§ 10.3 Chain Reaction

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§ 10.3 Chain Reaction pressu re temperatu re 600 o C

description

pressure. 600 o C. temperature. § 10.3 Chain Reaction. 3.1 basic concepts of chain reaction:. chain-initiating step. chain-propagating steps. chain-terminating step. - PowerPoint PPT Presentation

Transcript of § 10.3 Chain Reaction

§ 10.3 Chain Reaction

pre

ssu

re

temperature

600 oC

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.