14 FoC Catalysts - FofC p 212 Problem 1 Fireflies flash at...

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1 FofC p. 212, Problem 1 Fireflies flash at a rate that is temperature dependent. At 29 o C the average firefly flashes at a rate of 3.3 flashes every 10 seconds. At 23 o C the average rate is 2.7 flashes every 10 seconds. Use the Arrhenius equation to determine the activation energy for the reaction that causes the flashing. 0 o C = 273 K R = 8.314 J mol -1 K -1 Hint: Look at the ratio k 29 degrees /k 23 degrees ! a E RT k = A e A)2.5 kJ/mol B)12 kJ/mol C)25 kJ/mol D)150 kJ/mol E)250 kJ/mol 2 1 2 1 1 2 2 o -1 1 1 o -1 2 2 Ea - Ea 1 1 Ea Ea RT ) - - R T T RT RT 1 Ea - 2 RT a T = 29 C = 302 K, k 3.3/10 = 0.33 s T = 23 C = 296 K, k = 2.7/10 = 0.27 s k e = = e = e k e take the ln of both sides and solve for E (not log) k ln a 1 2 2 1 -1 -1 1 a 2 2 1 a E 1 1 = - k R T T k R E = ln substituting values and using R = 8.3 J mol K k 1 1 - T T E = 25 kJ/mol Summarize info Recognize use of Arrhenius eq.
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3 Model 3: Catalysts, FoC p. 213 KQ 10. What effect does a catalyst have on the activation energy of a reaction? A) increases the activation energy B) decreases the activation energy C) has no effect on the activation energy Energy diagram for a reaction with and without a catalyst. 4 KQ 11. What effect does a catalyst have on the change in free energy of a reaction, G, which is the difference in free energy between the products and reactants? A) increases G B) decreases G C) has no effect on G
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