1. Chemistry 1011 Slot 51 Chemistry 1011 TOPIC Rate of Reaction TEXT REFERENCE Masterton and Hurley Chapter 11

2. Chemistry 1011 Slot 52 11.4 Models for Reaction Rate YOU ARE EXPECTED TO BE ABLE TO: Apply collision theory and the concept of activation energy to explain reaction rates. Apply the transition state model to explain reaction rates. Draw a potential energy diagram to show the activation energy, energy change, reactants and products of a chemical system given appropriate data.

3. Chemistry 1011 Slot 53 Collision Theory Molecules must collide before they can react In a gas, molecules are traveling with different velocities and have different kinetic energies. The average kinetic energy is dependent on temperature (Kinetic Theory of Gases) Colliding molecules must together have enough kinetic energy to break existing bonds. The minimum kinetic energy required for a reaction to occur is the activation energy, symbol E a

4. Chemistry 1011 Slot 54 Energy diagram

5. Chemistry 1011 Slot 55 Collision Theory Molecules must also collide with a favorable orientation for reaction to occur The factors affecting the rate of reaction are: –Fraction of collisions which occur with required orientation (p) –Collision frequency (Z) –Fraction of collisions with required energy (f) Most collisions do not result in reaction

6. Chemistry 1011 Slot 56 Effective vs. ineffective collisions

7. Chemistry 1011 Slot 57 Collision Theory and the Rate Constant For elementary processes, where the rate law expression is: Rate = k[A][B] k = p x Z x f It can be shown that the fraction of collisions with sufficient energy is: f = e -E a /RT –R = gas constant –T = temperature in Kelvin

8. Chemistry 1011 Slot 58 Collision Theory and the Rate Constant Substitute into the collision theory equation for the rate constant: k = p x Z x e -E a /RT The larger the activation energy, the smaller the fraction of molecules having enough energy to react, so the slower the rate

9. Chemistry 1011 Slot 59 Limitations of Collision Theory Collision theory does not provide any prediction of p, the “steric factor” Calculated values for the rate constant are usually too high compared with measured values Measured activation energies are lower than the energies of the bonds that have to be broken in reactions

10. Chemistry 1011 Slot 510 Transition State Theory At the moment of collision, molecules with enough energy and the right orientation combine to form an activated complex The activated complex can either revert back to reactants or decompose to products The exact nature of the activated complex is difficult to determine Less energy is required to form an activated complex than to beak bonds

11. Chemistry 1011 Slot 511 Energy diagram