Measuring Reaction Rates

Polarimetry measures reaction rates by tracking optical rotation changes in optically active substances. Plane-polarized light passes through the reacting solution, and optically active compounds rotate this light clockwise or counterclockwise. By measuring the degree of rotation at set time intervals, the relative concentrations of reactants or products can be calculated. For example, during sucrose hydrolysis, sucrose causes clockwise rotation while glucose and fructose cause counterclockwise rotation, allowing rate determination.

The Beer-Lambert law relates light absorbance to concentration. In spectrophotometry, reactants or products absorb light at specific wavelengths; higher concentrations produce more intense absorbance. During reactions, a spectrophotometer measures light absorbed at different time points. Using the Beer-Lambert law, absorbance values are converted to concentration changes. For instance, as iodine vapor reacts with hydrogen gas, decreasing iodine concentration is observed through reduced yellow-green light absorbance.

Pressure measurements track reaction rates when gases are reactants or products. A manometer monitors pressure changes as the reaction progresses; reactant pressure decreases while product pressure increases. Using the ideal gas law, which relates gas concentration to partial pressure, pressure values recorded at different time points are converted to concentrations. This concentration change over time yields the reaction rate, as demonstrated during hydrogen peroxide decomposition when oxygen gas evolution is monitored.

For prolonged reactions, aliquots are sampled at regular time intervals and analyzed using instrumental techniques. Gas chromatography separates and quantifies gas-phase components, mass spectrometry identifies molecular species and their abundances, and titration chemically determines reactant or product concentrations. These methods provide relative concentration measurements at each time point, enabling calculation of reaction rates from the concentration change data.

Optical activity—the ability of compounds to rotate plane-polarized light—is essential for polarimetry-based rate measurements. Optically active substances rotate polarized light based on their molecular structure and concentration. Optically inactive substances produce no rotation. By monitoring rotation angle changes during a reaction, the concentrations of optically active reactants or products change as a function of time, directly revealing reaction progress without disturbing the system.

Spectrophotometry exploits differences in light absorption between reactants and products at specific wavelengths. If a reactant absorbs light strongly but the product does not, decreasing absorbance indicates reaction progress. Conversely, if a product absorbs light while the reactant does not, increasing absorbance shows product formation. A detector measures transmitted light intensity at periodic intervals, converting absorbance data to concentration values and enabling rate calculation through the integrated rate law the dependence of concentration on time.

Polarimetry measures optical rotation of plane-polarized light by optically active compounds, tracking concentration changes through rotation angle. Spectrophotometry measures light absorbance at specific wavelengths by reactants or products, tracking concentration through absorbance intensity. Polarimetry works only for optically active substances, while spectrophotometry applies to any compound absorbing light at measurable wavelengths. Both methods provide concentration data as a function of time, enabling reaction rate determination.