Chain Reaction in Chemistry: Explained for Students

There are various types of reactions prevalent in Chemistry, Physics, and Biology, one of the most commonly used reactions is a chain reaction. Let us discuss the chain reaction definition, A chain reaction is a series of reactions in which a reactive substance or by-product initiates further reactions. Positive feedback in a chain reaction causes a self-amplifying chain of events.

From the above-mentioned definition, the chain reaction meaning becomes more clear. It gives us ideas about the thermodynamic stability of the reaction. 

• These reactions are one way for non-thermodynamically balanced systems to release energy or increase entropy in order to achieve a higher entropy state.

For example, a device may be unable to achieve a lower energy state by releasing energy into the environment because it is hindered or prevented from taking the path that will result in the energy release in some way. If a reaction results in a limited energy release, the device will normally collapse explosively before most or all of the accumulated energy has been released.

Role of Potential Energy in Chain Reaction

A snowball triggering a larger snowball before an avalanche occurs is a macroscopic term for chain reactions ("snowball effect"). This is due to gravitational potential energy being retained and finding a direction of release over friction. A spark triggering a forest fire is the chemical equivalent of a snow avalanche. A single stray neutron can cause a rapid critical event in nuclear physics, which may eventually be energetic enough to cause a nuclear reactor failure or (in the case of a bomb) a nuclear explosion.

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Chain Reaction Chemistry

The following are the most common forms of chain reaction phases.

• Initiation (formation of active particles or chain carriers, often free radicals, in either a thermal or a photochemical step)

• Propagation (can contain many elementary or simple steps in a cycle, where the reactive particle through chemical reaction forms another reactive particle that continues the chain of reaction by entering the next elementary or simple step). In addition, the active particle acts as a catalyst for the propagation cycle's overall reaction. The following are examples of special cases: 

Chain Branching - It's a phase in the propagation process where one reactive particle enters and two or more are formed.

Chain transfer (a propagation step in which the active particle is a growing polymer chain which reacts to form an inactive polymer whose growth is terminated and an active small A radical, for example, is a particle that can react to produce a new polymer chain.

• Termination (simple or elementary step in which the reactive particle loses its reactivity; e. g. by recombination of two free radicals).

The chain length is equal to the overall reaction rate divided by the initiation rate and is defined as the average number of times the propagation cycle is repeated.

Complex rate equations of fractional order or mixed order kinetics can be used in certain chain reactions.

Examples of Chain Reactions

1. Formation of hydrogen Bromide from hydrogen and bromine gas. The reaction H\[_{2}\] + BR\[_{2}\] → 2HBr proceeds by the following mechanism:

• Initiation

Br\[_{2}\] → 2Br• (This step require thermal energy) 

or

Br\[_{2}\] + hv → 2Br• (This step takes place in the presence of photons)

each Br atom is a free radical, indicated by the symbol (• ) representing an unpaired electron.

• Propagation (here a cycle of two steps)

Br• + H\[_{2}\] → HBr + H•

H• + Br\[_{2}\] → HBr + Br•

The sum of the above two stages reaction corresponds to the overall reaction as: H\[_{2}\] + Br\[_{2}\] → 2HBr, with catalysis by Br• which takes part in the first step and is regenerated in the second step. Retardation (inhibition)

H• + HBr → H\[_{2}\] + Br•

This step is unique to this example and corresponds to the reverse of the first propagation step.

• Termination 2 Br• → Br\[_{2}\]

In this case, the recombination of two radicals corresponds to initiation in reverse.

The thermal reaction has a fractional order (3/2) initial rate and a full rate equation with a two-term denominator, as demonstrated by the steady-state approximation (mixed-order kinetics).

2. Chain branching is shown in reaction 2H\[_{2}\] + O\[_{2}\] → 2H\[_{2}\]O. The propagation is a two-step process that results in the replacement of an H atom by another H atom plus two OH radicals. Under certain temperature and pressure conditions, this results in an explosion.

