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Chemistry · Glossary

What is Collision cross-section, steric factor?

Also known as: collision cross-section · steric factor

Definition 12.1 University Chemistry — Year 3 · Chapter 12 — Theories of Reaction Rates

The collision cross-section of two molecules is the area σ=πd2\sigma = \pi d^2 of the disc, perpendicular to their relative velocity, inside which the centre of one must pass for it to hit the other. The steric factor PP is the ratio of the measured pre-exponential factor of a reaction to the one predicted by collision theory.

The collision cylinder. Moving at the relative speed v_ rel, molecule A sweeps in a time t a cylinder of cross-section = π d2, with d the sum of the two radii; every B whose centre lies inside is hit.
The collision cylinder. Moving at the relative speed vrelv_{\mathrm{rel}}, molecule A sweeps in a time Δt\Delta t a cylinder of cross-section σ=πd2\sigma = \pi d^2, with dd the sum of the two radii; every B whose centre lies inside is hit.
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