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Chemistry LibreTexts

Groupwork 13 Kinetics 2

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    Work in groups on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help.

    Elementary reaction = rate law follows the stoichiometry

    Complex reaction = rate law is derived from a series of individual elementary reaction steps.

    The complex reaction

    \[2H_2(g)+2NO(g)\overset{k_{obs}}\rightarrow N_2(g)+2H_2O(g)\]


    is proposed to occur via the two step reaction mechanism

    \(H2(g)+NO(g)+NO(g)\overset{k_1}\Rightarrow N_2O(g)+H_2O(G)\).............(1)

    \(H_2(g)+N_2O(g)\overset{k_2}\Rightarrow N_2(g)+H_2O(g)\)....................(2)

    If the reaction \(2H_2(g)+2NO(g)\overset{k_{obs}}\rightarrow N_2(g)+2H_2O(g)\) were an elementary reaction, what would be the rate law describing the appearance of N2 (g)? 


    What is the rate law for elementary reaction (1), \(H2(g)+NO(g)+NO(g)\overset{k_1}\Rightarrow N_2O(g)+H_2O(G)\)? Write rate laws describing the appearance of the two products. 


    What is the rate law for elementary reaction (2), \(H_2(g)+N_2O(g)\overset{k_2}\Rightarrow N_2(g)+H_2O(g)\)?

    Write rate laws for the disappearance of reactants and for the appearance of products. 


    In the rate laws, molecules that appear in elementary reactions that are not in the original complex reaction are called intermediates.  What are the intermediates for the original complex reaction? 


    Rate laws describing complex reactions cannot include intermediates in the overall rate law.  To attain the correct rate law, we must find a way to describe those components from other reactions.  One way assumes that the concentration of the intermediate does not change appreciably over the course of the reaction, that is [intermediate]=constant.  Consider your rate laws for elementary reactions (1) and (2); if [intermediate]=constant, what must be true about the rates for appearance and disappearance of the intermediate? 


    Experimentally, the rate law is observed to follow



    Under what conditions would this rate law be observed?  What would kobs be for the reaction in terms of k1 and k2