Thermodynamic & Kinetic Control (College Board AP® Chemistry)

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Alexandra Brennan

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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 energyE 

    • Reactions which have a high Ewill 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 Ea 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 Ea 

    • 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

energy-profile-with-and-without-catalystenergy-profile-with-and-without-catalyst

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

Examiner Tip

Remember: A catalyst reduces the activation energy by providing an alternative pathway for the reaction

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.