1. §10.1 Typical complex reactions
Chapter X
Kinetics of Complex Reactions
§10.1 Typical complex reactions
Outside classroom reading:
Levine: p

2. Typical complex reactions
1) Opposing Reaction
2) Parallel Reaction
3) Consecutive Reaction

3. 1.1 Opposing Reaction / reversible reaction
The forward and the backward / reverse reaction take place simultaneously.
for opposing reaction consisting of elementary reactions:
As reaction proceeds, r+ increases while r decreases. When r+ becomes equal to r, equilibrium is reached.

4. Can we extend this discussion to ammonia synthesis?
therefore
The connection between the equilibrium constant (Kc) and the rate coefficients of simple reactions. This relation, named as kinetic equilibrium constant.
Can we extend this discussion to ammonia synthesis?
It is correct only for elementary reactions!

5. Under equilibrium conditions
(2) rate equation
For first-first order
opposing reaction:
t = 0 a
t = t
a-x x
t = te
a-xe xe
The total rate is
Under equilibrium conditions

6. Principle of relaxation method for studying fast reaction
The first equilibrium gives a; the second equilibrium gives xe.
k+ and k can be determined by measuring x at t and at equilibrium concentration.
Principle of relaxation method for studying fast reaction

7. Why? 1-2 opposing reaction 2-2 opposing reaction
Other opposing reactions
1-2 opposing reaction
2-2 opposing reaction

8. 1.2 Parallel reaction / Competing reaction
When
When
The rate of parallel reaction is determined mainly by the faster one.

9. t x = y + z a For production of B and C: A B C a a-x y z
Integration of the equation yields:
For production of B and C: A B C a
a-x y z
x = y + z

10. A B C t c
The composition of the final products is fixed.
selectivity of the reaction.

11. Optimum temperature for better selectivity
Example
A  B A1
Ea, 1
A  C A2
Ea, 2
logA2
1/T
logA1
log k B C
logA2
1/T
logA1
logk B C

12. Selectivity: Main reaction and Side reaction:
reaction with higher k is taken as the main reaction, while others side reactions.
Reaction that produces the demanded product is the main reaction.
Selectivity:

13. 1.3 Consecutive reaction a = x + y + z CH4 + Cl2 CH3Cl CH2Cl2 CHCl3
Some reactions proceed through the formation of intermediate.
CH4 + Cl2
CH3Cl
CH2Cl2
CHCl3
CCl4
General reaction A B C
t = 0 a
t = t x y z
a = x + y + z

14. C
tmax t A B

15. At tmax, the concentration of B = ?
shows that the intermediate’s concentration rises from zero to a maximum and then drops back to zero as A is depleted and C dominates in the mixture.
If C is the demanded product, the reaction time should be prolonged. If B is the demanded product, the reaction should be interrupted at optimum time, i.e., tmax.
At tmax, the concentration of B = ?

17. k2/k1
1/5 5 10
100
103
108
tmax
2.01
0.40
0.25
0.047
710-3
10-7
ymax/a
0.67
0.13
0.08
7 10-3
10-3
Ea,1Ea,2
-0.4
4.0
5.7
11.5
17.2
46.1 t y
k1/k2
When k2 >> k1, ymax would be very small, and the tmax would be very short.

18. Steady-state approximation
Physical meaning of k2 >> k1
B is a active intermediate (Such as active atom: Cl, H, etc., radicals: CH3•, H2C:, C+, C-, etc., activated molecules: A*), it is difficult to form but easy to decompose to product.
For consecutive reaction with large k2/k1 ratio, once the reaction take place, the active intermediate (B) rapidly attains its maximum concentration and its concentration keeps nearly unchanged during the whole reaction.
Steady-state approximation

19. When k2 >> k1
The total rate is determined mainly by k1
When k2 << k1
The total rate is determined mainly by k2
The rate of the overall consecutive reaction depends only on the smaller rate constant (rate-determining step).

20. rate-determining step (r. d. s.): the step with the slowest rate.
?? !! 
It’s a r.d.s


patient !

21. Rate-determining step approximation
The rate of the elementary step with the lowest rate constant, i.e., r.d.s., can be used to express the actual rate of the overall reaction.
What is a eligible r. d. s.?
Its activation energy should be 10 kJmol-1 more than that of other steps.