1. Chemical Kinetics and Rates of Reaction McGraw-Hill Chemistry Text Read Pg: 462-467 and 481-482

2. Chemical Kinetics The study of rates of chemical reactions and the mechanisms (or steps) by which a chemical reaction takes place. Reaction rates vary greatly – some are very fast (burning) and some are very slow (disintegration of a plastic bottle in sunlight).

3. Rate of reaction: the change in concentration of a reactant or product per unit of time. Rxn Rate (avg) = Δ [reactant or product] Δ time Δ time Note: [square brackets] = mol/L

4. Example problem: If the concentration of reactant X after 50 s in a reaction was 0.0079 mol/L and after 100 s was 0.0065 mol/L, calculate the average reaction rate. Rate = Δ[X] / Δt = (0.0065 – 0.0079 M) / (100-50 s) = - 0.000028 M/s

5. Concentration of reactant decreases with time, as it is being used up The rate is fastest when concentration of reactants is greatest; and slows when concentration of reactants are less

6. The reactants are being used up as the reaction takes place. What’s happening to the concentration of products over time? It is increasing.

7. In the beginning of the reaction, product is formed quickly, but slows over time because less reactants means less reactants collisions producing the new product. Time (s) Concentration of product (mol/L)

8. Factors that affect the Rate: Concentration (and pressure) Temperature Amount of surface area Catalysts Nature of reactants

9. Concentration A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increasing reaction rate We are not increasing the amount being made for a given balanced equation with limiting reactants, we are only speeding up how quickly those products are made.

10. Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater concentration of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction

11. Temperature Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time.

12. Surface Area Reducing the size of particles increases the rate of a reaction because it increases the surface area available for collisions to take place. This increases the number of collisions.

13. Catalysts A catalyst is a substance that speeds up a reaction without being used up itself. Some reactions have catalysts that can speed them up, but for many reactions there is no catalyst that works. How do they work?

14. 1. A catalyst provides a surface on which the reaction can take place. One or more reactants are adsorbed to the surface (stick to it) Let’s look at the example of ethene undergoing hydrogenation (breaking double bond and making single bonds while adding hydrogen) Usually VERY slow

15. Interaction between the catalyst and the reactant (maybe weakening bonds, or an actual reaction with the surface) The reactants may collide on the catalyst or one is prepared and then collide together

16. The product is desorbed, breaks away from the catalyst once formed Nickel and platinum are excellent heterogeneous catalysts. They don’t adsorbed too much or too little. Hydrogenation of vegetable oil to make margarine uses a nickel catalyst

17. 2) A catalyst lowers the activation energy AE: the minimum amount of energy needed for a reaction to take place. Reactants require less energy to react. By providing an alternative mechanism (step in the reaction) Sometimes catalysts seem to be used up, but they will regenerate at the end of the reaction

18. Nature of Reactants Reactants with large number of chemical bonds that have to be broken, or with strong chemical bonds that have to be broken, will lead to a slower rate. The strong bonds will cause a high activation energy, leading fewer collisions being successful.

19. More complex reactant molecules with more atoms, will have more difficulty lining up properly to have a successful reaction. This will also lead to a lower reaction rate.

20. Good rule of thumb… –Ions will react more quickly, because already broken up. –Simple molecules (non-metals covalently bonded) will take more time –Complex molecules and ionic compounds (metal and non-metal bonded) will take the most time because the strong ionic bonds need lots of energy to be broken, and the complex molecules will be hard to have proper geometry (line up)

21. Practice Place the following reactions from fastest to slowest, based on the previous slides. 1. 2Cu +2 + 2I -  2 Cu + + I 2 2. 2NaCl  2Na + Cl 2 3. Cl 2  2Cl Ans: 1 is fastest, then 3, and 2 is slowest