1. Explaining Reaction Rates Chapter 6.3

2. Explaining Reaction Rates We will further explain the factors affecting reaction rate with respect to Collision Theory

3. Collision Theory Collision Theory states that chemical reactions can occur only if reactant atoms, molecules or ions collide with proper orientation and with enough kinetic energy to break reactant bonds and form product bonds Rate of a Reaction depends on the frequency and the proportion of collisions that convert reactants into products.

4. Explaining Collision Theory There are two important factors of Collision Theory: 1.Orientation: Collisions will result in reactions only if the molecules colllide in the proper geometry

5. Explaining Collision Geometry 1.D 2.Activation Energy (E a ): the minimum amount of energy that reactant molecules must possess for a collision to be effective Activation energy serves two purposes: Required to overcome electrostatic repulsive forces between colliding entities Required to weaken the bonds of the reactants

6. Explaining Collision Geometry Activated Complex (transition state): the unstable arrangement of atoms containing partially formed and partially broken bonds that represent the maximum potential energy point in the change In a chemical reaction, potential energy is stored in the bonds within the reactants; kinetic energy is their movement ΔHΔH Consider the following reaction: 2BrNO  2NO + Br 2 Ea is the activation energy required for reaction to proceed The activated complex is Since the reactants have a higher potential energy than the products, it is an exothermic reaction The net change in potential energy between reactants and products is ΔH (change in enthalpy)

7. Exothermic vs Endothermic Reactions Exothermic reactions: since reactants have higher E P than products, they release energy and have a negative ΔH Endothermic reactions: since reactants have lower E P than products, they absorb energy and have a positive ΔH

8. Temperature of the Reaction System Increased temperature increases the rate of reaction by increasing both the collision frequency and the fraction of effective collisions Maxwell-Boltzmann Distribution

9. Temperature of the Reaction System The distribution curve changes when temperature is changed: At lower temperatures, the fraction of entities that have kinetic energy equal to or greater than the Ea is small At higher temperatures, the fraction of entities that have kinetic energy equal to or greater than the Ea is large

10. Chemical Nature of Reactants For any reactant, the activation energy required for a successful collision depends on: 1.the bond type 2.the bond strength 3.Size and complexity of molecule Example: a large, complex molecule will have more bonds to break and less chance of proper orientation during collision

11. Concentration Increased concentration increases the rate of reaction by increasing the collision frequency

12. Surface Area Increased surface area also increases the rate of reaction by increasing the collision frequency

13. Catalyst Theory Catalysts increase the rate of reaction by providing an alternate pathway for the reaction with a lower activation energy

14. Catalyst Theory The lowered activation energy increases the fraction of collisions that are effective

15. Homework Supplementary your notes! Read page 366-372 Questions p. 372 #1-6