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D08 abbrev arrhenius and catalysts_alg

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D08 abbrev arrhenius and catalysts_alg

  1. 1. General Chemistry II CHEM 152 Week 3
  2. 2. Week 3 Reading Assignment Chapter 13 – Sections 13.5 (temperature), 13.7 (catalysts)
  3. 3. SOLUTION A key reaction in the upper atmosphere is The E a(fwd) is 19 kJ, and the  H rxn ( ∆E) for the reaction is -392 kJ. Draw a reaction energy diagram for this reaction, postulate a transition state, and calculate E a (rev) . Your Turn O 3 ( g ) + O( g ) 2O 2 ( g ) transition state E a = 19kJ  H rxn = -392kJ E a (rev) = (392 + 19)kJ = 411kJ
  4. 4. The Arrhenius Equation ln k = ln A - E a /RT where k is the rate constant at T E a is the activation energy R is the energy gas constant = 8.3145 J/(mol K) T is the Kelvin temperature A is the collision frequency factor Temperature Effects ln k 2 k 1 = E a R - 1 T 2 1 T 1 -
  5. 5. ln k = -E a /R (1/T) + ln A Graphical Analysis . Y = m X + b
  6. 6. Typical Problems Suppose a chemical reaction has an activation energy of 76 kJ/mol. By what factor is the rate of reaction at 50 o C increased over its rate at 25 o C? k 2 =rate constant @ 50ºC k 1 =rate const @ 25ºC k 2 /k 1 = 10.7 over 10 times faster! = -76000 J 8.3145 J/mol K ln k 2 k 1 = 2.37 ln k 2 k 1 1 323.15K 1 298.15K -
  7. 7. The decomposition of hydrogen iodide, E a = ___________ kJ/mol 2HI( g )  H 2 ( g ) + I 2 ( g ) has rate constants of 9.51x10 -9 L/mol*s at 500. K and 1.10x10 -5 L/mol*s at 600. K. Find E a .
  8. 8. Series of plots of concentra-tion vs. time Initial rates Reaction orders Rate constant ( k ) and actual rate law Integrated rate law (half-life, t 1/2 ) Rate constant and reaction order Activation energy, E a Plots of concentration vs. time Overview Find k at varied T Determine slope of tangent at t 0 for each plot Compare initial rates when [A] changes and [B] is held constant and vice versa Substitute initial rates, orders, and concentrations into general rate law: rate = k [A] m [B] n Use direct, ln or inverse plot to find order Rearrange to linear form and graph Find k at varied T
  9. 9. <ul><li>Catalysts speed up reactions by altering the mechanism to lower the activation energy barrier, but they are not consumed. </li></ul>MnO 2 catalyzes decomposition of H 2 O 2 2 H 2 O 2  2 H 2 O + O 2 Catalysis Uncatalyzed reaction Catalyzed reaction
  10. 10. <ul><li>Imagine trying to get a bunch of cattle to where you want them to go without any help… </li></ul>“ Cattle-ists” Will they get there very quickly on their own? Will they take the shortest path do get there? Is there a way to help?
  11. 11. <ul><li>What if there was someone there to herd the cattle and guide them along a more efficient path? </li></ul>“ Cattle-ists” The end result is the same – but the reaction takes a more efficient path.
  12. 12. The metal-catalyzed hydrogenation of ethylene
  13. 13. Catalytic Converters 13.6 CO + Unburned Hydrocarbons + O 2 CO 2 + H 2 O catalytic converter 2NO + 2NO 2 2N 2 + 3O 2 catalytic converter
  14. 14. Catalysis <ul><li>3) Enzymes — biological catalysts </li></ul>Enzymes are specialized organic substances, composed of polymers of amino acids ( proteins ), that act as catalysts to regulate the speed of the many chemical reactions involved in the metabolism of living organisms.
  15. 15. Enzyme Catalysis
  16. 16. Enzymes uncatalyzed enzyme catalyzed
  17. 17. Enzymes Saturation Effects Denaturization
  18. 18. Summary Activity: Fireflies flash at a rate that is temperature dependent. At 29 ˚C the average firefly flashes at a rate of 3.3 flashes every 10. seconds. At 23 ˚C the average rate is 2.7 flashes every 10. seconds. Use the Arrhenius equation to determine the activation energy (kJ/mol) for the flashing process.
  19. 19. The activation energy of the firefly flashing process is: E a = ___________ kJ/mol

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