The Collision Model

1. The Collision Model • The reaction rate depends on: • collision frequency • a probability or orientation factor • activation energy (Ea) • The reaction rate increases as the number of collisions between reacting species increase. • Concentration • temperature

2. Cl . Cl . Cl . Br Br Br H H H The Collision Model • Collisions must occur in a particular orientation for reactions to occur. • For the reaction: Cl. + H - Br  H - Cl + Br. Desired rxn cannot occur. Desired rxn cannot occur. Desired rxn can occur.

3. The Collision Model • Collisions must occur with a specific minimum amount of energy in order for a reaction to take place. • Activation energy (Ea) • the minimum energy the reactants must have for a reaction to occur • the energy difference between the reactants and thetransition state

4. The Collision Model • Transition state: • a particular arrangement of atoms of the reacting species in which bonds are partially broken and partially formed • the state of highest energy between reactants and products • a relative maximum on the reaction-energy diagram.

5. Reaction Energy Diagrams • Reaction energy diagram: • a plot of potential energy changes that occur as reactants are converted to products

6. Reaction Energy Diagrams • Given a reaction energy diagram for a chemical reaction, you should be able to identify the reactants, products, transition state, activation energy, the heat of reaction, and whether the reaction is endothermic or exothermic.

7. Reaction Energy Diagrams Example: For each reaction energy diagram below, mark the location of the reactants, products and transition state. Identify the magnitude of Ea and DHrxn. Is each reaction endothermic or exothermic?

8. Arrhenius Equation • Reaction rate increases with temperature because: • molecules have more kinetic energy • more collisions occur • greater number of collisions occur with enough energy to “get over the hill” • i.e. with energy greater than or equal to Ea

9. Arrhenius Equation • The Arrhenius Equation relates the value of the rate constant to Ea and the temperature: k = Ae wherek = rate constant Ea = activation energy R = gas constant (8.314 J/mol.K) T = temperature in Kelvin A = frequency factor (a constant) A is related to the frequency of collisions and the probability that the collisions are oriented favorably for reaction. -Ea/RT

10. Arrhenius Equation • The activation energy of a reaction can be found by measuring the rate constant at various temperatures and using another version of the Arrhenius equation. ln k1 = Ea1 - 1 k2 R T2 T1 • You do not need to memorize this equation. You must be able to use it, however, to solve for Ea.

11. Arrhenius Equation Example: At 189.7oC, the rate constant for the rearrangement of methyl isonitrile to acetonitrile is 2.52 x 10-5 s-1. At 251.2oC, the rate constant for the reaction is 3.16 x 10-3 s-1. Calculate the activation energy for this reaction. ln k1 = Ea1 - 1 k2 R T2 T1

12. Arrhenius Equation

13. Arrhenius Equation • Once you find the value for Ea, you can use the Arrhenius Equation to find the frequency factor (A) for the reaction. • Once you have the value for Ea and A, you can calculate the value for the rate constant at any temperature. • The following two examples illustrate this process. • Be prepared for similar problems on your exam.

14. Arrhenius Equation Example: Using the activation energy obtained in the previous example, calculate the value for the frequency factor using k = 2.52 x 10-5 s-1 at 189.7oC. (Note: You could also have used the other set of conditions.)

15. Arrhenius Equation Example: Use the value for the frequency factor (A) and the activation energy obtained in the previous two examples to calculate the value of the rate constant at 25oC.