Equilibrium in a closed vessel and the rate of reaction

Reactions differ hugely in speed. A silver chloride precipitate forms at once, soap-making from oil and alkali is slow, and the rusting of iron takes months. The rate of reaction is the change in concentration per unit time: average rate = change in concentration ÷ time, in mol/(L·s) or mol/(L·min). For a reactant the concentration falls, so we use the size of the change and the rate is positive. Six factors change the rate: the nature of the reactants, their concentration, the temperature, the pressure (for gases), a catalyst, and light. As a rule of thumb, reactions between ions are very fast and reactions between covalent substances, such as organic ones, are slow. On a concentration–time graph the curve of a reactant is steepest at the start, because the concentration is highest, and it flattens as the reaction slows. In a reversible reaction it becomes flat when equilibrium is reached. Surface area, which makes powder dissolve faster than a lump, is an extra idea outside the book’s list of six.