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.