159_326_spring_08_chapt_16_kineti

159_326_spring_08_chapt_16_kineti - Chem 160 Ch 16 32 The...

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Unformatted text preview: Chem 160 Ch 16 32 The Effect of Temperature on Reaction Rate Temperature often has a major effect on reaction rate. In fact for many reactions near room temperature, an increase of 10C causes a doubling or tripling of the rate. In other words, temperature affects the rate by affecting the rate constant. Lizards and crocodiles bask in the sun or lie in the shade to regulate their internal reaction rates. Sec 16.5 Chem 160 Ch 16 33 The Effect of Temperature on Reaction Rate In 1889, Svante Arrhenius proposed the following mathematical expression for the effect of temperature on the rate constant, k k = Ae E a / RT EOS ln k = E a / RT + ln A Chem 160 Ch 16 34 The Effect of Temperature on Reaction Rate The Arrhenius Equation The constant A , called the frequency factor , is the product of the collision frequency and a probability factor that takes into account the orientation required for effective molecular collisions EOS The expression e E a / RT represents the fraction of molecular collisions sufficiently energetic to produce a reaction Chem 160 Ch 16 35 Arrhenius Equation Dependence of k on Temperature and Activation Energy k = A e-Ea/RT l n k =- E a /RT + ln A l n k 2 = -E a2 /RT 2 + ln A (Ea, T are variables) l n k 1 = -E a1 /RT 1 + ln A l n k 2 ln k 1 = -E a2 /RT 2 + ln A +E a1 /RT 1- ln A l n k 2 /k 1 = -E a2 /RT 2 +E a1 /RT 1 l n k 2 /k 1 = -1/RT(Ea 2 E a1 ) (T 1 =T 2 =T, constant temperature) l n k 2 /k 1 = -E a /R (1/T 2 1/T 1 ) (Ea 1 =Ea 2 =Ea,constant Ea) Chem 160 Ch 16 36 Figure 16.9 The Effect of Temperature on Reaction Rate - Review Graphical determination of the activation energy ln k = -E a /R (1/T) + ln A Chem 160 Ch 16 37 Typical Exam Problem 20 The rate of a reaction increases by a factor of 1000 in the presence of a catalyst at 298 K. The activation energy of the original pathway is 98 kJ/mol. What is the activation energy in kJ/mol of the new pathway, all other factors being equal? Chem 160...
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159_326_spring_08_chapt_16_kineti - Chem 160 Ch 16 32 The...

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