8.1.1—Terms: rate of reaction, frequency of collisions

Rate of reaction is the change in concentration of a reactant or product per unit time. It can be expressed as a positive product-formation rate or a negative reactant-consumption change with the sign interpreted.

Collision theory explains rate: particles must collide with enough energy and suitable orientation. Increasing concentration, pressure, surface area or temperature changes the frequency or success of collisions.

A gas reaction speeds up when pressure rises because particles collide more often in the smaller volume. A powdered solid reacts faster than the same mass in lumps because more surface is exposed.

A faster rate does not mean a larger final yield. Rate describes time dependence; equilibrium or limiting reactants determine how far a reaction proceeds.