# Lec 12 - Problem From Last Time Fireflies flash at a rate...

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1 1 Problem From Last Time! 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. R = 8.314 J mol -1 K -1 a E RT k = A e Hint: Look at the ratio k 29 degrees /k 23 degrees ! A) 52 kJ/mol B) 43 kJ/mo/ C) 25 kJ/mol D) 34 kJ/mol 3 o- 1 11 1 22 T = 29 C = 302 K, k 3.3/10 = 0.33 s T = 23 C = 296 K, k = 2.7/10 = 0.27 s -1 -1 1 a 2 21 a k R E = ln substituting values and using R = 8.3 J mol K k - TT E = 25 kJ/mol    a E RT k = A e

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2 4 1 21 12 2 o- 1 11 1 22 Ea - 1 1 RT ) - RT T RT 1 - 2 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 ke = = e k e take the ln of both sides and solve for E (not log) k ln    a 1 1 -1 1 a 2 a E kR T T k R E = substituting values and using R = 8.3 J mol K k TT E = 25 kJ/mol 6 Reaction Mechanisms & Rate Laws A reaction mechanism is a series of elementary reactions. Elementary reactions are either a unimolecular (a single molecule) or a bimolecular (two molecules) reaction. Its very improbable for more than 2 molecules to collide simultaneously with enough energy and orientations favorable for reaction.
3 7 Rate Law for a Unimolecular Elementary Reaction First order Occurs when one molecule dissociates or rearranges into a different structure.

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Lec 12 - Problem From Last Time Fireflies flash at a rate...

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