WEBVTT

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Reaction rates can be studied
by determining the change

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in concentrations of
reactants or products

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as a function of time.

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Concentration changes
can be measured

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by experimental techniques
like polarimetry, spectroscopy,

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or pressure measurements.

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Polarimetry uses
plane-polarized light

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with an electric field
oriented along only one plane.

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It measures the
ability of compounds

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to rotate polarized
light, which depends

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on the molecular structure
of the compound present.

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Consider the hydrolysis
of sucrose, which

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yields glucose and fructose.

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A polarimeter is used to
measure the degree of rotation

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of plane-polarized light coming
through the reacting sucrose

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solution.

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Sucrose causes
clockwise rotation,

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whereas glucose
and fructose cause

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counterclockwise rotation.

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By measuring the degree
of rotation of light

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at set time intervals, the
relative concentrations

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of sucrose, glucose, or
fructose can be calculated

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and the reaction
rate determined.

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Reaction rates can
also be measured

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using spectrophotometric
methods,

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utilizing the ability
of reactants or products

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to absorb light of
specific wavelengths.

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The higher the concentration
of the substance-of-interest,

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the more intense its
light-absorbance will be.

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For instance,
colorless hydrogen gas

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reacts with violet iodine vapor
to form colorless hydrogen

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iodide.

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Iodine vapor absorbs light
in the yellow-green region

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and reflects violet light.

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During the reaction,
a spectrophotometer

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measures the amount
of light absorbed

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by the sample and analyses
the light transmitted.

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Thus, as the
reaction progresses,

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the decrease in the
iodine vapor concentration

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is observed by the reduction
of the yellow-green light

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absorbance.

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Using the Beer--Lambert law,
the intensity of light absorbed

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at different time points can
be calculated and related

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to changes in concentration.

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Alternatively, if one of the
reactants or products is a gas,

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pressure measurements are used
to determine reaction rates

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by monitoring pressure changes.

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For example, during hydrogen
peroxide decomposition,

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the reaction rate is
studied using a manometer

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to monitor the pressure
of oxygen gas released.

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As the reaction progresses
and more oxygen gas evolves,

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the pressure rises.

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Using the ideal gas
equation, pressure values

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recorded at
different time points

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are converted to concentrations.

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The change in concentration
as a function of time

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is used to determine
the reaction rate.

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For prolonged reactions,
samples, or aliquots,

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can be taken from
the reaction mixture

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at regular time intervals.

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The relative
concentrations are then

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measured using
instrumental techniques

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like gas chromatography, mass
spectrometry, or titration,

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to compute reaction rates.

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