The document provides a tutorial on reaction mechanisms, rate determining steps, order of reactions, and key concepts like molecularity, intermediates, and transition states. It discusses how to propose a reaction mechanism based on the overall stoichiometry and experimentally determined rate law. The criteria for a proposed mechanism are that each step must add up to the overall balanced equation, the steps must be reasonable involving unimolecular or bimolecular collisions, and the slow step must match the experimental rate law. Several examples of mechanisms are provided and checked against these criteria.

1. http://lawrencekok.blogspot.com
Prepared by
Lawrence Kok
Tutorial on Rate Determining Step, Reaction Mechanism and
Order of Reaction

2. Reaction Mechanism
Reaction Mechanism
• Order can only be determined experimentally, cant be predicted from stoichiometry
• Mechanism cant be proven, can only suggest a most likely mechanism
• Sequence of rxn steps involve individual/elementary steps from reactant to product
• Elementary steps involve slow and fast steps, may contain intermediate species.
• Mechanism must account for overall stoichiometry of rxn0
Keywords
• Molecularity – Number species involve in elementary step
• Unimolecular – 1 species involve in elementary step
A → B + C Rate = k[A]
• Bimolecular – 2 species involve in elementary step
A + B → C + D Rate = k[A][B]
•Termolecular – 3 species involve in elementary step
A + B + C → D Rate = k[A][B][C]
Intermediates Transition State
Exist a while Only in transient state
Occurs at PE minimum Occurs at PE maximum
Formed in 1 step and
consumed in next step
Exist through every step
involve bond
break/make
Diff bet Intermediates and Transition states
Rate Determining Step (RDS)
Slowest step, highest activation energy
Limit overall rate. Its rate law represent rate law for overall rxn
Criteria for proposed mechanism
• Mechanism must account for overall stoichiometry rxn and experimentally determined Rate Law
• Elementary step MUST add up to overall balanced eqn
• Elementary step must be reasonable/likely, involve unimolecular/bimolecular collision
(Termolecular collision, 3 molecules colliding together is unlikely due to probability/ orientation factor)
• Mechanism MUST correlate with Rate Law
Video on rxn mechanism

3. Criteria for proposed mechanism
• Mechanism must account for overall stoichiometry rxn and experimentally determined Rate Law
• Elementary step MUST add up to overall balanced eqn
• Elementary step must be reasonable/likely, involve unimolecular/bimolecular collision
(Termolecular collision, 3 molecules colliding together is unlikely due to probability/ orientation factor)
• Mechanism MUST correlate with Rate Law
Propose mechanism : Step Molecularity Rate elementary step
• NO2 + F2 → NO2F + F Slow (RDS) 2 Rate = k[NO2]1
[F2]1
• NO2 + F → NO2F Fast 2 Rate = k[NO2]1
[F]1
2NO2 + F2 →2NO2F (overall eqn)
2NO2 + F2 → 2NO2F
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
Rate slow step (RDS) = Expt Rate Law
k[NO2]1
[F2]1
= k[NO2]1
[F2]1
+
Mechanism is correct!
2NO2 + F2 → 2NO2F
Rate Law = k[NO2]1
[F2]1
Rate 1st order to NO2 and F2
Mechanism which correlate with Rate Law
2ICI + H2 → 2HCI + I2
Rate Law = k[ICI]1
[H2]1
Rate 1st order to ICI and H2
Mechanism which correlate with Rate Law
Propose mechanism : Step Molecularity Rate elementary step
• ICI + H2 → HCI + HI Slow (RDS) 2 Rate = k[ICI]1
[H2]1
• ICI + HI → HCI + I2 Fast 2 Rate = k[ICI] [HI]1
2ICI + H2 →2HCI + I2 (overall eqn)
2ICI + H2 → 2HCI + I2
+
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
Rate slow step (RDS) = Expt Rate Law
k[ICI]1
[H2]1
= k[ICI]1
[H2]1
Mechanism is correct!

4. CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
Rate slow step (RDS) = Expt Rate Law
k[NO2]1
[F2]1
= k[NO2]1
[F2]1
+
Mechanism is incorrect!
+
Mechanism is correct!
Mechanism whereby Step 2 rate determining step
Rate Law = k[NO]2
[O2]1
Rate 2nd order NO and 1st order O2
Mechanism which correlate with Rate Law
2NO + O2 → 2NO2
Propose mechanism : Step Molecularity Rate elementary step
• NO + NO + O2 → 2NO2 Slow (RDS) 3 Rate = k[NO]2
[O2]1
• 2NO + O2 → 2NO2 (overall eqn)
Proposed Mechanism possible but UNLIKELY due to 3 molecule colliding together
Propose mechanism : Step Molecularity Rate elementary step
• NO + O2 → NO3 Fast 2 Rate = k[NO]1
[O2]1
• NO3 + NO → 2NO2 Slow (RDS) 2 Rate = k[NO3]1
[NO]1
• 2NO + O2 → 2NO2 (overall eqn)
2NO + O2 → 2NO2
2NO + O2 → 2NO2
2NO + O2 → 2NO2
Rate Law dont contain intermediates - NO3
NO3 , intermediate depend on NO and O2
Conc [NO3] = [NO][O2] from step 1
Rate = k[NO3]1
[NO]1
= k [NO][O2][NO] = k[NO]2
[O2]1
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
Rate slow step (RDS) = Expt Rate Law
k[NO2]1
[F2]1
= k[NO2]1
[F2]1
Rate Law = k[NO]2
[O2]1
Rate 2nd order NO and 1st order O2
Mechanism which correlate with Rate Law

5. Propose mechanism : Step Molecularity Rate elementary step
• A + B → X Fast 2 Rate = k[A]1
[B]1
• X + C → D Slow(RDS) 2 Rate = k[X]1
[C]1
• A + B + C → D (overall eqn)
Propose mechanism : Step Molecularity Rate elementary step
•A + B + C → D Slow(RDS) 3 Rate = k[A]1
[B]1
[C]1
•A + B + C → D (overall eqn)
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
+
Mechanism is incorrect!
+
Mechanism is correct!
Mechanism whereby Step 2 rate determining step
Proposed Mechanism possible but UNLIKELY due to 3 molecules colliding together
Rate Law dont contain intermediate - X
X, intermediate depend on A and B
Conc [X] = [A][B] from step 1
Rate = k[X]1
[C]1
= k [A][B][C]
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
Rate Law = k[A]1
[B]1
[C]1
Rate 1st order to A, B and C
Mechanism which correlate with Rate Law
A + B + C → D A + B + C → D
Rate slow step (RDS) = Expt Rate Law
k[A]1
[B]1
[C]1
= k[A]1
[B]1
[C]1
A + B + C → D A + B + C → D
Rate slow step (RDS) = Expt Rate Law
k[A]1
[B]1
[C]1
= k[A]1
[B]1
[C]1
Rate Law = k[A]1
[B]1
[C]1
Rate 1st order to A, B and C
Mechanism which correlate with Rate Law

6. Propose mechanism : Step Molecularity Rate elementary step
•A + B → AB Fast 2 Rate = k[A]1
[B]1
•AB+ B → AB2 Slow(RDS) 2 Rate = k[AB]1
[B]1
•AB2 + B → AB3 Fast 2 Rate = k[AB2]1
[B]1
•A + 3B → AB3 (overall eqn)
Propose mechanism : Step Molecularity Rate elementary step
•A + B + B → AB2 Slow(RDS) 3 Rate = k[A]1
[B]2
•AB2 + B → AB3 Fast 2 Rate = k[AB2]1
[B]1
•A + 3B → AB3 (overall eqn)
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
+
Mechanism is incorrect!
+
Mechanism is correct!
Mechanism whereby Step 2 rate determining step
Proposed Mechanism possible but UNLIKELY due to 3 molecules colliding together
Rate Law dont contain intermediate - AB
AB, intermediate depend on A and B
Conc [AB] = [A][B] from step 1
Rate = k[AB]1
[B]1
= k [A][B][B] = k[A]1
[B]2
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
Rate Law = k[A]1
[B]2
Rate 1st order A and 2nd order B
Mechanism which correlate with Rate Law
A + 3B → AB3
A + 3B → AB3
Rate slow step (RDS) = Expt Rate Law
k[A]1
[B]2
= k[A]1
[B]2
A + 3B → AB3
Rate Law = k[A]1
[B]2
Rate 1st order A and 2nd order B
Mechanism which correlate with Rate Law
A + 3B → AB3
Rate slow step (RDS) = Expt Rate Law
k[A]1
[B]2
= k[A]1
[B]2

7. Propose mechanism : Step Molecularity
•CH3COCH3 + H+
→ CH3C(OH+
)CH3 Slow(RDS) 2
•CH3C(OH+
)CH3 → CH2=C(OH)CH3 + H+
Fast 2
•CH2=C(OH)CH3 + I2 → CH3 COCH2I + H+
+ I-
Fast 2
• CH3COCH3 + I2 → CH3 COCH2I + H+
+ I-
(overall eqn)
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
+
Mechanism is correct!
Mechanism Iodination of Propanone with Intermediates
CH3COCH3 + I2 → CH3COCH2I + H+
+ I-
CH3COCH3 + I2 → CH3COCH2I + H+
+ I-
Rate Law = k[CH3COCH3 ][H+
]
Rate 1st order to CH3COCH3 and H+
Mechanism which correlate with Rate Law
Rate slow step (RDS) = Expt Rate Law
k[CH3COCH3]1
[H+
]1
= k[CH3COCH3]1
[H+
]1
Intermediates Transition State
Exist a while Only in transient state
Occurs at PE minimum Occurs at PE maximum
Formed in 1 step and
consumed in next step
Exist through every step involve
bond break/make
CH3COCH3 + I2 → CH3COCH2I + H+
+ I-
CH3COCH3 + I2
CH3COCH2I + H+
+ I-

