CH353 - Physical Chemistry I
Spring 2012, Unique 52135

Homework, Week 14

Homework, week 14
Answer key

1. The following reaction:

            2 H2(g) + 2 NO(g) --> N2(g) + 2 H2O(g)


has an experimentally observed rate law of:

            d[N2]/dt = kobs[H2][NO]2

A proposed mechanism is:

            1)  H2(g) + NO(g) + NO(g) --> N2O(g) + H2O(g)

            2)  H2(g) + N2O(g) --> N2(g) + H2O(g)

Determine the conditions under which this mechanism gives the experimentally observed rate law, and express kobs in terms of the rate constants or equilibrium constants of the individual elementary reactions.

2.  The thermal decomposition of ethylene oxide occurs, H2COCH2, through the following chain reaction:

            1)  H2COCH2(g) --> H2COCH(g) + H(g)

            2)  H2COCH(g) --> CH3(g) + CO(g)

            3)  CH3(g) + H2COCH2(g) --> H2COCH(g) + CH4(g)

            4)  CH3(g) + H2COCH(g) --> products

a) Determine which steps are the initiation, propagation, and termination steps.

b) Determine that if the intermediates CH3 and H2COCH are treated by the steady-state approximation, then the rate law expressed as the appearance of products is first order in ethylene oxide. 

3.  The reaction of atomic I to form molecular iodine is catalyzed by the presence of any inert gas, Ar:

            I(g) + I(g) + Ar(g) --> I2(g) + Ar(g)

The observed rate law of the reaction is:

            v(t) = kobs[Ar][I]

Propose a series of elementary steps that explain this rate law.  State all of your assumptions.

4.  The reaction

            NO2(g) + CO(g) --> CO2(g) + NO(g)

has the experimentally determined rate law

            v(t) = kobs[NO2]2

The following mechanism has been proposed for this reaction:

            1)  NO2(g) + NO2(g) --> NO3(g) + NO(g)

            2)  NO3 + CO(g) --> CO2(g) + NO2(g)

where the first elementary step is rate determining.  Show that this mechanism is consistent with the experimentally determined rate law and express kobs in terms of the rates of each individual steps.  State all your assumptions clearly.

5.  The rate law for the reaction of carbon monoxide and chlorine gas to form phosgene (Cl2CO) has the experimentally determined rate law

            v(t) = kobs[Cl2]3/2[CO]

Show that the following mechanism is consistent with this rate law and express obs in terms of rate or equilibrium constants for individual steps of the mechanism.

            1)  Cl2(g) + M(g) --> 2 Cl(g) + M(g)                         (fast equilibrium)

            2)  Cl(g) + CO(g) + M(g) --> ClCO(g) + M(g)          (fast equilibrium)

            3)  ClCO(g) + Cl2(g) --> Cl2CO(g) + Cl(g)                (rate-determining)