The explosion of gas mixture has two different processes: thermal reaction and chain reaction
(1) Thermal reaction mechanism
Combustion and explosion of gas mixture are essentially the same from the point of view of chemical reaction
When the combustion is carried out in a certain air direction, if the heat dissipation is not good, the reaction temperature will continue to rise, and the increase in temperature will accelerate the reaction rate, so that the cycle continues to cause an explosion
(2) Chain reaction mechanism
Some explosion phenomena cannot be explained by the thermal reaction mechanism
For example, when a mixture of bromine and hydrogen explodes at a lower temperature, the reaction formula is: H 2 +Br 2 =2HBr+3.
5kJ/mol, the heat of reaction is only 3.
5kJ/mol; while the reaction between sulfur dioxide and hydrogen, the reaction The formula is: SO 2 +3H 2 =H 2 S+2H 2 O+12.
6kJ/mol, the heat of reaction is 12.
6kJ/mol but it will not explode
There are many examples like this.
Therefore, some explosion phenomena cannot be explained by the thermal reaction mechanism, but need to be explained from the viewpoint of chemical kinetics.
Such explosions are the result of chain reactions
According to the chain reaction theory, after explosive mixtures (such as flammable explosive gases and oxygen) are in contact with external energy (such as fire sources), the activated molecules will absorb energy and dissociate into free radicals, that is, free radicals.
The free radicals are used as the chain reaction The active center is thus formed
On this basis, the heat and the chain carrier spread outwards, prompting the mixture of the adjacent layer to chemically react, and then this layer becomes the source of heat and the chain carrier to cause the reaction of the other layer of the mixture and interact with other molecules.
The interaction forms a series of chain reactions
This cycle continues, releasing the heat of combustion, until all the explosive mixture has reacted
The flame of the explosion spreads in all directions in the form of layers of concentric spherical surfaces
The whole process was completed in a very short time
There are two types of chain reactions: straight-chain reactions and branched-chain reactions
The linear reaction means that each free radical undergoes its own chain reaction.
For example, chlorine and oxygen belong to this type of reaction, that is, the chlorine molecule is activated into two chlorine radicals under the action of light.
The free radicals all carry out their own chain reaction, and each reaction only leads to a new free radical
Branched chain reaction
Branched chain reaction means that one free radical can generate more than one new free radical in the reaction.
Because the branched chain reaction has more active particles-the generation of free radicals, the reaction rate can be accelerated rapidly, the temperature rises rapidly, and the volume expands.
, The pressure in the confined space increases rapidly, causing an explosion
For example, the chain reaction of hydrogen and oxygen belongs to this type of reaction.
Its characteristic is that a free radical (active center) can generate more than one free radical in the reaction: H·+O 2 =OH·+O·, O· +H 2 =OH·+H·, so the reaction chain will branch
The branching reaction is shown in Figure 13-1
In the case that chain growth means that the reaction can proliferate free radicals, if the reaction rate of destroying free radicals (chain termination) at the same time is not high, the number of free radicals will increase, and the number of reaction chains will also increase.
The rate will increase accordingly, which will add more free radicals, so the cycle progresses, and the reaction rate can be accelerated to an explosive level in an instant
Figure 13-1 Branched chain reaction
The chain reaction process is roughly divided into three stages: ① chain initiation, free radical generation; ② chain transfer, the free radical acts on other compounds involved in the reaction to generate new free radicals; ③ chain termination, that is, the consumption of free radicals, so that The chain reaction ended