8. Propose mechanism : Step Molecularity
•H2O2 + I-
→ H2O + IO-
Slow(RDS) 2
•IO-
+ H+
→ HIO Fast 2
•HIO + H+
+ I-
→ H2O + I2 Fast 2
•H2O2 + 2H+
+ 2I-
→ 2H2O + I2 (overall eqn)
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
+
Mechanism is correct!
Mechanism Iodine Clock with Intermediates
Intermediates Transition State
Exist a while Only in transient state
Occurs at PE minimum Occurs at PE maximum
Formed in 1 step and
consumed in next step
Exist through every step involve
bond break/make
H2O2 + 2H+
+ 2I-
→ 2H2O + I2 H2O2 + 2H+
+ 2I-
→ 2H2O + I2
H2O2 + 2H+
+ 2I-
→ 2H2O + I2
Rate Law = k[H2O2 ][I-
]
Rate 1st order to H2O2 and I-
Mechanism which correlate with Rate Law
Rate slow step (RDS) = Expt Rate Law
k[H2O2 ]1
[I-
]1
= k[H2O2 ]1
[I-
]1
H2O2 + 2H+
+ 2I-
2H2O + I2
IO-
HIO

9. Propose mechanism : Step Mol Rate elementary step
•(CH3)3CBr → (CH3)3C+
+ Br -
Slow (RDS) 1 Rate = k[(CH3)3CBr]1
•(CH3)3C+
+ OH-
→ (CH3)3COH Fast 2 Rate = k[(CH3)3C+
]1
[OH-
]1
(CH3)3CBr + OH-
→ (CH3)3COH + Br -
(overall eqn)
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
+
Mechanism is correct!
Intermediates Transition State
Exist a while Only in transient state
Occurs at PE minimum Occurs at PE maximum
Formed in 1 step and
consumed in next step
Exist through every step involve
bond break/make
Hydrolysis 2-bromo -2methylpropane by SN1 mechanism with Intermediates
(CH3)3CBr + OH -
→ (CH3)3COH + Br-
(CH3)3CBr + OH -
→ (CH3)3COH + Br-
(CH3)3CBr + OH -
→ (CH3)3COH + Br-
Rate Law = k[(CH3)3CBr]1
Rate 1st order (CH3)3CBr
Mechanism which correlate with Rate Law
Rate slow step (RDS) = Expt Rate Law
k[(CH3)3CBr]1
= k[(CH3)3CBr]1
(CH3)3CBr + OH -
(CH3)3COH + Br-
(CH3)3C+

10. Propose mechanism : Step Mole Rate elementary step
•CH3Br + OH -
→ CH3OH + Br -
Slow(RDS) 2 Rate = k[CH3Br]1
[OH-
]1
CH3Br + OH -
→ CH3OH + Br -
(overall eqn)
CHECK if proposed mechanism correct?
1. Each step add up to overall balanced eqn
2. Each step likely (involve Uni/Bimolecular collision)
3. Slow step (RDS) same as expt Rate Law
Mechanism is correct!
Intermediates Transition State
Exist a while Only in transient state
Occurs at PE minimum Occurs at PE maximum
Formed in 1 step and
consumed in next step
Exist through every step involve
bond break/make
Hydrolysis bromomethane by SN2 mechanism with Transition State
CH3Br + OH -
→ CH3OH + Br- CH3Br + OH -
→ CH3OH + Br-
Rate Law = k[CH3Br]1
[OH -
]1
Rate 1st order CH3Br and OH -
Mechanism which correlate with Rate Law
Rate slow step (RDS) = Expt Rate Law
k[CH3Br]1
[OH-
]1
= k[CH3Br]1
[OH-
]1
CH3Br + OH -
→ CH3OH + Br-
CH3Br + OH-
CH3OH + Br-
HO---CH3--- Br

11. Acknowledgements
Thanks to source of pictures and video used in this presentation
Thanks to Creative Commons for excellent contribution on licenses
http://creativecommons.org/licenses/
Prepared by Lawrence Kok
Check out more video tutorials from my site and hope you enjoy this tutorial
http://lawrencekok.blogspot.com