As reaction Htwo+Cltwo→ The mechanism of 2HCl is as follows:
Cltwo+M→2Cl+M （1）
Cl+Htwo→HCl+H （2）
H+Cltwo→HCl+Cl （3）
……………
2Cl+M→Cltwo+M（4）
In reaction (1), the active component chlorine atom is generated by thermal, optical, electrical or chemical action, and then in reaction (2) and (3), the active component interacts with the reactant molecule to alternately and repeatedly generate new active component chlorine atom and hydrogen atom, so that the reaction can continue to circulate until the active component disappears, which is known as chain reaction.The active component in the reaction is called chain carrier.
mechanism
The mechanism of chain reaction generally includes three steps:
①Chain initiation。It is a reaction that generates the first chain carrier in the reaction system by thermal, optical, electrical, chemical and other actions. It is generally a reaction that stabilizes molecules to decompose into free radicals, such as reaction (1).
② Chain transmission.The reaction of new chain carrier and new saturated molecule is generated by the interaction of chain carrier and saturated molecule, such as reaction (2), (3).
③Chain termination。The death process of chain carrier, such as reaction (4).Where M is the third body (other molecules or reactor wall, etc.) that receives the energy released by chain termination.
In the chain transfer stage, if an old chain carrier disappears, only a new chain carrier will be generated, which is calledLinear chain reaction；If the disappearance of an old chain carrier leads to the generation of two or more new chain carriers, it is called a branched chain reaction.[1]
classification
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Linear chain reaction
In the chain reaction, the free radicals of the active component are constantly regenerated, and the free valence remains unchanged, such as the following reactions:
Cl·+Htwo→HCl+H·
H·+Cltwo→HCl+Cl·
In the reaction, the chlorine atom (or hydrogen atom) with single free price generates the hydrogen atom (or chlorine atom) with single free price.
Branched chain reaction
In chain reaction, when a single free valence chain carrier reacts with a molecule, more than one new free radical can be produced, which is a branched chain reaction, such as the following reactions:
H+Otwo→HO+O
O+Htwo→OH+H
Plus other chain transfer processes: OH+Htwo→HtwoO+H，Then HtwoThe diagram of O reaction is as follows:
It is not difficult to see that in the above reactions, just as one neutron can produce three new neutrons in a nuclear reaction, one H can produce three new H, and so on.Therefore, under certain conditions, the rate of the reaction can become infinite, so explosion can occur.
Degenerate branched chain reaction
In the process of chain branching, more stable active molecules (such as organic peroxides) are generated than the chain carrier, and this active molecule can decompose more than one branched chain carrier, so that the branching process can be carried out.However, the process of molecular decomposition to produce chain carrier is much slower than the reaction of chain carrier.This is called degenerate branched chain reaction.Liquid phase oxidation of some organic substances belongs to this kind of reaction.[2]
Main steps
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Chain initiation
Chain initiation:
Molecules in stable state absorb external energy, such as heating, lighting or adding initiators, so that they can be decomposed into free atoms or free radicals and other active transmitters.The activation energy is equivalent to the bond energy of the broken bond.
Chain transmission
Chain propagation:
The active transporter generated by chain initiation interacts with another stable molecule to form a product while generating a new active transporter, so that the reaction continues to develop like a chain.
Termination of chain
Chain termination:
Two active transmitters collide to form stable molecules or disproportionation, losing the transmission activity;Or it collides with the vessel wall to form stable molecules, and the released energy is absorbed by the vessel wall, causing the reaction to stop.[3]
