Advanced Topic 443 of 1102 Kinetics & Equilibrium
Activated Complex
The unstable arrangement of atoms at the energy maximum between reactants and products.
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In-depth explanation of Activated Complex
The activated complex is the arrangement of atoms at the maximum of the energy profile along the reaction coordinate, with partly formed and partly broken bonds. It is an unstable configuration rather than a stable intermediate, so it cannot be isolated or stored. Its lifetime is extremely short, of the order of a molecular vibration, roughly 10^-13 s. The structure of the activated complex is inferred from rate data, isotope effects and quantum chemical calculations rather than observed directly. Its energy relative to the reactants determines the activation energy of the reaction.
Going deeper: Activated Complex
Key ideas: On a potential energy surface the activated complex is the saddle point, a maximum along the reaction coordinate but a minimum in all other directions. One vibration of the complex is replaced by translation along the coordinate, and this mode becomes the decomposition into products. Its structure is estimated by computation and by isotope effects.
Common mistakes and connections: Students call it an intermediate or imagine it can be isolated, which is wrong because it lies at an energy maximum. Another error is to treat it as a collision pair that is merely close together, when bonds are partially formed. Related concepts are transition state, Hammond postulate and activation entropy, which describes how ordered the complex is.
Visualization of Activated Complex
In SN2 the activated complex has the nucleophile and leaving group both partly bonded to carbon. It sits at the barrier top and lives about 10^-13 s. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In the reaction of a hydrogen atom with H2 the activated complex is a linear H-H-H arrangement with stretched bonds.
Frequently asked questions about Activated Complex
What is Activated Complex?
The unstable arrangement of atoms at the energy maximum between reactants and products.
Is Activated Complex a beginner, intermediate or advanced topic?
Activated Complex 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 Activated Complex?
Study these earlier topics in Kinetics & Equilibrium first: Collision Theory, Activation Energy, Transition State Theory.
What comes after Activated Complex?
The next topics in Kinetics & Equilibrium are Arrhenius Equation and Arrhenius Plot.
Can I practise Activated Complex with quizzes?
Yes. The interactive lesson includes a self-check question on Activated Complex, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Activated Complex fits in Kinetics & Equilibrium
Activated Complex is topic 24 of 91 in Kinetics & Equilibrium.