Chem.414 - Physical Chemistry II

1. Chem.414 - Physical Chemistry II

2. Chemical Kinetics

3. Study of Chemical Kinetics • Rate of reaction • Dependence of concentration of species • Dependence of temp., pressure, catalyst • Control of reactions • Mechanisms [Dominating step (fast vs. slow)] • Guide to chemical intuition

4. Reaction Rates Reaction Rate and Stoichiometry • For the reaction C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) we know • In general for aA + bB cC + dD

5. C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq)

6. 4. Consider the following N2O5 reaction:  2 N2O5(soln)  ---->  4 NO2(soln)  +  O2(g)     Let:    C = [N2O5]     (a) Using a graph of C vs. t, obtain tangential slopes and plot dC/dt vs. C. Calculate k after fitting with linear regression.    (b) Plot ln C vs. t. Calculate k after fitting with linear regression.    (c) Plot C vs. t. Fit the data with an appropriate function.  Display the equation in standard IRL form with the appropriate variable names for this reaction.    (d) Calculate half-live (t2) and life-time (t).   Compare them to the interpolated values from the plot of C vs. t. EXCEL

8. The Change of Concentration with Time Isomeric Transformation of Methyl Isonitrile to Acetonitrile First Order Reactions (to one component)

10. Differential and Integrated Rate Laws n-th Order to One Component (Generalized Rate Laws) Let: C = concentration of reactant A remaining at time t Co = initial concentration of reactant A (i.e. t=0) k = rate constant (units depends on n) DRL: IRL:

11. Differential and Integrated Rate Laws

12. Rate Law: First Order to One Component

14. The Change of Concentration with Time Second Order Reactions

15. Rate Law: Second Order to One Component

16. Gas-Phase Decomposition of Nitrogen Dioxide Is this reaction first or second order? k = 0.543 unit?

17. Half-Lives, Rate Constants and Co

18. Half-Lives, Rate Constants and Co - II

19. Zeroth Order to One Component - Catalysis • Provide the DRL. • Determine the IRL. • Sketch the IRL: Co=1.00 mol L-1 , k = 5.00x10-3 mol L-1 s-1 . • Use Mathcad (or EXCEL) to generate the IRL graph. • Obtain the half-life expression. • How many half-lives would it take for the reaction to reach equilibrium (i.e. completion)? [ Hint: Solve the IRL for time when C=0. Confirm by graph. ]

21. Summary of Rate Laws to One-Component m = -k b = ln Co m = k b = 1/Co m = -k b = Co

22. Concentration and Rate Exponents in the Rate Law • For a general reaction with rate law we say the reaction is mth order in reactant 1 and nth order in reactant 2. • The overall order of reaction is m + n + …. • A reaction can be zeroth order if m, n, … are zero. • Note the values of the exponents (orders) have to be determined experimentally. They are not simply related to stoichiometry.

23. Method of Initial/Comparative Rates

25. Three Component Rate Law

26. Techniques for Multiple Component Rate Laws • Integration Approach: Second Order – First Order to each of two components • Flooding Technique: Rate = k [A]x [B]y [C]z

28. Applications of First-Order Processes • Radioactive Decay • Bacterial Growth • Interest and Exponential Growth [Credit Card] • Loan Balance

31. Temperature and Rate The Arrhenius Equation • Arrhenius discovered most reaction-rate data obeyed the Arrhenius equation: • k is the rate constant, Eais the activation energy, R is the gas constant (8.3145 J K-1 mol-1) and T is the temperature in K. • A is called the frequency factor. • A is a measure of the probability of a favorable collision. • Both A and Ea are specific to a given reaction.

32. Temperature and Rate

33. Reaction Mechanisms • The balanced chemical equation provides information about the beginning and end of reaction. • The reaction mechanism gives the path of the reaction. • Mechanisms provide a very detailed picture of which bonds are broken and formed during the course of a reaction. Elementary Steps • Elementary step: any process that occurs in a single step.

34. Reaction Mechanisms Elementary Steps • Molecularity: the number of molecules present in an elementary step. • Unimolecular: one molecule in the elementary step, • Bimolecular: two molecules in the elementary step, and • Termolecular: three molecules in the elementary step. • It is not common to see termolecular processes (statistically improbable).

35. Reaction Mechanisms Rate Laws for Elementary Steps • The rate law of an elementary step is determined by its molecularity: • Unimolecular processes are first order, • Bimolecular processes are second order, and • Termolecular processes are third order. Rate Laws for Multistep Mechanisms • Rate-determining step is the slowest of the elementary steps. [example]

36. Reaction Mechanisms Rate Laws for Elementary Steps

37. Rate Expressions If elementary steps: • -d[A]/dt = vk1[A]v[B]w – vk-1[C]x[D]y • -d[B]/dt = wk1[A]v[B]w – wk-1[C]x[D]y • d[C]/dt = xk1[A]v[B]w – xk-1[C]x[D]y • d[D]/dt = yk1[A]v[B]w – yk-1[C]x[D]y

38. Reaction Mechanisms Mechanisms with an Initial Fast Step 2NO(g) + Br2(g)  2NOBr(g) • The experimentally determined rate law can be: d[NOBr]/dt = kobs[NO]2[Br2] (or) = kobs’[NO][Br2] • Consider the following mechanism

39. Spring 2014

40. Spring 2014

41. General Mechanism Overall Reaction: Proposed Mechanism: Where: D = observable product M = intermediate

44. Spring 2014

45. Spring 2014

46. Hydrogen-Iodine Reaction Overall Reaction: Proposed Mechanism: Where: I• = free radical

49. Spring 2012

50. Spring 2012