Theories of Reaction Rates

1. Theories of Reaction Rates

2. Collision Theory In order for a reaction to occur, reacting particles (atoms, molecules or ions) must collide with one another.

3. The Effect of Concentration on Reactant Rate.

4. Effect of Concentration

5. Collision Theory and Surface Area. With greater surface area, more collisions can occur. Example: To start a campfire, you use kindling and paper, then add a log to have a slow burning fire.

6. Surface Area

7. Beyond Collision Theory Not every collision between reactants results in a reaction. In order for a reaction to occur a collision must be effective

8. The criteria to be effective Correct orientation of reactants Sufficient collision energy

9. Orientation of Reactants Reacting particle must collide with the proper orientation relative to one another or correct collision geometry.

10. Orientation of Reactants

11. Activation Energy (Ea) the energy of the collision The reactants must collide with sufficient energy to break the bonds in the reactants and begin to form the bonds in the products

12. In most reactions, only a fraction of the collisions have enough energy for the reaction to occur. Collision Energy is dependent on the kinetic energy of the system. Temperature is a measure of the average kinetic energy of the particles.

13. Plot the number of collisions in a substance at a given temperature against the kinetic energy of each collision called Maxwell–Boltzman Distribution

14. Maxwell-Boltzman Distribution

15. The dotted line represent the activation energy. The shade part of the graph indicates the collisions with energy that is equal to or greater than the activation energy.

16. The Effect of Temperature

17. Temperature con’t at both temperatures a relatively small fraction of collisions have sufficient kinetic energy –the activation energy– to result in a reaction. As the temperature of a sample increases, the fraction of collisions with sufficient energy to cause a reaction increases significantly. For many reactions, the rate roughly doubles for every 10oC rise in temperature.

18. Transition State Theory used to explain what happens when molecules do collide in a reaction. it examines the change or transition from reactants to products kinetic energy of Reactants?potential energy as reactants collide (law of conservation of energy)

19. Potential Energy Diagram the potential energy of the a reaction versus the progress of the reaction y-axis=potential energy, x-axis= the progress of reaction over time.

20. Exothermic Reaction

21. the hill represents the activation energy barrier slow reaction: high activation energy barrier, i.e. few reactants have sufficient kinetic energy for a successful reaction fast reaction: low activation energy barrier

22. Exothermic Reaction the reactants are at a higher energy level than the products the overall difference in potential energy is the enthalpy change ?H There is no way to predict the activation energy of a reaction from its enthalpy change. ?H of a reaction is determined by finding the overall energy that is transferred. Activation energy is determined by analyzing the reaction rate Earev=Eafwd + ?H

23. Endothermic Reaction

24. Endothermic Reaction the reactants have a lower energy level than the products the overall difference in potential energy between reactants and products is the enthalpy change Earev = Eafwd -?H

25. The top of the activation energy barrier on a potential energy diagram represents the transition state or change over point of the reaction. The chemical species that exist at the transition state is referred to as the activated complex (neither reactant or product-has partial bonds and unstable)

26. Tracing a Reaction with a Potential Energy Diagram Carbon monoxide reacts with nitrogen dioxide, carbon dioxide and nitric oxide are formed. Draw a potential energy diagram to illustrate the progress of the reaction. You do not need to draw the diagram to scale. Label the axis, the transition state and the activated complex. Indicate the activation energy for the forward reaction. Eafwd =134 KJ and ?H=-226 kJ. Calculate the activation energy of the reverse reaction and show it on the graph