Assignment4Solutions - Chemistry 350 Assignment 4:...

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Chemistry 350 Assignment 4: Solutions November 21 st 2005 Question 1 Q18.1 Why is the stoichiometry of a reaction generally not sufficient to determine reaction order? The order of a rate equation for a chemical reaction indicates the concentration dependence of the rate. The order is dependent on the mechanism of the reaction, and not on the proportionality of the reaction (i.e. the number of reactant molecules required to generate a specific number of product molecules). Q18.2 What is an elementary chemical step, and how is one used in kinetics? An elementary reaction step is a chemical process that occurs in a single step. Elementary reaction steps are combined to construct complex reaction mechanisms. Q18.15 What is the relationship between the parameters in the Arrhenius equation and in the Eyring equation? The principle connection between the Arrhenius and Eyring equations is the relationship between the activation energy ( E a ) in the Arrhenius expression and the enthalpy difference between the reactants and transition state ( ! H ) which is: "# with a E RT U U H PV $ % ! ! $ ! % !
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Where "# PV ! is the difference in PV between reactants and the transition state. This term depends on the phase and type of reaction being considered. Question 2 18.1 Express the rate of reaction with respect to each species in the following reactions: a) 2 2 4 2NO( ) O ( ) N O ( ) g g g % &&’ b) 22 H ( ) I ( ) 2HI( ) g g g % c) 2 ClO( ) BrO( ) ClO ( ) Br( ) g g g g %% a) ( ) () 2 24 NO O 2 NO P 11 P 1 P 1 d NO d R dt RT dt d dO dt RT dt d d N O dt RT dt $ * $ * $ * $ * $$ b) ( ) 2 2 H 2 I 2 HI P 1 P 1 P d dH R dt RT dt d dI dt RT dt d HI d dt RT dt $ * $ * $ * $ * c)
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( ) () 2 ClO BrO ClO 2 Br P 1 P 1 P 1 P 1 d ClO d R dt RT dt d BrO d dt RT dt d d ClO dt RT dt d Br d dt RT dt $ * $ * $ * $ * $$ 18.2 Consider the first-order decomposition of cyclobutane at 438°C at constant volume: 4 8 2 4 C H ( ) 2C H ( ) gg &&’ a) Express the rate of the reaction in terms of the change in total pressure as a function of time. b) The rate constant for the reaction is 2.48 + 10 –4 s –1 . What is the half-life? c) After initiation of the reaction, how long will it take for the initial pressure of C 4 H 8 to drop to 90% of its initial value? a) The rate is given as 48 24 11 2 CH dP Rate RT dt RT dt $ * $ The pressure at time t is given as 4 8 4 8 2 4 C H ( ) C H ( ) C H . t initial reacted P P P P $ *% By stoichiometry 4 8 2 4 C H ( ) C H 1 2 reacted PP $ so that: 4 8 2 4 2 4 4 8 2 4 C H ( ) C H C H C H ( ) C H 22 t initial initial P P P P P P $ * % $ % Since P is dependent on time and C H ( ) initial P is time independent: 1 2 t dP dt dt $
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and 1 t dP Rate RT dt
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This note was uploaded on 04/11/2010 for the course CHEM 1101 taught by Professor Leroy during the Spring '10 term at University of Toronto- Toronto.

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Assignment4Solutions - Chemistry 350 Assignment 4:...

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