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Rates of Chemical Reactions

Rates of Chemical Reactions. Consider this reaction:. N 2 (g) + 3H 2 (g) ----> 2 NH 3 (g) + 92 kJ D G 298K = -16.8 kJ. What does this equation tell us?. N 2 (g) + 3H 2 (g) ----> 2 NH 3 (g) + 92 kJ D G 298K = -16.8 kJ. The reaction is spontaneous at 298K It is exothermic

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Rates of Chemical Reactions

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  1. Rates of Chemical Reactions Consider this reaction: N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ DG298K = -16.8 kJ What does this equation tell us?

  2. N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ DG298K = -16.8 kJ • The reaction is spontaneous at 298K • It is exothermic • entropy is decreasing • enthalpy is decreasing • the names and states of reactants and products are given • for every 3 mol of H2(g) and 1 mol of N2(g) consumed 2 mol of NH3(g) is produced • 92 kJ of energy is produced

  3. N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ DG298K = -16.8 kJ What does this equation NOT tell us? • How much of each substance we started with • how much of each substance is consumed • How fast the reaction is • Whether or not the reaction proceeds directly as shown in the equation or if intermediate steps occur (reaction mechanism)

  4. Consider this reaction: 4 HBr(g) + O2(g) --> 2 H2O + 2 Br2(g) This reaction proceeds through these steps HBr + O2 ---> HOOBr (slow) HOOBr + HBr ---> 2 HOBr (fast) 2 HOBr + 2 HBr --> 2 H2O + 2 Br2(g) (fast) In this case the equation simply tells us the reactants and products. It doesn’t tell us the series of steps by which the reaction goes.

  5. When we refer to reaction rates we are referring to how fast a reaction goes. Rates can be expressed both qualitatively and quantitatively. In a qualitative way one would describe a reaction as fast or slow. H2(g) + O2(g) ----> H2O (g) fast Fe + 1/2O2 + H2O ---> Fe(OH)2slow Quantitatively rates are expressed by observing the rate at which a reactant disappears or a product appears.

  6. It may be expressed as the change in mol/unit time or the change in concentration/unit time 2.4 g/24 g/mol # mol of Mg consumed = Time required to disappear 45 s = 2.2 x 10-3 mol/s Mg(s) + 2 HCl(aq) ---> MgCl2(aq) + H2(g) Rate =

  7. See Saunders - Chapter 15-Lesson 2 For Measuring Rates then conduct an exercise using burettes

  8. Energy Changes During Chemical Reactions

  9. H2 H2 Cl2 Cl2 Cl2 H2 Chemical reactions only occur when reacting particles collide with sufficient energy, and at a favourable geometry.

  10. Fast enough, but the wrong geometry

  11. Fast enough, but the wrong geometry

  12. Fast enough, but the wrong geometry

  13. Fast enough, but the wrong geometry

  14. Fast enough, but the wrong geometry

  15. Fast enough, but the wrong geometry

  16. Fast enough, but the wrong geometry

  17. Right Geometry, too slow

  18. Right Geometry, too slow

  19. Right Geometry, too slow

  20. Right Geometry, too slow

  21. Right Geometry, too slow

  22. Right Geometry, too slow

  23. Right Geometry, too slow

  24. Right Geometry, too slow

  25. Right Geometry, too slow

  26. Right Geometry, too slow

  27. Right Geometry, too slow

  28. Right Geometry, too slow

  29. Right Geometry, too slow

  30. Right Geometry, too slow

  31. Right Geometry, too slow

  32. Right Geometry, too slow

  33. Right Geometry, too slow

  34. Right Geometry, too slow

  35. Right Geometry, too slow

  36. Right Geometry, too slow

  37. Right Geometry, too slow

  38. Right Geometry, sufficient speed Activation energy reached

  39. Right Geometry, sufficient speed Activation energy reached

  40. Right Geometry, sufficient speed Activation energy reached

  41. Right Geometry, sufficient speed Activation energy reached

  42. Right Geometry, sufficient speed Activation energy reached

  43. Right Geometry, sufficient speed Activation energy reached

  44. 4 - If the molecules have sufficient energy to react a short lived activated complex is formed. At this point no bond breaking or bond making is occuring. Potential Energy Reaction Coordinate 3 - At the same time new bonds are starting to form between H and Cl as H nuclei attract Cl electrons and Cl nuclei attract H electrons 2 - As the reacting molecules approach their electrons start to repel so the H-H and Cl-Cl bonds stretch, Ek decreases and Ep increases. 5 - If the reaction proceeds the bonds between H-Cl continue to shrink until they reach a stable state 1 - The reacting molecules are sufficiently far apart so they have no influence on one another

  45. 2. Potential Energy (kJ) Reaction Coordinate 7.6 Eaf = energy of activation, forward Eaf= 7.6 kJ - 4.0 kJ = 3.6 kJ Ear= 7.6 kJ - 2.0 kJ = 5.6 kJ 1. 4.0 DH = 2.0 kJ - 4.0 kJ = - 2.0 kJ 3. 2.0

  46. Construct a potential energy vs. reaction coordinate curve for an endothermic reaction

  47. 2. Potential Energy (kJ) Reaction Coordinate 5.6 Eaf = energy of activation, forward Eaf= 5.6 kJ - 1.0 kJ = 4.6 kJ Ear= 5.6 kJ - 3.0 kJ = 2.6 kJ 3.0 3. DH = 3.0 kJ - 1.0 kJ = 2.0 kJ 1. 1.0

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