)
  • This shows a linear relationship between log(K) and 1/T, where the slope equals -Eₐ/R, allowing for experimental determination of activation energy.
  • Experimentally:
  • Obtain K values at different temperatures.
  • Plot log(K) vs. 1/T, calculate slope to find Eₐ.

Reaction Profile Examples and Calculations

  1. Exothermic Reaction:
  • Positive ΔH results in a downward slope on the reaction profile.
  • Activation energy forward (Eₐ) is added to the reaction path for reverse energy use.
  1. Endothermic Reaction:
  • Products are higher in energy than reactants.
  • ΔH of the reverse reaction is equal in magnitude but opposite in sign to ΔH forward.
  • Activation energy for reverse reactions is calculated from Eₐ forward and ΔH.

Factors Affecting Reaction Rate

  • Molecular orientation during collisions can significantly impact reaction rates.
  • Example: Specific orientation required for a reaction to proceed; affects probability of successful collisions.
  • Fewer viable collisions slow down the reaction rate.

Conclusion

  • Understanding activation energy and temperature's role in reaction rates is critical for predicting and controlling chemical behavior.
  • The lab activities will allow for hands-on experimentation and validation of kinetic concepts learned in theoretical discussions.