1. Reaction mechanisms and catalysts
6.4

2. Reaction Mechanisms
The transition state theory is used to explain the process in a reaction that results from an effective collision.
The collision of two particles may result in the formation of a new temporary unstable substance that behaves as a temporary complex in the formation of a more stable product
This high energy unstable state is called an activated complex.
Within the activated complex the bonds are not completely formed or broken which represents a maximum potential energy state called the transition state.

3. Transition state Threshold energy
Activated complex
Threshold energy
Potential Energy (kJ) Ea
Products DH
Add Ea forward and Ea reverse
Reactants
Reaction Progress (s)

4. Reaction Mechanisms
We tend to focus on the start and end of chemical reactions. The pathway from start to finish is often comprised of intermediate or elementary steps.
Elementary steps only involve one, two or possibly three particles in a collision.
Unstable compounds may be produced in these elementary steps and are called reaction intermediates.
The slowest of these elementary steps is the rate determining step.
The sum of these elementary steps represents the overall sequence or reaction mechanism.
Elementary Reaction: each step of a reaction involving a single molecular event.
May involve the formation of different molecules or ions
May involve the formation of different molecules or ions, or may involve a change in the energy or geometry of starting molecules.
Cannot be broken down into further, simpler steps.

5. The overall reaction may be comprised of individual elementary reaction steps. Each involving their own energy changes and rates.
4 HBr + O2
2 H2O + 2 Br2
Overall impression of the reaction’s progress
Potential Energy (kJ)
Reaction Progress (s)
Actual progression of the reaction through 3 intermediate reactions
4 HBr + O2
2 H2O + 2 Br2

6. The overall reaction may be comprised of individual elementary reaction steps. Each involving their own energy changes and rates.
4 HBr + O2
2 H2O + 2 Br2
Activated complexes
Potential Energy (kJ)
Reaction Progress (s)
Reaction intermediates
4 HBr + O2
2 H2O + 2 Br2

7. HBr(g) + O2(g)  HOOBr(g) (slow) HOOBr(g) + HBr(g)  2 HOBr(g) (fast)
2x(HOBr(g) + HBr(g)  H2O(g) + Br2(g)) (fast)
4 HBr(g) + O2(g)  2 H2O(g) Br2(g)
HOOBr
HOBr
Potential Energy (kJ)
Reaction Progress (s)
4 HBr + O2
2 H2O + 2 Br2

8. HBr(g) + O2(g)  HOOBr(g) (slow) HOOBr(g) + HBr(g)  2 HOBr(g) (fast)
Elementary Reaction Steps
Rate Determining Step
HBr(g) + O2(g)  HOOBr(g) (slow)
HOOBr(g) + HBr(g)  2 HOBr(g) (fast)
2x(HOBr(g) + HBr(g)  H2O(g) + Br2(g)) (fast)
4 HBr(g) + O2(g)  2 H2O(g) Br2(g)
HOOBr
HOBr
Reaction Intermediates
Potential Energy (kJ)
Reaction Progress (s)
4 HBr + O2
2 H2O + 2 Br2 Ea DH
Elementary reactions in mechanisms all have different rates.
Rate-determining step: slowest elementary reaction.
This reaction is what the rate of the overall reaction depends on.

9. Rate Law Equations for Elementary Reactions
For elementary reactions, the exponents in the rate law equation are the same as the stoichiometric coefficients for each reactant in the chemical equation.

10. Proposing and Evaluating Mechanisms
When chemists propose a mechanism, they must satisfy the following criteria:
The equation for the elementary steps must combine to give the equation for the overall reaction
The proposed elementary steps must be reasonable
The mechanism must support the experimentally determined rate law.

11. Rate-Determining Step and the Rate law
Example: The rate law will depend on the rate-determining reaction.

12. Uncatalyzed activation energy Catalyzed activation energy
A catalyst reduces the overall activation energy for the reaction by producing alternative reaction intermediates with lower energy states.
Overall reaction
Uncatalyzed reaction path
Catalyzed reaction path
Potential Energy (kJ)
Uncatalyzed activation energy
Catalyzed activation energy
Reaction Progress (s)

13. Example:
Reaction of sodium potassium tartrate with H2O2, catalyzed by cobaltous chloride.
CoCl2 is pink in solution.
Contents of beaker turn dark greed, suggesting the formation of a reaction intermediate.
CoCl2 is regenerated when the reaction is over.

14. Explaining & Applying Chemical Kinetics
Increasing the temperature of the system increases the number of particles with sufficient energy to react. ET
New particles with sufficient energy to effectively collide in the chemical reaction.
Number of particles
Particles with sufficient energy to effectively collide in the chemical reaction.
Kinetic energy (kJ)

15. Explaining & Applying Chemical Kinetics
An increase in concentration results in more particles with the required threshold energy being added to the system.
New particles with sufficient energy to effectively collide in the chemical reaction.
Increased concentration ET
Number of particles
Particles with sufficient energy to effectively collide in the chemical reaction.
Kinetic energy (kJ)

16. Explaining & Applying Chemical Kinetics
A catalyst reduces the threshold energy and consequentially, activation energy .
New particles with sufficient energy to effectively collide in the chemical reaction.
ET Catalyzed ET
Numberof particles
Particles with sufficient energy to effectively collide in the chemical reaction.
Kinetic energy (kJ)

17. Explaining & Applying Chemical Kinetics
A catalyst reduces the activation energy and threshold energy. ET Ep
(kJ)
ET Catalyzed
Reaction Progress

18. Homogeneous Catalysts
Exists in the same phase as the reactant.
Usually catalyze gaseous and aqueous reactions.

19. Heterogeneous Catalysts
Exist in a phase different from the phase of the reaction.
Example: addition of hydrogen to an organic compound that contains C=C bonds: ___________.
Without a catalyst, the reaction is very slow.
Ethylene and hydrogen molecules form bonds with the metal surface. This weakens the bonds of the hydrogen and ethylene
The H-H bonds of the hydrogen molecules break and are somewhat stabilized because of their attraction to the metal.
Hydrogen atoms react with ethylene, forming ethane.