H + O\[_{2}\] → OH

O + H\[_{2}\] → OH + H

The propagation stage in chain-growth polymerization corresponds to the elongation of the growing polymer chain. Chain transfer refers to the transfer of operation from a growing chain that has reached the end of its growth cycle to another molecule, which may be a second growing polymer chain. The above-mentioned kinetic chain length can vary from the degree of polymerization of the product macromolecule during polymerization.

Polymerase chain reaction (PCR) is a molecular biology technique that uses a DNA polymerase to amplify (make several copies of) a piece of DNA in vitro.

3. Acetaldehyde Pyrolysis and Rate Equation

The Rice-Herzfeld process is used in the pyrolysis (thermal decomposition) of acetaldehyde, CH\[_{3}\]CHO(g) → CH\[_{4}\](g) + CO(g) :

• Initiation (formation of free radicals):

CH\[_{3}\]CHO(g) → •CH\[_{3}\](g) + •CHO(g)k1

Free radicals are the methyl and CHO groups.

• Propagation (two steps):

•CH\[_{3}\](g) + CH\[_{3}\]CHO(g) → CH\[_{4}\](g) + •CH\[_{3}\]CO(g) k2

Methane, one of the two key products, is generated during this reaction stage.

•CH\[_{3}\]CO(g) → CO(g) + •CH\[_{3}\](g) k3

The previous step's product •CH\[_{3}\]CO(g) produces carbon monoxide (CO), which is the second main product.

The overall reaction CH\[_{3}\]CHO(g) → CH\[_{4}\](g) + CO(g), catalysed by a methyl radical  •CH\[_{3}\], is equal to the number of the two propagation steps.

• Termination:

•CH\[_{3}\](g) + •CH\[_{3}\](g) → C\[_{2}\]H\[_{6}\] (g) k4 

This reaction produces only ethane (minor product) and is thought to be the main chain's final step.

While this process describes the main products, others, such as acetone (CH\[_{3}\]COCH\[_{3}\]) and propanal, are shaped in a minor way (CH\[_{3}\]CH\[_{2}\]CHO). The rate law for the formation of methane and the order of reaction was found using the Steady State Approximation for intermediate species CH\[_{3}\](g) and CH\[_{3}\]CO(g).

Types of Chain Reaction

Nuclear Chain Reaction (Controlled Chain Reaction)

Leo Szilard suggested a nuclear chain reaction in 1933, just after the neutron was discovered but more than five years before nuclear fission was discovered. Szilard was familiar with chemical chain reactions, and he had recently read about a 1932 demonstration by John Cockcroft and Ernest Walton of an energy-producing nuclear reaction involving high-energy protons bombarding lithium. Now, Szilard proposes that neutrons released theoretically by some nuclear reactions in lighter isotopes be used to cause further neutron-producing reactions in lighter isotopes. In theory, this will result in a chain reaction at the nucleus stage. Since he didn't know about fission at the time, he didn't think of it as one of these neutron-producing reactions. 

Later, after the discovery of fission in 1938, Szilard realised that neutron-induced fission could be used as the specific nuclear reaction needed to generate a chain reaction, as long as fission also emitted neutrons. Szilárd demonstrated this neutron-multiplying reaction in uranium with Enrico Fermi in 1939. A neutron plus a fissionable atom causes fission in this reaction, resulting in a greater number of neutrons than the single one absorbed in the initial reaction. By the mechanism of neutron-induced nuclear fission, the practical nuclear chain reaction was born.

If one or more of the emitted neutrons interact with other fissionable nuclei, and these nuclei also fission, there is a chance that the macroscopic overall fission reaction will not end, but will proceed in the reaction material. As a result, the chain reaction becomes self-propagating and thus self-sustaining. This is how nuclear reactors and atomic bombs work.

