Advanced Topic 442 of 1102 Kinetics & Equilibrium
Transition State Theory
Reactants pass through a high-energy activated complex at the top of the energy barrier.
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In-depth explanation of Transition State Theory
Transition state theory describes a reaction as the passage of reactants over an energy barrier through a high-energy arrangement of atoms called the activated complex, which then converts to products. The rate constant is related to the concentration of the activated complex and to a universal frequency kT/h, which leads to the Eyring equation. This connects kinetics directly with thermodynamic quantities, since the activated complex is treated as being in quasi-equilibrium with the reactants. The theory explains the effects of temperature, pressure and solvent on rates through changes in the energy and entropy of the transition state.
Going deeper: Transition State Theory
Key ideas: The rate is the concentration of activated complex times the frequency of crossing the barrier, kT/h, about 6 x 10^12 per s at 300 K. In thermodynamic form k = (kT/h) exp(-delta G double dagger/RT) and K double dagger is treated as an equilibrium constant. Relation: Ea = delta H double dagger + RT for solution reactions.
Common mistakes and connections: Students treat the transition state as a real molecule with a measurable lifetime, or confuse it with an intermediate. Another mistake is believing that all complexes passing over the barrier go on to products, which ignores the transmission coefficient. The theory generalizes collision theory and leads to the Eyring equation and activation entropy interpretation.
Visualization of Transition State Theory
Reactants pass through an activated complex at the energy maximum; its quasi-equilibrium with reactants gives the rate constant (Eyring equation). The interactive lesson shows this as a live animation or chart.
Worked example and use case
In an SN2 reaction the transition state has the nucleophile and the leaving group both partly bonded to the central carbon.
Frequently asked questions about Transition State Theory
What is Transition State Theory?
Reactants pass through a high-energy activated complex at the top of the energy barrier.
Is Transition State Theory a beginner, intermediate or advanced topic?
Transition State Theory is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Transition State Theory?
Study these earlier topics in Kinetics & Equilibrium first: Pre-equilibrium Approximation, Collision Theory, Activation Energy.
What comes after Transition State Theory?
The next topics in Kinetics & Equilibrium are Activated Complex and Arrhenius Equation.
Can I practise Transition State Theory with quizzes?
Yes. The interactive lesson includes a self-check question on Transition State Theory, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Transition State Theory fits in Kinetics & Equilibrium
Transition State Theory is topic 23 of 91 in Kinetics & Equilibrium.