The concept of reaction rate

Reactions go at very different speeds: the formation of a precipitate in solution is instantaneous, while rusting of iron takes months. The average rate of a reaction is the change in the concentration of a substance (reactant or product) per unit time: v = |ΔC|/Δt. Since ΔC = Δn/V, v = |Δn|/(V·Δt); the unit is mol/(L·s) or mol/(L·min). To find the rate you need the change in the amount of substance, the volume of the vessel and the time; if the volume is given in m³, convert it with 1 m³ = 1000 L. The rate depends on the nature of the substances, the concentration (in gases and solutions), the size of the surface of a solid, the temperature and a catalyst. For a one-step reaction aA + bB → products v = k·C(A)^a·C(B)^b, where k is the rate constant; for multi-step reactions the exponents are found by experiment. The concentration of a pure solid does not appear in this expression because the reaction takes place on its surface.