What you'll be able to do: Explain reaction rates using collision frequency, orientation and activation energy, and interpret a reaction energy profile.
Best after: Reaction Rates and How They Are Measured
Rate laws describe what happens; collision theory explains why. Particles must meet, meet hard enough, and meet the right way round. Every factor that changes a rate acts on one of those three requirements.
These are recommended, not required. You can start this lesson at any time.
Particles must collide, the collision must carry at least the activation energy, and the geometry must allow the correct bonds to form. Most collisions fail on the second or third requirement, which is why reactions are slower than the enormous collision frequency in a liquid or gas would suggest.
Plot energy against reaction progress. The rise from reactants to the peak is the forward activation energy; the fall from the peak to products is the reverse activation energy. Their difference is ΔH, negative for an exothermic reaction and positive for an endothermic one. The species at the peak is the transition state, which cannot be isolated.
ΔH = Ea(forward) - Ea(reverse)
At any temperature, molecular energies are spread over a wide distribution. Raising the temperature flattens and broadens the curve, shifting the peak to higher energy, and the area beyond the activation energy grows sharply. That growing tail, not a faster average speed alone, is what makes reactions accelerate so strongly with temperature.
Higher concentration or pressure increases collision frequency. Greater surface area on a solid increases the number of exposed particles available to collide. Higher temperature increases both frequency and, far more importantly, the fraction of collisions with sufficient energy. A catalyst lowers the activation energy so a larger fraction of existing collisions succeeds.
A catalyst provides an alternative pathway with a lower barrier and is regenerated by the end. It lowers the forward and reverse activation energies equally, so it speeds up the approach to equilibrium without shifting the position of equilibrium, and ΔH is unchanged.
The minimum energy a collision must supply for a reaction to occur.
The highest-energy arrangement along the reaction path, existing only momentarily.
A collision with sufficient energy and correct orientation to give products.
Ea forward and Ea reverse fall by the same amount, so ΔH and the equilibrium position are unchanged.
Temperature mostly matters because it increases the fraction of molecules above Ea, not the collision count.
ΔH = Ea(forward) - Ea(reverse)
A reaction has Ea(forward) = 85 kJ/mol and Ea(reverse) = 120 kJ/mol. Find ΔH and classify the reaction.
Try it first: Sketch the profile roughly before calculating; the higher reverse barrier tells you where the products sit.
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Explain why powdering a solid reactant speeds up its reaction with an acid, in collision-theory terms.
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A catalyst lowers Ea from 100 kJ/mol to 60 kJ/mol for an endothermic reaction with ΔH = +30 kJ/mol. What is the reverse activation energy with and without the catalyst?
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Why it's wrong: A catalyst changes the pathway, not the relative energies of reactants and products.
Check instead: Only Ea changes; ΔH and K stay the same.
Why it's wrong: A transition state is an energy maximum and cannot be isolated; an intermediate is a local minimum and can be.
Check instead: Peak means transition state, valley means intermediate.
Why it's wrong: The increase in collision frequency is modest; the dominant effect is the larger fraction of collisions above Ea.
Check instead: Refer to the growing high-energy tail of the distribution.
Why it's wrong: Orientation still matters, especially for large or asymmetric molecules.
Check instead: State both the energy and the orientation requirement.
No practice questions are available for this topic yet. You can still practice the whole unit.
Collision theory holds that a reaction occurs only when particles collide with at least the activation energy and with a suitable orientation. The fraction of collisions clearing the activation barrier grows steeply with temperature, which is why a modest temperature rise produces a large rate increase. An energy profile shows reactants, an activation-energy barrier topped by the transition state, and products; the forward and reverse activation energies differ by the enthalpy change, and a catalyst lowers both by providing a different pathway.
Chemistry 2e, OpenStax, Rice University, licensed CC BY 4.0. License