Fast Reactions: Characteristics, Techniques

Fast reactions are those types of chemical reactions that occur quickly, i.e., within a few seconds.

Chemical kinetics, as we all know, is a field of physical chemistry that deals with reaction speed or the rate at which reactant concentration changes over time. There are many different kinds of reactions, including chain reactions, polymerization reactions, rapid reactions, and simple chemical reactions. Some reactions are slow and take longer time to complete. On the other hand, some reactions are so quick that it completes in a short period. Fast reactions are those types of chemical reactions that occur in a very short period i.e. within a few seconds. They finish in mere 10^-13 sec. As a result, these reactions cannot be studied using conventional techniques.

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Characteristics of fast reactions

These reactions are so fast that they complete in a split second as soon as the reactants are brought together. These reactions can be completed in 10^-14 to 10^-16 sec. It is due to this fact that it is almost impossible to determine the rates of these reactions.
Various special analytical methods and techniques are being used and developed to study such reactions.
The half-life of such reactions is within a few milliseconds or less.
They take place instantaneously.
In comparison to the reaction’s half-life, the time required to combine the reactants or raise their temperature may be important.
NMR techniques are utilized to collect kinetic data in solutions. The technique is based on the observation that two substances with dissimilar NMR chemical shifts combine into one peak when they undergo fast changes in one direction to the other.

Examples:

Ionic reactions, for instance, can occur very quickly. Silver chloride can precipitate quickly when sodium chloride and silver nitrate are combined in aqueous solutions.
Neutralization reaction between acids and bases.
Combustion of natural gas.

Different methods for the study of fast reactions

Flow techniques

This technique is the extension of the classical mix and shake method where reactants are mixed within a fraction of a second.
Developed by Roughton and Hastridge in 1923.
Allows to measure reactions having half-life in the range of 10 sec to 10^-3 sec.
Involves a continuous flow technique and a stopped-flow technique.

Continuous flow technique:

The two reactants are allowed to pass to the mixing chamber from their respective reservoirs as shown in the figure.
The mixed solution then moves through the observation tube.
The reaction occurs in the mixing chamber. As the name suggests the flow in this technique is continuous at a constant rate.
Since the flow rate is constant, the concentration at a particular point along the observation tube does not change. This will provide an opportunity to make observations in a few milliseconds after missing.
Light is allowed to fall on the observation tube.
If the distance between the points at which the reaction is initiated and the product is known, then the time interval can be found from the flow rate as the flow rate is constant. By varying this distance, the time required to obtain the maximum yield can then be determined.
This technique can be coupled with spectrophotometric equipment at the point of mixing.
Measurement of light absorption may be used to determine the extent of reaction if the absorption spectrum of reactant and product differs.

Stopped flow technique:

This method is used to overcome the disadvantages of continuous flow techniques. The main problem of the continuous flow method is that it requires a large volume of reactants and is mostly suited for gas-phase reactions. The stopped-flow technique is the most common means of studying fast solid-phase reactions that complete within a millisecond.

The basic working principle of the stopped-flow technique is also similar to the continuous-flow technique. The difference is that in this technique, the flow is stopped suddenly so there is rapid change in the concentration of reactants. This change can be observed by coupling the instrument with the instrument that measures absorption, fluorescence, light scattering, or other optical or electrical properties of the solution. The change in the concentration of reactant before and after mixing will give kinetics of reactions.

Flash photolysis method

This method was given by George Porter and Ronald Norrish in 1949. This method is extensively used to study reaction kinetics in solution as well as in gaseous phases. The working principle of flash photolysis includes using a short pulse of light that is used to initiate a reaction. The progress of the reaction can be observed by optical and other means. A good example is the formation of hydrochloric acid by the combination of hydrogen and chloride, which proceeds explosively when the system is illuminated with visible light.

The reactants are kept in a cylindrical quartz vessel which is present next to the photolytic flash tube. They are coated with a MgO reflector.
A quartz tube filled with rare gas (xenon) is placed next to it.
The photoflash lamp is parallel to the reaction cell.
The reaction is initiated by the intense flash of visible or UV light that is generated by a photoflash lamp. The duration of flash is very short about 5 to 10 microseconds.
Monochromatic light from the lamp is passed through the sample and the wavelength is selected by the monochromator.
The intensity is measured by a photomultiplier and then displayed on an oscilloscope.
When a flash is applied to a system, the system gets perturbed and rapidly from equilibrium, and a change in concentrations is seen. This will help to ascertain the kinetics of fast reactions.

Chemical relaxation method

In the relaxation technique, a system is perturbed i.e. its equilibrium is disturbed by a rapid change in external parameters such as temperature, pressure, or electrified intensity. The time required to attain a new equilibrium known as relaxation time, is measured. The relaxation time can be measured by various methods.

Temperature jump:

In this method, a system is perturbed by changing the temperature to several degrees (10^OC) in 10^-5 sec. Then the relaxation time is measured.
The sudden change in temperature results in a change in the equilibrium concentration and this change can be observed by spectrophotometer.
It has been discovered that a 1^OC temperature change affects the equilibrium concentration by roughly 3%.

Pressure jump:

In this method sudden change is pressure is applied that disturbs the equilibrium.

A flexible cell is used to hold the sample.
After that, it is attached to a pressure vessel that has an inert liquid inside of it.
Next, the vessel is pressured to roughly 65 atmospheres.
Then, the pressure is reduced in roughly 10^-4 seconds to an atmospheric pressure by piercing a thin metal drill bit into the vessel’s wall.

Nuclear magnetic resonance

Zeeman splitting of nuclear energy levels is used in NMR technology. Typically, the systems under study are near equilibrium. The NMR technique is also a relaxation technique where molecules stimulated by radiation absorption shed their excess energy rapidly as a result of collisions.

Image source: https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/nmr/nmr1.htm

Coils emit a certain radio frequency.
Any radioactivity is picked up by a receiver and recorded.
Only radiations whose nuclear energy level matches with quanta of radiation of a specific frequency are absorbed when the field strength is increased.
Absorption takes place when two frequencies coincide.
These absorptions were captured and are used in the study of reaction kinetics.

References

https://byjus.com/question-answer/what-are-some-examples-of-fast-chemical-reactions/
https://www.slideshare.net/NITINOO/kinetic-of-fast-reaction
https://en.wikipedia.org/wiki/Chemical_kinetics
https://chem.libretexts.org/Courses/University_of_California_Davis/Chem_107B%3A_Physical_Chemistry_for_Life_Scientists/Chapters/2%3A_Chemical_Kinetics/2.10%3A_Fast_Reactions_in_Solution#:~:text=run%20to%20completion%3F-,Flow%20methods,the%20reaction%20as%20discussed%20below.
https://link.springer.com/chapter/10.1007/978-1-4020-4547-9_7