Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Collision Frequency
What is Collision Frequency?
The number of collisions between reacting particles per unit time. It rises when there are more particles in a given volume, as when a solution is made more concentrated or a gas is compressed, and it is one of the two routes by which any factor can change a reaction rate.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Questions students ask about Collision Frequency
Why does increasing concentration speed up a reaction?
A more concentrated solution packs more particles into the same volume, so collision frequency rises and more successful collisions — ones with combined energy at least equal to the activation energy — happen per second. Concentration does not change how fast individual particles move; only temperature does that.
Why doesn't grinding a solid increase its concentration?
Concentration is defined only for a solute dissolved in a solution, and a solid reactant is not in solution. Grinding a lump into powder increases its surface area instead, exposing particles that were previously buried inside the lump, which raises collision frequency at the surface without changing the mass or the amount of product formed.