Reaction rates can be studied by determining the change in concentrations of reactants or products as a function of time. Concentration changes can be measured by experimental techniques like polarimetry, spectroscopy, or pressure measurements. Polarimetry uses plane-polarized light with an electric field oriented along only one plane. It measures the ability of compounds to rotate polarized light, which depends on the molecular structure of the compound present. Consider the hydrolysis of sucrose, which yields glucose and fructose. A polarimeter is used to measure the degree of rotation of plane-polarized light coming through the reacting sucrose solution. Sucrose causes clockwise rotation, whereas glucose and fructose cause counterclockwise rotation. By measuring the degree of rotation of light at set time intervals, the relative concentrations of sucrose, glucose, or fructose can be calculated and the reaction rate determined. Reaction rates can also be measured using spectrophotometric methods, utilizing the ability of reactants or products to absorb light of specific wavelengths. The higher the concentration of the substance-of-interest, the more intense its light-absorbance will be. For instance, colorless hydrogen gas reacts with violet iodine vapor to form colorless hydrogen iodide. Iodine vapor absorbs light in the yellow-green region and reflects violet light. During the reaction, a spectrophotometer measures the amount of light absorbed by the sample and analyses the light transmitted. Thus, as the reaction progresses, the decrease in the iodine vapor concentration is observed by the reduction of the yellow-green light absorbance. Using the Beer–Lambert law, the intensity of light absorbed at different time points can be calculated and related to changes in concentration. Alternatively, if one of the reactants or products is a gas, pressure measurements are used to determine reaction rates by monitoring pressure changes. For example, during hydrogen peroxide decomposition, the reaction rate is studied using a manometer to monitor the pressure of oxygen gas released. As the reaction progresses and more oxygen gas evolves, the pressure rises. Using the ideal gas equation, pressure values recorded at different time points are converted to concentrations. The change in concentration as a function of time is used to determine the reaction rate. For prolonged reactions, samples, or aliquots, can be taken from the reaction mixture at regular time intervals. The relative concentrations are then measured using instrumental techniques like gas chromatography, mass spectrometry, or titration, to compute reaction rates.
