13.2: Measuring Reaction Rates

Polarimetry finds application in chemical kinetics to measure the concentration and reaction kinetics of optically active substances during a chemical reaction. Optically active substances have the capability of rotating the plane of polarization of linearly polarized light passing through them—a feature called optical rotation. Optical activity is attributed to the molecular structure of substances. Normal monochromatic light is unpolarized and possesses oscillations of the electrical field in all possible planes perpendicular to the direction of its propagation. When an unpolarized light passes through a polarizer, a linearly polarized light maintaining oscillations in one plane emerges out.

A polarimeter instrument determines the polarization direction of the light or the rotation produced by an optically active substance. In a polarimeter, the plane-polarized light is introduced to a tube containing the reacting solution, and the reaction can be followed without disturbing the system. If the sample contains optically inactive substances, there will be no change in the orientation of the plane of the polarized light. The light will be visible in the same intensity on the analyzer screen, and the angle of rotation reading (ɑ) will read zero degrees.

However, the presence of optically active compounds in the reacting sample causes the rotation of the plane of the polarized light passing through. The light emerging out will be less bright. The axis of the analyzer device will have to be rotated in a clockwise (dextrorotatory) or counter-clockwise (levorotatory) direction to observe the maximum brightness. The direction in which the analyzer needs to be rotated depends on the nature of the compound present. The optical rotation measured is proportional to the concentration of the optically active substances present in the sample. By analyzing the angle of rotation measurements at different time points, the concentrations of the optically active compounds can be determined as a function of time.

Spectrometry

Optical experimental techniques like spectrometry are also frequently employed to monitor chemical reactions and secure quantitative information on reaction kinetics. Using spectrometry, the light of a specific wavelength is made to pass through a reacting sample. The molecules or compounds (either a reactant or product) within the sample may absorb some light while transmitting the remaining amount, which is measured by a detector. The quantity of light absorbed depends on the concentration of the compound or molecule of interest. For instance, the higher the concentration of a compound, the larger its absorbance. From the absorbance, the instrument will be able to determine the concentration of the compound of interest. In a reacting sample, the absorbance measured at periodic intervals computes the concentrations of the reactant or product as a function of time.

Pressure Measurements

For reactions involving gas-phase substances, the reaction kinetics is followed by quantifying the changes in the number of moles of gases as a function of the changes in pressure. The experimental settings of a gas-phase reaction can be connected to a manometer that could measure the pressure of either a gaseous reactant or product. As the reaction progresses, the pressure of the reactants decreases, and(or) the products' pressure increases. This can be measured by the manometer as a function of time. By employing the ideal gas law—the concentration of a gas is proportional to its partial pressure—the rate of a chemical reaction can be calculated.

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.