1. Collision Theory and Reaction Rates

2. The Collision Theory is a theory which states how reacting particles (like atoms and molecules) must interact to start a chemical reaction.

3. The Collision Theory States 3 Things: 1 st : Reactant particles must collide in order for the reaction to occur.

4. 2 nd : Reactant particles must collide the “right way” with proper orientation. The Collision Theory Continued...

5. 3rd: Reactant particles must collide with enough energy to “stick together.” E a is called activation energy. Activation energy is the minimum amount of energy required for a reaction to occur.

6. High Activation Energy (E a ) LOTS of energy needed for the reaction to happen. It takes a long time to get this energy, so there is a slow reaction rate.

7. Low Activation Energy (E a ) LESS energy needed for the reaction to occur. It takes less time to get needed energy so there is a fast reaction rate.

8. If the reactant particles meet ALL THREE requirements then an activated complex forms when the reactant particles “stick together”.

9. An Activated Complex Represents: a temporary, unstable structure the transition between the breaking of old bonds and the forming of new bonds

10. 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.

11. Endo- or Exothermic? Exothermic

12. 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

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

14. Endo- or Exothermic? Endothermic

15. 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 Ea rev = Ea fwd -ΔH

16. 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)

17. What is E a ? oActivation energy – the minimum amount of energy required to start a reaction. Why does a high E a mean there will be a slow reaction? oIt takes a while for particles to get enough energy to overcome the E a barrier. Why does a low E a mean there will be a fast reaction? oIt won’t take as long for particles to get enough energy to overcome the E a barrier.

18. Which of the following reactions would occur fastest: 1 or 2 and why? Graph 2 has smaller E a barrier.

19. Catalyst Catalysts are substances added to a chemical reaction that speed up the rate of reaction to make it occur faster. How does it do this? It lowers activation energy! Draw the new E a on the diagram below WITH a catalyst.

20. What is “proper orientation?” oThe direction/orientation in which reactants must collide. Example: NO 3 + CO → NO 2 + CO 2 In this chemical reaction, when NO 3 and CO react together, they must collide with sufficient energy and with the nitrogen atom in NO 3 colliding with the carbon atom in CO. In other words, the nitrogen and carbon atoms must “bump” together!

21. Orientation of Reactants

22. What would happen if... particles collide with sufficient energy but do not have proper orientation?

23. What would happen if... particles collide with proper orientation but not enough energy?

24. a reaction were to occur?

25. Which of the parts of the collision theory can be manipulated by us to cause a reaction to occur faster (or slower)? Collision frequency Collision energy

26. How can collision frequency and collision energy be manipulated?

27. Reaction Rate A reaction rate is how fast or how slow a reaction occurs over time.

28. Certain factors can be changed to make the reaction occur faster or slower. *Concentration *Surface Area *Temperature *Catalyst or inhibitor

29.  Concentration is the amount of reactant(s) used measured in terms of molarity. oIncreasing the concentration of a substance increases the number of particles involved in the reaction. oThis makes it easier for the reacting substances to “find” each other which leads to an increase in the frequency of collisions. oMore frequent collisions means a higher probability of collisions with proper orientation!

30. Example : Will a strip of Mg have a faster rxn rate with 10. M HCl or 0.50 M HCl? Explain why – watch terminology! Answer: 10. M HCl will consume Mg faster because it is more concentrated. Because there are more moles of acid per liter, there will be more frequent collisions b/w the acid and Mg and more frequent collisions w/proper orientation as a result.

31. Surface area is the amount of the surface the reactant has exposed for collisions. oBy pulverizing or crushing a substance, you increase the amount of the substance’s surface that is exposed. oThis makes more surface area of the particles available for the reaction. oMore particles available for the reaction means more frequent collisions and more frequent collisions with proper orientation.

32. big chunk of Na crushed

33. Example Which would work faster: Aspirin in powder form or aspirin in tablet form? Explain why. Answer: The aspirin would work faster if taken in powder form because the fact that it has been crushed increases surface area. This will make more frequent collisions and more frequent collisions with proper orientation – which will make the aspirin work faster.

34.  Temperature is the measure of kinetic energy of particles in a substance. oAn increase in temperature increases the kinetic energy (or movement) of particles. oThis increases the frequency of collisions because particles are bouncing around more. oThis will increase the frequency of the collisions with proper orientation. oBecause the particles are moving faster, they are more likely to achieve enough energy to overcome the E a barrier.

35. Example : Will a strip of Mg react faster in 1 M HCl at 75  C or in 1 M HCl at 23  C? Explain. Answer: The Mg reacts faster in the acid at 75  C because the Mg will absorb more kinetic energy in the acid at the higher temperature causing more frequent collisions and more frequent collisions with proper orientation and more collisions with sufficient E a.

36. Catalyst Energy Diagram

37.  A catalyst is a substance that speeds up a reaction.  It does this by lowering the activation energy.  This makes the E a barrier easier to overcome so that that reaction can take place more quickly. Catalysts form an activated complex of lower energy than a reaction without a catalyst.

38. More Catalyst Info...  Catalysts are NOT used up or consumed in a chemical reaction so they can be used over and over again like recycling– very cost efficient for pharmaceutical companies!  Catalysts do NOT make more of a product, they only are used to speed up a chemical reaction.

39. inhibitors are catalysts that are used to SLOW DOWN reactions Draw an energy diagram showing how an inhibitor works to reduce a reaction rate.

40. Inhibitors RAISE the activation energy, which makes the reaction SLOW

41. Why would a reaction want to be slowed down? Food preservatives added to prevent/slow down spoilage. Weed killers slow down the growth of weeds. Medications used to treat HIV and cancer slow the spread of each.

42. Enzymes An enzyme is a biological catalyst found in nature – which means enzymes speed up chemical reactions by lowering activation energy. Example: Amylase is an enzyme in saliva that breaks down starches into sugars. Lipase is an enzyme that breaks down fats.

43. You Survived!