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.
What a learner can do afterwards
- 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
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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.
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.
Treat the barrier top as a balanced species, read order from the entropy term, and pull both parameters from one straight line.
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