D.I. Mendeleev A.M.Butlerov

The velocity of chemical reactions
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Influence of pressure on the velocity. Additional material.

Chain reaction includes in its mechanism a number of sequentially repeated elementary acts of the same type (chain).

Lets take the reaction:

H2 + Cl2 = 2HCl

It consists of following stages, that are common for all chain reactions:

1) Initiation, or the beginning of a chain

Cl2 = 2Cl

Decomposition of a molecule of chlorine on atoms (radicals) occurs at the UV-irradiation or at heating. The main point of a stage of initiation is formation of active, reactive particles.

2) Chain propagation

Cl + H2 = HCl + H
H + Cl2 = HCl + Cl

As a result of each elementary act of chain propagation the new radical of chlorine is formed, and this stage repeats again and again, theoretically - up to a full expenditure of reagents.

3) Recombination, or circuit opening

2Cl = Cl2
2H = H2
H + Cl = HCl

The radicals, appeared close to each other, can recombine, forming a steady particle (molecule). They give excess of energy to the "third particle" - for example, to walls of a vessel or to molecules of impurity.

Considered chain reaction is not branched since the amount of radicals does not increase in the elementary act of development of a chain. Chain reaction of interaction between hydrogen and oxygen is branched since the amount of radicals in the elementary act of development of a chain increases:

H + O2 = OH + O
O + H2 = OH + H
OH + H2 = H2O + H

Many reactions of burning are related to the branching-chain reactions. Uncontrollable growth of number of free radicals (both as a result of a branching of a chain, and for not branched reactions in case of too fast initiation) can lead to strong acceleration of reaction and explosion.

It seems to, the more pressure we give, the more concentration of radicals we have and explosion is more probable. But actually for reaction of hydrogen with oxygen explosion is possible only in the certain range of pressure: from 1 up to 100 mm hg and above 1000 mm hg. It follows from the mechanism of reaction. At small pressure the most part of formed radicals recombines on walls of a vessel, and reaction goes slowly. At increase of pressure up to 1 mm hg radicals reach walls rarely since they react with molecules more frequently. In these reactions radicals are being multiplied that leads to explosion. However at pressure above 100 mm hg concentration of substances increases so much, that radicals begin to recombine as a result of threefold impacts (for example, with a molecule of water), and reaction proceeds easy, without explosion (stationary current). Above 1000 mm hg concentration become very great, and even threefold impacts are not sufficient to prevent multiplication of radicals.

It is known to you that the branching-chain reaction of division of uranium-235, where in each elementary act 1 neutron (playing a role of a radical) is trapped and up to 3 neutrons are emitted. Depending on conditions (for example, concentration of neutrons absorbers) it is also probable stationary current or explosion. It is one more example of correlation of chemical and nuclear processes kinetics.


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