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CAS CH 110
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Pinghua Liu
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Lecture 31

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Chemistry

CAS CH 110

Pinghua Liu

Spring

Description

Chain Reactions
Chain reactions have highly reactive intermediates that produce more
highly reactive intermediates…and so on.
These reaction intermediate are called a chain carrier.
Radical chain reaction have radical intermediates.
The rate laws are very complex, and not derived here.
The formation of HBr in the reaction takes place by a chain reaction.
H 2g) + Br (2) ® 2 HBr(g)
The first step called, intiation, produces chain carriers H· and Br·, Δ is heat
hv is light.
Br (g) →┴(∆ or hv) Br· + Br·
2
The second step, propagation, creates more chain carriers, in this case
radicals.
Br· + H 2 HBr + H·
H· + Br 2 HBr + Br·
The final step, termination, occures when two chain carriers combine to
form products.
Br· + Br· ® Br 2
H· + Br· ® HBr
Rates and Equilibrium
The equilibrium constant for an elementary reaction is equal to the forward
and reverse rate constants of the reaction, we can show K = "k 1" /" -1"2"
/" -2"
For A + B ⇌C + D Rate = k 1A][B]
C + D ⇌A + B Rate = k -1][D]
k1[A][B] = k-1C][D]
Or "k 1" /" -1""[C][D]" /"[A][B]"
K = "k1" /" -1"
So for multiply steps we get K = "k k ×"k k …
1" /" -1" 2" /" -2"
Models of Reactions: Effects of Temperature In the late nineteenth century, the Swedish chemist Svante Arrhenius found
that the logarithm plot or rate constant (ln k) against the inverse of the
absolute temperature (1/T) is a straight line.
ln k = intercept + slope × "1" /"T"
The intercept is denoted ln A, the slope is denoted -E /R (mora on this
later), where R is the gas constant. With this notation, the empirical
Arrhenius equation is,
Arrhenius equation ln k = ln A - "E a" /"" or k = Ae^("-E a" ⁄" )
The two constants, A and E , areaknown as the Arrhenius parameters for
the reaction and are found from experiment.
A is called the pre-exponential factor.
E as the activation energy.
Both A and E ara nearly independent of temperature but have values that
depend on the reaction being studied.
Arrhenius behavior
"E a"d temperatures are closely tied to each other.
-1
Low E ,a10 kJ·mol , have a low slope, and are not so entirely dependent
on temperature.

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