1. Reaction Mechanisms Learning Goals:
Thermochemistry – the study of the energy associated with chemical changes.
Chemical Kinetics – the study of the rate and speed of chemical reactions.
Learning Goals:
I will become familiar with reaction mechanisms and how they relate to the rate law

2. A series of steps by which a chemical reaction occurs
Reaction Mechanisms
A series of steps by which a chemical reaction occurs
Elementary step: only involves one, two or possibly three particles in a collision.
Rate Determining Step: slowest step; determines the rate of the overall reaction
Reaction Intermediate : not a reactant or a product but is an unstable compounds formed and used up in a reaction.
Activation Complex: high energy unstable state

3. 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

4. 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

5. HOOBr HOBr 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

6. HBr(g) + O2(g)  HOOBr(g) (slow) HOOBr(g) + HBr(g)  2 HOBr(g) (fast)
Rate Determining Step
Elementary Reaction Steps
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

7. 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)

8. Let’s Try It:
Write out the elementary steps for the reaction above and find the overall reaction equation.

9. Example A proposed mechanisms for the reaction is show below:
I2(g)  2I(g) slow H2(g) + 2I(g) 2HI(g) fast
(a) what is the overall reaction?
(b) are there any intermediates?
(c) what is the rate law equation?

10. 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)

11. 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)

12. 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)

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