§ 10.3 Chain Reaction
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Transcript of § 10.3 Chain Reaction
3.1 basic concepts of chain reaction:
chain-initiating step
chain-propagating steps
chain-terminating step
The chain-initiating step is usually a reaction in which a molecule is
decomposed by thermal collision or radiation. In some cases, a
molecule named as initiator, which can readily decompose and
produces radicals, is introduced into the system. Some activated
species, such as free atoms, radicals, carbonium ions, act as chain
initiating species, which is usually called chain carriers.
1
2
3
4
5
2
2
2
2
2
Br 2Br
Br H HBr H
H Br HBr+Br
H +HBr H +Br
2Br Br
k
k
k
k
k
Owing to the participation of active intermediate, the activation
energy of chain reactions, usually ranging between 0 ~ 40 kJ mol-1, is
much lower than that of other kinds of reactions of 40 ~ 400 kJ mol-1.
Therefore, chain reactions can proceed rapidly.
chain-propagating steps
A chain reaction is one in which some intermediates are consumed
and regenerated in a cycle of reactions the net result of which is to
carry forward the overall reaction.
2
3
2
2
Br +H HBr+H
H +Br HBr+Br
k
k
Basic characteristics of chain reaction:
1) with active radicals participating;
2) low activation energy of chain propagation
3) wide existence
Basic characteristics of chain reaction:
1) with active radicals participating;
2) low activation energy of chain propagation
3) wide existence
Examples for Chain reactions:
1) H2 + Cl2 ; H2 + Br2 ;
2) Pyrolysis (thermal decomposition) of many organic molecules.
3) polymerization:
3.2 rate equation of chain reactionPolymerization initiated by initiator
X-X X
X + RCH=CH2 XRCH-CH2
XRCH-CH2 + RCH=CH2 XRCH-CH2-RCH-CH2
……
XR(CH-CH2)n-1CH-CH2 + RCH=CH2 XR(CH-CH2)nCH-CH2
XR(CH-CH2)nCH-CH2 +X XR(CH-CH2)nCH-CH2X
XR(CH-CH2)n + XR(CH-CH2)m XR(CH-CH2)n (CH-CH2) mRX
k2
k3
112
1 22 2
3
[R-CH=CH ][X-X]k
r kk
Just likes that of H2+Cl2
3.3 Branched chain reactions and explosion
According to Arrhenius equation:aE
RTk Ae
the rate constant of the reaction increases exponentially with temperature.
If an exothermic reaction is constrained to
take place in a limited volume and the heat
cannot be dissipated, temperature will
increase. The higher temperature will, in turn,
accelerate the reaction and produce more
heat, which causes a still higher temperature. This will finally result in a catastrophe-an explosion, which is named as thermal explosion.
p1 p2 p3
The most extensively studied branched chain reaction is the formation
of water. 2 H2(g) + O2(g) 2 H2O (g)
It was found that, the rate of the
reaction is highly dependent on the
surface of container, temperature, and
pressure.
Explosion of H2-O2 mixture
explosion limit;
region of explosion;
region of normal reaction.
Stable peninsula
Explosion peninsula
The complete mechanism for the reaction is still not
unambiguously established. One simple sequence consists of the
following elementary steps was proposed:
pre
ssu
re
temperature
600 oC400 oC
Initiation: 2H 2H
Propagation:
2 2 2H H O H O HO
2k2 2HO H H O H
Branching:
gaseous termination:
Wall termination:
k32H O HO O
4k2O H HO H
5k22H M H M
2HO H M H O M
H wall destroyed
HO wall destroyed
For certain reaction under appropriate conditions, propagating steps occur in
which the loss of a single propagator is overcompensated for by the production of
two or more other propagating species, such chain reaction is branched chain
reaction.
Initiation: 1kA R
Propagation: 2kR A P R
gaseous termination: gkR
wkR Wall termination:
21 2
2
[A]
[A](1 ) w g
k kr
k k k
Under low pressure: the free range of molecules is large, the wall termination is dominate, therefore, the explosion depends on the volume and the nature of the vessel.
At high pressure: the free range of molecules is small, the gaseous termination is dominate, therefore, the explosion does not depend on the volume and the nature of the vessel.
21 2
2
[A]
[A](1 ) w g
k kr
k k k
21 2
2
[A]
[A](1 ) w g
k kr
k k k
When = 1, the denominator (kw + kg) has certain value, and the
reaction will take place at a moderate rate.
When > 1, i.e., branched chain reaction, k2[A](1-) is of minus
value, if k2[A](1-) + kw + kg 0, the reaction rate , and explosion
occurs.