The rate of a chemical reaction is expressed as the change in concentration of a substance during a given time interval. It can be either the rate of disappearance of a reactant or the rate of appearance of a product: rate = `Delta` [C]/`Delta` t
There are several methods of measuring a rate, depending on the type of reaction.
If a gas is produced, the rate can be measured by measuring the amount of gas that forms in specific time intervals. This can be accomplished by collecting the gas in a syringe with a moveable piston.
If a sufficient mass of gas is evolved the change in mass of the reactants can be monitored.
If the reaction is sufficiently exothermic or endothermic the rate can be measured by measuring the change in temperature in specific time intervals.
If the reaction involves a color change the rate of change in color can be measured using a spectrophotometer or colorimeter.
If the reaction produces a precipitate, the rate of precipitate formation can be determined by timing the formation of enough solid to obscure an mark on the bottom of the flask and weighing the precipitate.