Enrico Fermi and others demonstrated a self-sustaining nuclear chain reaction in the successful operation of Chicago Pile-1, the first artificial nuclear reactor, in late 1942.

Electron Avalanche in Gases

When an electric field reaches a certain threshold, an electron avalanche occurs between two unconnected electrodes in a gas. In a process known as impact ionisation, random thermal collisions of gas atoms can result in a few free electrons and positively charged gas ions. When these free electrons are accelerated in a strong electric field, they gain energy, and this energy induces the release of new free electrons and ions (ionisation), which fuels the same process. If this mechanism occurs faster than it is naturally quenched by ions recombining, new ions multiply in successive cycles until the gas is broken down into plasma and current flows freely in a discharge.

The dielectric breakdown mechanism in gases relies on electron avalanches. Corona discharges, streamers, leaders, or a spark or continuous electric arc that fully bridges the gap are all possible outcomes of the operation. Streamers in lightning discharge spread by forming electron avalanches in the high potential gradient ahead of the streamers' advancing tips, and this mechanism has the potential to extend massive sparks. The creation of photoelectrons induced by UV radiation emitted by the excited medium's atoms in the aft-tip field often amplifies avalanches once they have started. The resulting plasma's incredibly high temperature cracks the surrounding gas molecules, allowing the free ions and recombine to form new chemical compounds.

Since the passing of a single particle can be intensified to massive discharges, the process can also be used to detect radiation that initiates the process. This is how a Geiger counter works, as well as how a spark chamber and other wire chambers can be visualized.

Avalanche Breakdown in Semiconductor

In semiconductors, which conduct electricity in some ways like a slightly ionised gas, an avalanche breakdown process may occur. Conduction in semiconductors is based on free electrons knocked out of the crystal by thermal vibration. As a result, unlike metals, semiconductors improve their conductivity as the temperature rises. The same form of positive feedback is set up here; heat from current flow causes the temperature to rise, which increases charge carriers, lowers resistance, and allows more current to flow. This can go on until the usual resistance at a semiconductor junction completely breaks down, resulting in the system failing (this may be temporary or permanent depending on whether there is physical damage to the crystal). Certain instruments, such as avalanche diodes, make use of the effect on purpose.

History of Chemical Chain Reaction

Max Bodenstein, a German chemist, first proposed the concept of chemical chain reactions in 1913. When two molecules react, they produce not only molecules of the final reaction products, but also certain unstable molecules that have a far higher chance of reacting with the parent molecules than the initial reactants. 

To understand the quantum yield phenomenon, Walther Nernst suggested in 1918 that the photochemical reaction between hydrogen and chlorine is a chain reaction. This means that one photon of light will result in the formation of up to 106 molecules of the HCl substance. According to Nernst, the photon splits a Cl2 molecule into two Cl atoms, each of which starts a long chain of reactions that leads to the formation of HCl.

In a 1923 paper on polymer formation, Danish and Dutch scientists Christian Christiansen and Hendrik Anthony Kramers pointed out that a chain reaction need not start with a molecule excited by light, but could instead begin with two molecules colliding violently due to thermal energy, as suggested by van't Hoff for chemical reaction initiation.

Christiansen and Kramers also noted that if two or more unstable molecules are formed in one link of the reaction chain, the reaction chain will branch and expand. The effect is exponential growth, which leads to explosive changes in reaction rates and, in some cases, chemical explosions. The mechanism of chemical explosions was first proposed in this way.

Did You Know?

• In 1934, Soviet physicist Nikolay Semyonov established a quantitative chain chemical reaction theory. In 1956, Semyonov shared the Nobel Prize with Sir Cyril Norman Hinshelwood, who formed many of the same quantitative principles independently.

• In his laboratory underneath the bleachers of Stagg Field at the University of Chicago, Enrico Fermi, an Italian-born Nobel Prize-winning physicist, guides and controls the first nuclear chain reaction, ushering in the nuclear era.