Chemistry: An Atoms-Focused Approach (Second Edition)
Chemistry: An Atoms-Focused Approach (Second Edition)
2nd Edition
ISBN: 9780393614053
Author: Thomas R. Gilbert, Rein V. Kirss, Stacey Lowery Bretz, Natalie Foster
Publisher: W. W. Norton & Company
Question
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Chapter 13, Problem 13.109QA
Interpretation Introduction

To explain:

Which of the following mechanisms is for thermal decomposition of NO2 and which is for the photochemical decomposition of NO2?

Expert Solution & Answer
Check Mark

Answer to Problem 13.109QA

Solution:

Mechanism a is the photochemical decomposition of NO, while mechanism b and c are thermal decomposition of NO2.

Explanation of Solution

1) Concept:

We are calculating the rate law for slow step for each given mechanism. If it is the same as the given rate of thermal decomposition of NO2, then that mechanism is for thermal decomposition of NO2. If it is same as the given rate of photochemical decomposition of NO2, then that mechanism is for photochemical decomposition of NO2.

2) Given:

thermal decomposition of NO2, Rate=kNO22

photochemical decomposition of NO2, Rate=k[NO2]

a.  NO2gNO(g)+O(g)slow

 Og+NO2gNOg+O2gfast

b.NO2g+NO2gN2O4gfast

N2O4gNO(g)+NO3gslow

NO3g NOg+O2gfast

c.NO2g+NO2gNOg+NO3gslow

NO3g NOg+O2gfast

3. Calculations:

Rate law for mechanism a:

Rate1=ka[NO2]

Here Ka is the rate constant for slow step of reaction a. This rate is the same as the given rate of photochemical decomposition of NO2. Therefore, mechanism a is the mechanism of photochemical decomposition of NO2.

Rate law for mechanism b:

Rate2=kb[N2O4]

Here, N2O4 is the intermediate formed in the step (1) (fast), so we need to write the rate law for both forward and backward reaction of step (1):

Rate1=k1NO2NO2=k-1[N2O4]

Here k1 is the rate constant of forward reaction, and k-1 is the rate constant of backward reaction.

Rearranging above rate equation for [N2O4], we get

N2O4=k1NO2NO2k-1=k1k-1NO22

Plugging the value of [N2O4] in Rate2 equation, we get

Rate=kb.k1k-1NO22

Rate=kNO22

where, k=kb.k1k-1

This rate of mechanism b is the same as the given rate of thermal decomposition of NO2. Therefore, mechanism c is the mechanism of thermal decomposition of NO2.

Rate law for mechanism c:

Rate=kcNO2NO2

Rate=kcNO22

Here Kc is the rate constant for slow step of mechanism c. This rate is the same as the given rate of thermal decomposition of NO2. Therefore, mechanism c is the mechanism of thermal decomposition of NO2.

Conclusion:

Mechanism a is the photochemical decomposition of NO2, while mechanism b and c are thermal decomposition of NO2.

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Chapter 13 Solutions

Chemistry: An Atoms-Focused Approach (Second Edition)

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