UNIT: 9.4 In-Depth Notes on Thermodynamic and Kinetic Control

Thermodynamic and Kinetic Control

• Definition of Thermodynamic and Kinetic Control

  • Thermodynamic Control: Refers to reactions that are favorable based on Gibbs free energy, indicated by a negative $\Delta G$.
  • Kinetic Control: Refers to reactions that, despite being thermodynamically favored, have a slow rate of reaction due to high activation energy.

• Key Concepts

  • Activation Energy:
  • Definition: The minimum energy required for a reaction to occur, represented as the height on the potential energy diagram from reactants to the activated complex.
  • Impact: High activation energy leads to slower reaction rates.

• Reaction Examples

  • Example: Diamond to Graphite
  • Despite thermodynamic favorability, this reaction occurs very slowly due to its high activation energy (kinetic control).

• Catalysts

  • Role: Catalysts lower the activation energy, increasing the reaction rate without affecting the thermodynamic properties of the reaction (i.e., $\Delta H$, $\Delta S$, $\Delta G$).
  • Important Note: Adding a catalyst does not make a reaction more thermodynamically favorable or alter $\Delta G$ values.

• Review of Reaction Favorability

  • Factors Affecting Thermodynamic Favorability:
  • Delta H: Negative $\Delta H$ (exothermic) supports favorability.
  • Delta S: Negative $\Delta S$ (decreasing entropy), does not support favorability.
  • Problem Example:
  • Given a reaction with negative $\Delta H$ and its associated entropy change can determine driving forces.
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• Impact of Temperature

  • Increase in temperature effects on $\Delta G$:
  • For reactions with negative $\Delta H$ and negative $\Delta S$, an increase in temperature can cause $\Delta G$ to increase, making the reaction less favorable (favored at low temperatures).

• Apparent Contradiction in Reaction Rates

  • Some thermodynamically favored reactions do not proceed at measurable rates due to high activation energy.
  • Example: Diamond converting to graphite demonstrates that even thermodynamically favorable reactions may be slow.

• Final Takeaways

  • Favorability of a reaction does not guarantee a fast reaction rate.
  • Kinetic control can make reactions appear to be not progressing despite being favored.
  • A catalyst is effective in increasing rate without altering thermodynamic favorability.