Name: tracking reaction kinetics with light and pressure
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 9 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Measuring Reaction Rates

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&#160; yellow-green light absorbance.
Using the Beer&#8211;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.

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&#8212;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 (&#593;) 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&#39; pressure increases. This can be measured by the manometer as a function of time. By employing the ideal gas law&#8212;the concentration of a gas is proportional to its partial pressure&#8212;the rate of a chemical reaction can be calculated.

Polarimetry finds application in chemical kinetics to measure the concentration and reaction kinetics of optically active substances during a chemical ...

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Chemistry

Chemical Kinetics

KEY TERMS AND CONCEPTS

tracking reaction kinetics with light and pressure

Reaction kinetics can be tracked with polarimetry, spectrometry, and pressure measurements. These methods let scientists follow a reaction without disturbing the sample. They also help determine how reactant or product concentrations change over time.

Polarimetry is used for optically active substances. These substances rotate the plane of polarized light, which is called optical rotation. The effect comes from molecular structure. Normal monochromatic light is unpolarized, but after it passes through a polarizer, it becomes linearly polarized with vibrations in one plane.

In a polarimeter, this plane-polarized light is sent through a tube of reacting solution. The instrument then shows how much the light has been rotated. If the sample contains optically inactive substances, the light keeps the same orientation, the analyzer screen stays equally bright, and the rotation angle is zero degrees. If optically active compounds are present, the light becomes dimmer and the analyzer must be turned clockwise or counter-clockwise to restore maximum brightness. Clockwise rotation is dextrorotatory, and counter-clockwise rotation is levorotatory.

The amount of optical rotation is proportional to the concentration of the optically active substance in the sample. By taking rotation readings at different times, the concentration can be followed as the reaction proceeds. Spectrometry gives similar kinetic information by passing light of a specific wavelength through a reacting sample. Some of the light is absorbed by the reactant or product, and a detector measures the remaining light. Higher concentration gives higher absorbance, so repeated absorbance measurements can be used to find concentration over time.

Pressure measurements are useful for gas-phase reactions. A manometer can be connected to the reaction setup to monitor the pressure of a gaseous reactant or product. As the reaction goes on, reactant pressure may decrease and product pressure may increase. Using the ideal gas law, the concentration of a gas is proportional to its partial pressure, so reaction rate can be calculated from the pressure change.

Tracking Reaction Kinetics with Light and Pressure

tracking-reaction-kinetics-with-light-and-pressure

reaction rate

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts. For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes...

Reaction kinetics can be tracked with polarimetry, spectrometry, and pressure measurements. These methods let scientists follow a reaction without disturbing the sample. They also help determine how reactant or product concentrations change over time. Polarimetry is used for optically active substances. These substances rotate the plane of polarized light, which is called optical rotation. The effect comes from molecular structure. Normal monochromatic light is unpolarized, but after it passes...

concentration and rate law

Reaction Order from Concentration

Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k. In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n...

determining order of reaction

Finding Rate Law from Initial Rates

Rate laws connect the speed of a chemical reaction to the concentration of its reactants. The rate constant, k, and the reaction orders are not taken from the chemical equation. They are found experimentally by watching how the rate changes when the reactant concentrations change. One common way to do this is the method of initial rates. In this method, several trials are run with different starting concentrations. The measured rates are then compared to determine the reaction orders and,...

the integrated rate law: the dependence of concentration on time

the-integrated-rate-law-the-dependence-of-concentration-on-time

Reading Reaction Progress with Integrated Laws

Integrated rate laws connect reaction concentration to time. They help show how much reactant or product is present after a certain period. They can also help estimate how long a reaction must run to reach a given extent. A common use is finding how long radioactive material must be stored before its radioactivity drops to a safe level. These equations come from the differential rate law after it is integrated with respect to time. For a first-order reaction, where rate = k[A], the integrated...

half-life of a reaction

Reaction Half-Life in Kinetics

Reaction half-life is the time needed for half of a reactant to be consumed. After each half-life, half of the remaining reactant is used up. A hydrogen peroxide decomposition example shows this pattern clearly. The concentration drops from 1.000 M to 0.500 M in the first 6.00 hours, then to 0.250 M in the next 6.00 hours, and then to 0.125 M in the third period. Each later 6.00-hour interval removes half of what is left. For first-order reactions, the half-life does not depend on the reactant...

temperature dependence on reaction rate

Collision Theory and Reaction Speed

Collision theory explains why reaction speed depends on how particles collide. Atoms, molecules, or ions must come close together before they can react. The theory says the reaction rate depends on the collision rate, the orientation of the colliding species, and whether the collision has enough energy for bonds to change. A gas-phase reaction between carbon monoxide and oxygen shows why orientation matters. The molecules can collide in more than one way. If the oxygen side of carbon monoxide...

arrhenius plots

Determining Activation Energy from Temperature

The Arrhenius equation links activation energy and the rate constant, k, for chemical reactions. It is written as k = Ae−Ea/RT. In this equation, R is the ideal gas constant with a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor depends on collision frequency and the orientation of reacting molecules. Experimental kinetic data can be used to find the activation...

reaction mechanisms

Elementary Steps in Reaction Mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

rate-determining steps

Reaction Bottlenecks in Chemical Kinetics

Reaction bottlenecks help explain rate-determining steps in multistep reaction mechanisms. In a mechanism with several elementary steps, one step is often much slower than the others. That slowest step is called the rate-limiting step, or rate-determining step. A reaction cannot move faster than its slowest step, so this step controls the overall reaction rate. The idea is like traffic on a four-lane freeway with one lane closed. Cars can only move as fast as the bottleneck allows. In the same...

catalysis

Catalyst Action in Reaction Rates

Catalysts change the rate of a chemical reaction by giving it a faster path. A catalyst increases reaction rate without being consumed during the process. Its effect is often explained with reaction mechanisms and energy diagrams. For an endothermic reaction, the reactants and products start and end at the same energies in both cases. The uncatalyzed path is a one-step mechanism with one transition state. The catalyzed path follows a different mechanism with two transition states and a lower...

enzymes

Enzyme Action and Inhibition

Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase. Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and...

key terms and concepts

See how polarimetry, spectrometry, and pressure measurements monitor reaction kinetics by tracking optical rotation, absorbance, and gas pressure changes.

Tracking Reaction Kinetics with Light and Pressure | JoVE K12

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North Carolina K-12 Science Standard Course of Study

9-12

Matter and its Interactions

PS.Chm.5.1

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

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CH.7

CH.7.e

Minnesota Academic Standards in Science

3.2.1 9C.3.1.1.2

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.d

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-4

HS-PS1-5

Florida Department of Education

SC.912.P.12

SC.912.P.12.12

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New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-5.

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Tracking Reaction Kinetics with Light and Pressure

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