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 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.
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Q1: How does polarimetry measure 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.
Q2: What is the Beer-Lambert law and how does it apply to spectrophotometric rate measurements?
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.
Q3: How are pressure measurements used to determine reaction rates for gas-phase reactions?
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.
Q4: What experimental techniques are used to measure concentration changes in prolonged reactions?
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.
Q5: Why is optical activity important for measuring reaction kinetics with polarimetry?
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.
Q6: How does spectrophotometry distinguish between reactants and products during a reaction?
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.
Q7: What is the difference between polarimetry and spectrophotometry for measuring reaction rates?
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.