1. Chemical Kinetics Chemical Kinetics or Rates of reaction

2. Chemical Kinetics Studies the rate at which a chemical process occurs. Looks at the reaction mechanism (exactly how the reaction occurs).

3. Chemical Kinetics Outline: Kinetics (do not copy) Reaction Rates How we measure rates. Rate Laws How the rate depends on amounts of reactants. Integrated Rate LawsHow to calc amount left or time to reach a given amount. Half-lifeHow long it takes to react 50% of reactants. Arrhenius EquationHow rate constant changes with T. Mechanisms Link between rate and molecular scale processes.

4. Chemical Kinetics The Collision Model In a chemical reaction, bonds are broken and new bonds are formed. Molecules can only react if they collide with each other.

5. Chemical Kinetics Conditions for a Reaction? Proper orientation – if molecules are not properly oriented then there will be no reaction Required energy – When molecules come together they must have a minimum amount of energy required before they can react. This energy is called the Activation energy. E A.

6. Chemical Kinetics Reaction Coordinate Diagrams It shows the energy of the reactants and products (and, therefore,  H). The high point on the diagram is the transition state. The species present at the transition state is called the activated complex. The energy gap between the reactants and the activated complex is the activation energy barrier.

7. Chemical Kinetics The Collision Model Furthermore, molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation.

8. Chemical Kinetics

9. Chemical Kinetics Factors That Affect Reaction Rates Concentration of Reactants  As the concentration of reactants increases, so does the likelihood that reactant molecules will collide. Temperature  with higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. 10 o C rise in temperature doubles the rate. Catalysts  Substance that alters the rate of the reaction and it is regenerated at the end. Surface area  increase surface area causes an increase in the collision and hence the rate. Pressure For gases - increase in pressure causes an increase in the collision and hence the rate

10. Chemical Kinetics Reaction Rates Rates of reactions can be determined by monitoring  the change in concentration of either reactants(being used up)  or products (being formed) as a function of time. Rate =  [A] /  t Rate = Mol/L sec or MolL -1 S -1 or Ms -1 Rxn Movie

11. Chemical Kinetics Reaction Rates (do not copy) In this reaction, the concentration of butyl chloride, C 4 H 9 Cl, was measured at various times, t. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) [C 4 H 9 Cl] M

12. Chemical Kinetics Types of rate Average Rate Change in concentration over a given period of time. Draw a secant through the two points Find the slope of the line Average rate Instantaneous rate

13. Chemical Kinetics Types of Rate cont’d Instantaneous Rate Change in concentration at a specific time Draw a tangent through the point Find the slope of the line.

14. Chemical Kinetics Reaction Rates The average rate of the reaction over each interval is the change in concentration divided by the change in time: C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) Average Rate, M/s

15. Chemical Kinetics Reaction Rates Note that the average rate decreases as the reaction proceeds. This is because as the reaction goes forward, there are fewer collisions between reactant molecules. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

16. Chemical Kinetics Reaction Rates A plot of concentration vs. time for this reaction yields a curve like this. The slope of a line tangent to the curve at any point is the instantaneous rate at that time. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

17. Chemical Kinetics Reaction Rates The reaction slows down with time because the concentration of the reactants decreases. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

18. Chemical Kinetics Monitoring reaction rates

19. Chemical Kinetics Reaction Rates and Stoichiometry In this reaction, the ratio of C 4 H 9 Cl to C 4 H 9 OH is 1:1. Thus, the rate of disappearance of C 4 H 9 Cl is the same as the rate of appearance of C 4 H 9 OH. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) Rate = -  [C 4 H 9 Cl]  t =  [C 4 H 9 OH]  t Negative sign indicates that it is a reactant and it is used up. RATE is ALWAYS POSITIVE

20. Chemical Kinetics Reaction Rates and Stoichiometry What if the ratio is not 1:1? H 2 (g) + I 2 (g)  2 HI (g) Only 1/2 HI is made for each H 2 used.

21. Chemical Kinetics Reaction Rates and Stoichiometry To generalize, for the reaction aA + bBcC + dD Reactants (decrease) Products (increase)

22. Chemical Kinetics Temperature and Rate Generally, as temperature increases, so does the reaction rate. This is because k is temperature dependent.

23. Chemical Kinetics Maxwell–Boltzmann Distributions At any temperature there is a wide distribution of kinetic energies. As the temperature increases, the curve flattens and broadens. Thus at higher temperatures, a larger population of molecules have higher energy. If the dotted line represents the activation energy, as the temperature increases, so does the fraction of molecules that can overcome the activation energy barrier As a result, the reaction rate increases.

24. Chemical Kinetics Catalysts Catalysts increase the rate of a reaction by decreasing the activation energy of the reaction. Catalysts change the mechanism by which the process occurs.

25. Chemical Kinetics Catalysts One way a catalyst can speed up a reaction is by holding the reactants together and helping bonds to break.

26. Chemical Kinetics Enzymes Enzymes are catalysts in biological systems. The substrate fits into the active site of the enzyme much like a key fits into a lock.

27. Chemical Kinetics Reaction Mechanisms The sequence of events that describes the actual process by which reactants become products is called the reaction mechanism.

28. Chemical Kinetics Reaction Mechanisms Reactions may occur all at once or through several discrete steps. Each of these processes is known as an elementary reaction or elementary process.

29. Chemical Kinetics Reaction Mechanisms The molecularity of a process tells how many molecules are involved in the process. Coefficients in the elementary reaction becomes exponent in the rate equation The rate law for an elementary step is written directly from that step.

30. Chemical Kinetics Multistep Mechanisms In a multistep process, one of the steps will be slower than all others. The overall reaction cannot occur faster than the slowest, rate-determining step.(RDS)

31. Chemical Kinetics Slow Initial Step The rate law for this reaction is found experimentally to be Rate = k [NO 2 ] 2 CO is necessary for this reaction to occur, but the rate of the reaction does not depend on its concentration. This suggests the reaction occurs in two steps. NO 2 (g) + CO (g)  NO (g) + CO 2 (g)

32. Chemical Kinetics Slow Initial Step A proposed mechanism for this reaction is Step 1: NO 2 + NO 2  NO 3 + NO (slow) Step 2: NO 3 + CO  NO 2 + CO 2 (fast) Slow step used to determine the rate  rate = k[NO 2 ] 2 The NO 3 intermediate is consumed in the second step. Intermediate  specie that is formed in one step and used up in another step As CO is not involved in the slow, rate-determining step, it does not appear in the rate law. Catalyst is substance that is put in at one step and is regenerated at the end