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CHEM 001B Lecture Notes - Lecture 21: Elementary Reaction, Rate Equation, Reaction Step

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6 Mar 2018

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Increased temperature increases kinetic energy of molecules and. The minimum energy of molecular collisions required to break bonds in reactants, leading to formation of products. These factors are incorporated into rate constant k by arrhenius equation. R = universal gas constant [in j/(mol*k)] Includes frequency of collisions and an orientation factor. As temperature increases, the fraction of collisions exceeding this minimum energy also increases, leading to an increase in k and an increase in the rate of the reaction. Low e a = large k, fast forward reaction. High e a = small k, slow reverse reaction. Consider determination of k at two different t: The compound 1, 1-difluoroethane decomposes at elevated temperatures to give fluoroethylene and hydrogen fluoride: At 460 , k = 5. 8 x 10 -6 s -1 and e a = 265 kj/mol. Reaction mechanisms molecular level; must be consistent with the rate law for the reaction.

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In the collision model it is assumed that reactions occur as a result of collisions between molecules with the correct kinetic energy and orientation. The minimum energy required for a reaction to occur when molecules collide is called the activation energy, E_a. The number of collisions that are favorably oriented for a reaction is described by the frequency factor, A. The Arrhenius equation relates both of these factors to the rate constant, k. k = A middot e^-E_a/(R middot T) where R = 8.314 J/(mol middot K) and T is the temperature in kelvins. Based on the activation energies and frequency factors, rank the following reactions from fastest to slowest reaction rate, assuming they are all at the same temperature. Rank from fastest to slowest reaction rate. To rank items as equivalent, overlap them. Fraction of molecules The exponential term in the Arrhenius equation is equal to the fraction of molecules, f, with kinetic energy greater than or equal to the activation energy: f = e^-E_a/(R middot T) Most scientific calculators have an e^x function as the second function of the LN button. Part B A certain reaction with an activation energy of 145 kJ/mol was run at 495 K and again at 515 K. What is the ratio of f at the higher temperature to f at the lower temperature? Express your answer numerically using one significant figure. In the collision model it is assumed that reactions occur as a result of collisions between molecules with the correct kinetic energy and orientation. The minimum energy required for a reaction to occur when molecules collide is called the activation energy, E_a. The number of collisions that are favorably oriented for a reaction is described by the frequency factor, A. The Arrhenius equation relates both of these factors to the rate constant, k: k = A middot e^-E_a/(R middot T) where R = 8.314 J/(mol middot K) and T is the temperature in kelvins. Part A Based on the activation energies and frequency factors, rank the following reactions from fastest to slowest reaction rate, assuming they are all at the same temperature. Rank from fastest to slowest reaction rate. To rank items as equivalent, overlap them. Fraction of molecules The exponential term in the Arrhenius equation is equal to the fraction of molecules, f, with kinetic energy greater than or equal to the activation energy: f = e^E_a/(R middot T) Most scientific calculators have an e^x function as the second function of the LN button. Part B A certain reaction with an activation energy of 145 kJ/mol was run at 495 K and again at 515 K. What is the ratio of f at the higher temperature to f at the lower temperature? Express your answer numerically using one significant figure. In the collision model it is assumed that reactions occur as a result of collisions between molecules with the correct kinetic energy and orientation. The minimum energy required for a reaction to occur when molecules collide is called the activation energy, Ea. The number of collisions that are favorably oriented for a reaction is described by the frequency factor, A. The Arrhenius equation relates both of these factors to the rate constant, k: k = A middot e^-E_a/(R middot T) where R = 8.314 J/(mol middot K) and T is the temperature in kelvins. Fraction of molecules The exponential term in the Arrhenius equation is equal to the fraction of molecules, f, with kinetic energy greater than or equal to the activation energy: f = e^-E_a/(R middot T) Most scientific calculators have an e^x function as the second function of the LN button. Part B A certain reaction with an activation energy of 14 kJ/mol was run at 495 K and again at 515 K. What is the ratio of f at the higher temperature to f at the lower temperature? Express your answer numerically using one significant figure.