Reaction Mechanisms

1. Learning Goals: I will become familiar with reaction mechanisms and how they relate to the rate law Reaction Mechanisms

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

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

5. Potential Energy (kJ) 4 HBr + O2 2 H2O + 2 Br2 Reaction Progress (s) 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) + 2 Br2(g)

6. Rate Determining Step Elementary Reaction Steps Potential Energy (kJ) Reaction Intermediates Ea HOOBr HOBr 4 HBr + O2 DH 2 H2O + 2 Br2 Reaction Progress (s) • 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) + 2 Br2(g)

7. Overall reaction Uncatalyzed reaction path Catalyzed reaction path A catalyst reduces the overall activation energy for the reaction by producing alternative reaction intermediates with lower energy states. 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) slowH2(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