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Chemical Kinetics

Chemical Kinetics. The area of chemistry that concerns reaction rates and reaction mechanisms. Reaction Rate. The change in concentration of a reactant or product per unit of time. 2NO 2 (g)  2NO(g) + O 2 (g). Reaction Rates:. 1. Can measure disappearance of reactants.

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Chemical Kinetics

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  1. Chemical Kinetics The area of chemistry that concerns reaction rates and reaction mechanisms.

  2. Reaction Rate The change in concentration of a reactant or product per unit of time

  3. 2NO2(g)  2NO(g) + O2(g) Reaction Rates: 1. Can measure disappearance of reactants 2. Can measure appearance of products 3. Are proportional stoichiometrically

  4. 2NO2(g)  2NO(g) + O2(g) Reaction Rates: 4. Are equal to the slope tangent to that point 5. Change as the reaction proceeds, if the rate is dependent upon concentration [NO2] t

  5. Rate Laws Differential rate laws express (reveal) the relationship between the concentration of reactants and the rate of the reaction. The differential rate law is usually just called “the rate law.” Integrated rate laws express (reveal) the relationship between concentration of reactants and time

  6. Writing a (differential) Rate Law Problem- Write the rate law, determine the value of the rate constant, k, and the overall order for the following reaction: 2 NO(g) + Cl2(g)  2 NOCl(g)

  7. Writing a Rate Law Part 1– Determine the values for the exponents in the rate law: 2 1 Rate = k[NO]x[Cl2]y In experiment 1 and 2, [Cl2] is constant while [NO] doubles. The rate quadruples, so the reaction is second order with respect to [NO] In experiment 2 and 4, [NO] is constant while [Cl2] doubles. The rate doubles, so the reaction is first order with respect to [Cl2]

  8. Writing a Rate Law Part 2– Determine the value for k, the rate constant, by using any set of experimental data: Rate = k[NO]2[Cl2]

  9. Writing a Rate Law Part 3– Determine the overall order for the reaction. Rate = k[NO]2[Cl2] = 3 2 + 1  The reaction is 3rd order Overall order is the sum of the exponents, or orders, of the reactants

  10. Determining Order withConcentration vs. Time data (the Integrated Rate Law) Zero Order: First Order: Second Order:

  11. Solving an Integrated Rate Law Problem: Find the integrated rate law and the value for the rate constant, k A graphing calculator with linear regression analysis greatly simplifies this process!! (Click here to download Rate Laws program for theTi-83 and Ti-84)

  12. Time vs. [H2O2] Regression results: y = ax + b a = -2.64 x 10-4 b = 0.841 r2 = 0.8891 r = -0.9429

  13. Time vs. ln[H2O2] Regression results: y = ax + b a = -8.35 x 10-4 b = -.005 r2 = 0.99978 r = -0.9999

  14. Time vs. 1/[H2O2] Regression results: y = ax + b a = 0.00460 b = -0.847 r2 = 0.8723 r = 0.9340

  15. And the winner is… Time vs. ln[H2O2] 1. As a result, the reaction is 1st order 2. The (differential) rate law is: 3. The integrated rate law is: 4. But…what is the rate constant, k ?

  16. Finding the Rate Constant, k Method #1: Calculate the slope from the Time vs. ln[H2O2] table. Now remember:  k = -slope k = 8.32 x 10-4s-1

  17. Finding the Rate Constant, k Method #2: Obtain k from the linear regresssion analysis. Regression results: y = ax + b a = -8.35 x 10-4 b = -.005 r2 = 0.99978 r = -0.9999 Now remember:  k = -slope k = 8.35 x 10-4s-1

  18. Rate Laws Summary

  19. Reaction Mechanism The reaction mechanism is the series of elementary stepsby which a chemical reaction occurs. • The sum of the elementary steps must give the overall balanced equation for the reaction • The mechanism must agree with the experimentally determined rate law

  20. Rate-Determining Step In a multi-step reaction, the slowest stepis the rate-determining step. It therefore determines the rate of the reaction. The experimental rate law must agree with the rate-determining step

  21. Identifying the Rate-Determining Step For the reaction: 2H2(g) + 2NO(g)  N2(g) + 2H2O(g) The experimental rate law is: R = k[NO]2[H2] Which step in the reaction mechanism is the rate-determining (slowest) step? Step #1 H2(g) + 2NO(g)  N2O(g) + H2O(g) Step #2 N2O(g) + H2(g)  N2(g) + H2O(g) Step #1 agrees with the experimental rate law

  22. Identifying Intermediates For the reaction: 2H2(g) + 2NO(g)  N2(g) + 2H2O(g) Which species in the reaction mechanism are intermediates (do not show up in the final, balanced equation?) Step #1 H2(g) + 2NO(g)  N2O(g) + H2O(g) Step #2 N2O(g) + H2(g)  N2(g) + H2O(g) 2H2(g) + 2NO(g)  N2(g) + 2H2O(g)  N2O(g) is an intermediate

  23. Collision Model Key Idea: Molecules must collide to react. However, only a small fraction of collisions produces a reaction. Why?

  24. Collision Model Collisions must have sufficient energy to produce the reaction (must equal or exceed the activation energy). 1. Colliding particles must be correctly oriented to one another in order to produce a reaction. 2.

  25. Factors Affecting Rate Increasing temperature always increases the rate of a reaction. • Particles collide more frequently • Particles collide more energetically Increasing surface area increases the rate of a reaction Increasing Concentration USUALLY increases the rate of a reaction Presence of Catalysts, which lower the activation energy by providing alternate pathways

  26. Endothermic Reactions

  27. Exothermic Reactions

  28. The Arrhenius Equation • k = rate constant at temperature T • A = frequency factor • Ea = activation energy • R = Gas constant, 8.31451 J/K·mol

  29. The Arrhenius Equation, Rearranged • Simplifies solving for Ea • -Ea / R is the slope when (1/T) is plotted against ln(k) • ln(A) is the y-intercept • Linear regression analysis of a table of (1/T) vs. ln(k) can quickly yield a slope • Ea = -R(slope)

  30. Catalysis • Catalyst: A substance that speeds up a reaction without being consumed • Enzyme: A large molecule (usually a protein) that catalyzes biological reactions. • Homogeneous catalyst: Present in the same phase as the reacting molecules. • Heterogeneous catalyst: Present in a different phase than the reacting molecules.

  31. Lowering of Activation Energy by a Catalyst

  32. Catalysts Increase the Number of Effective Collisions

  33. Heterogeneous Catalysis Carbon monoxide and nitrogen monoxide adsorbed on a platinum surface Step #1: Adsorption and activation of the reactants.

  34. Heterogeneous Catalysis Carbon monoxide and nitrogen monoxide arranged prior to reacting Step #2: Migration of the adsorbed reactants on the surface.

  35. Heterogeneous Catalysis Carbon dioxide and nitrogen form from previous molecules Step #3: Reaction of the adsorbed substances.

  36. Heterogeneous Catalysis Carbon dioxide and nitrogen gases escape (desorb) from the platinum surface Step #4: Escape, or desorption, of the products.

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