1. Reaction Kinetics

2. Reaction Kinetics
Main types of questions:
- Determine rate eqn using (1) continuous mtd, (2) initial rate
mtd or both
- Elucidate mechanism based on rate eqn
- Use collision theory to explain how rate depends on (1)
temp (2) conc (3) surface area (4) catalyst

3. Reaction Kinetics
- study of the rate of reactions
- rate of reaction can be described as how fast products are
formed or reactants are reacted.

4. Reaction Kinetics
- we have learnt in ‘O’ levels that rate of rxn depends on conc.
of reactants
- the mathematical dependence of rate on conc. of each
reactant is known as the rate law.

5. Reaction Kinetics
Consider reaction where A and B reacts to form P:
A+B→P
The general rate eqn:
Rate = k[A]m[B]n
m: order w.r.t. A
m+n: overall order
n: order w.r.t. B
Most qns require finding m and n.

6. Determining
Rate Law
Rate = k[A]m[B]n
m and n can only be found experimentally.
There are two methods:
1. Continuous method
2. Initial rate method
continuous method

7. Determining
Rate Law
1. Continuous method
- Only one experiment
- Plot [rxt] or [pdt] versus t
- Look for t1/2 (1st order) or shape of plot (0 order)
-
Generally used for 0 or 1st order kinetics
continuous method

8. Determining
Rate Law
A+BP
-
t1/2 constant
-
if [B]>>[A]:
1st order w.r.t A
if [A]>>[B]:
1st order w.r.t B
-
continuous method

9. Determining
Rate Law
When 2nd t1/2 cannot be found
at the ¼x, start with any y value
and find time taken to become
y/2.
y
y/2
t2
continuous method

10. Determining
Rate Law
A+BP
-
t1/2 constant
-
[P
]
if [B]>>[A]:
1st order w.r.t A
if [A]>>[B]:
1st order w.r.t B
-
continuous method

11. Determining
Rate Law
A+BP
[P]
Note: For [P] vs t,
1. You will need to determine the max
conc. of pdt first (using molar
calculation or extrapolation)
2. The two t1/2 are measured from 0 to ½
and ½ to ¾
continuous method

12. Determining
Rate Law
A+BP
-
straight line
if [B]>>[A]:
0 order w.r.t A
if [A]>>[B]:
0 order w.r.t B
[P
]
continuous method

13. Determining
Rate Law
2. Initial Rate Method
- More than one expt
- Change initial conc of rxt each time; measure initial rate
i.e. gradient at t=0
- Compare for different expts
initial rate method

14. Determining
Rate Law
2. Initial Rate Method (Example)
Run
[A]/ moldm–3
[B]/ moldm–3
time/ s
1/time /s–1
1
0.1
0.1
5
0.2
2
0.2
0.1
2.5
0.4
3
0.1
0.2
1.25
0.8
Compare expt 1&2:
[A] x 2, rate x 2
Compare expt 1&3:
[B] x 2, rate x 4
 Rate ∞ [A]1
 Rate ∞ [B]2
initial rate method

15. Application of Rate Laws
Mechanism
Rate laws can be used to evaluate reaction mechanism.
From a given rate law, you can determine if:
1. the reaction is single step or multi-step.
2. what species react in the slow step and in what ratio

16. Application of Rate Laws
Mechanism
From a given rate law, you can determine if:
1. the reaction is single step or multi-step
If the reactants in the overall equation matches that in the rate
law in the exact reacting ratio, the mechanism could be single
step.
Otherwise, it is definitely multi step.

17. Application of Rate Laws
Mechanism
Example: A + 2B  P
Rate Law
Rate = k [A]
Rate = k [A][B]
Rate = k [A][B]2
Conclusion
Mechanism is
multistep
 B does not
appear in rate law
Mechanism is
multistep
 although both
A & B appear in
rate law, it is not
in the reacting
ratio i.e. 1:2
Mechanism could
be single step
 both A & B
appear in rate law in
the exact reacting
ratio

18. Application of Rate Laws
Mechanism
From a given rate law, you can determine if:
2. what species react in the slow step and in what ratio
For a multi-step mechanism, the rate law shows the species
reacting in the slow step (r.d.s.); the order shows the reacting
ratio.

19. Application of Rate Laws
Mechanism
Example: A + 2B  P
Rate Law
Rate = k [A][B]
Conclusion
(1) Mechanism is multistep
(2) In the slow step, one atom/ molecule/ ion of A reacts
with one atom/ molecule/ ion of B
Possible mechanism:
A+BI
I+BP
overall
A + 2B  P
I : intermediate
(slow)
(fast)

20. Factors affecting Rate
Rate =
k [A]m[B]n
k = Ae
surface area
–Ea/RT
catalyst
concentration
/pressure
temperature

21. Collision Theory
At the atomic/ molecular level, the rate of reaction depends on:
1.
2.
freq of collision
3. freq of effective
collisions
fraction of particles having E
≥ Ea
- If 1 or 2 increases, 3 will increase.
- All the factors that affect rate can be explained using 1
and/ or 2

22. Collision Theory
Factors affecting Rate
Concentration/ pressure ↑
 no. of particles per unit volume ↑
 freq of collision ↑
 freq of effective collision ↑

23. Collision Theory
Factors affecting Rate
Particle size ↓
 surface area of reactant ↑
 freq of collision ↑
 freq of effective collision ↑

24. Collision Theory
Factors affecting Rate
Temperature ↑
 avg K.E. of particles ↑
 freq of collision ↑
 fraction of particles having E ≥ Ea ↑
 freq of effective collision ↑

25. Collision Theory
Factors affecting Rate

26. Collision Theory
Factors affecting Rate
Use of catalyst
 provide alternative pathway with lower Ea
 fraction of particles having E ≥ Ea ↑
 freq of effective collision ↑

27. Collision Theory
Factors affecting Rate