--- title: "Transition State Theory and the Eyring Equation" description: "Collision theory counts encounters and gets the size of a rate constant badly wrong. Treating the reacting pair as a species in equilibrium with the reactants, poised at the top of the barrier, gives " canonical: https://lightmysky.com/learn/science/transition-state-theory-and-the-eyring-equation-mt_JVjDMcXds1 source: https://lightmysky.com/learn/science/transition-state-theory-and-the-eyring-equation-mt_JVjDMcXds1.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Transition State Theory and the Eyring Equation Collision theory counts encounters and gets the size of a rate constant badly wrong. Treating the reacting pair as a species in equilibrium with the reactants, poised at the top of the barrier, gives a rate constant built from thermodynamic quantities. Subject: Science · Area: Chemistry · Ages 20 to 22 Page: https://lightmysky.com/learn/science/transition-state-theory-and-the-eyring-equation-mt_JVjDMcXds1 ## Ready when they can - Writes the Eyring equation and identifies the enthalpy and entropy of activation in it - Explains what a large negative entropy of activation says about the geometry of the transition state - Compares an Arrhenius activation energy with an activation enthalpy and states how they differ - Extracts activation parameters from a plot of the appropriate function against reciprocal temperature ## Lesson: Counting reactions at the barrier top Every step in a chain reaction runs at a rate set by its own rate constant. Collision theory tries to predict that constant by picturing reaction as hard, well aimed crashes. It splits the rate into how often molecules meet and what fraction clears the barrier. For bulky molecules the aiming part goes badly wrong, and the predicted constant misses by far. **Example.** Transition state theory counts differently. It treats the reacting pair at the top of the barrier as a real species, the activated complex, in equilibrium with the reactants. The Eyring equation builds the rate constant from that balance: it holds the enthalpy and the entropy of activation. The entropy of activation reports on geometry. A large negative value means the complex is tight and ordered, because the partners lost freedom on the way up. The Arrhenius activation energy is always a little larger than the activation enthalpy for a gas reaction, so state which one you mean. To extract both parameters, plot the natural log of k over T against one over T. The slope gives the enthalpy of activation and the intercept gives the entropy. A steep slope means a tall enthalpy barrier. **Recap.** Treat the barrier top as a balanced species, read order from the entropy term, and pull both parameters from one straight line. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [The Molecular Partition Function](https://lightmysky.com/learn/science/the-molecular-partition-function-mt_-TPTlb7zQ2) - [Chain Reactions, Branching and Explosion Limits](https://lightmysky.com/learn/science/chain-reactions-branching-and-explosion-limits-mt_8SDv_5iRMu) - [The Arrhenius Equation and Finding Activation Energy](https://lightmysky.com/learn/science/the-arrhenius-equation-and-finding-activation-energy-mt_QBQDSNwbXK) ## Opens up - [Electrode Kinetics: Exchange Current and Overpotential](https://lightmysky.com/learn/science/electrode-kinetics-exchange-current-and-overpotential-mt__z7Y780pyo) - [Designing a Safer Process: Hazard, Scale and Waste](https://lightmysky.com/learn/science/designing-a-safer-process-hazard-scale-and-waste-mt_5zhSYCYU_o) - [Reaction Dynamics: Trajectories Across a Potential Energy Surface](https://lightmysky.com/learn/science/reaction-dynamics-trajectories-across-a-potential-energy-surface-mt_6ctTUYqYtS) - [Computed Geometries, Frequencies and Reaction Barriers](https://lightmysky.com/learn/science/computed-geometries-frequencies-and-reaction-barriers-mt_pdiFjK2wPe) - [Chemical Potential and Gibbs Energy That Depends on Composition](https://lightmysky.com/learn/science/chemical-potential-and-gibbs-energy-that-depends-on-composition-mt_Tdl6K-gFO3) - [Asymmetric Catalysis: Where Enantioselectivity Comes From](https://lightmysky.com/learn/science/asymmetric-catalysis-where-enantioselectivity-comes-from-mt_vFpTH27iHP)