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.