Thermodynamic & Kinetic Control (College Board AP® Chemistry)

Thermodynamic & Kinetic Control

• A common misconception is that thermodynamically favorable reactions occur quickly 

• Actually, some processes that are thermodynamically favored either occur at a very slow rate or do not occur to any measurable extent

  • These reactions are said to be under kinetic control 

• A common reason this happens is due to the reaction having a high activation energy, E_a  
  

  • Reactions which have a high E_a will not take place at room temperature 

• Some reactions under kinetic control can be sped up by using a catalyst which reduces the activation energy and allows the reaction to proceed at a measurable rate

• For example, the decomposition of hydrogen peroxide at 298 K

  H₂O₂ (l) → H₂O (l) + ½O₂ (g)

  • This reaction has a very large E_a so must be catalysed using manganese dioxide, MnO₂

  • If the reaction was left for long enough, the hydrogen peroxide would eventually decompose

  • However, the addition of the MnO₂ allows the reaction to take place via an alternative route with a lower E_a 

  • Reaction energy profiles can be used to show how the activation energy changes when a catalyst is used 

Reaction Energy Profile for an Exothermic Reaction

A catalyst will lower the activation energy, E_a, and help speed up some reactions that are under kinetic control