Structure and Bonding: The Kinetics of Dinitrogen Pentoxide Decomposition and the Ozone Hole

Let's examine the structure of dinitrogen pentoxide, an important nitrogen reservoir in polar stratospheric clouds found in Antarctica and involved with the ozone hole.

Molecule produced using Spartan Plus and energy minimized by an AM1 calculation.

Make the following measurements in the N2O5 molecule.  Right click and click on rotation to stop the rotation for the measurements.  Right click, go to select and then mouse click action, and then select distance (or angle).  

For distance, place the cursor on the first atom, click and then select a second atom and click.
For angles, select and click on three consecutive connected atoms.

Watch the lower left of your browser screen to get the results.  The distance x 100 will give you picometers (pm).  See the Chime Guide for help.  Estimate the bond orders.  Refer to The Carbon-Carbon Bond activity for a discussion of bond order and to Oxides of Nitrogen for some needed data about nitrogen-oxygen bonds.  The oxygen between the two nitrogens will be referred to as the bridging oxygen.  The other four oxygens in the molecule are terminal (at the end) oxygens.   

Measurement Value Bond Order
N-O terminal oxygen bond distance    
N-O bridging oxygen bond distance    
N-O-N bond angle   ------
O-N-O bond angle*   ------

* use the terminal oxygens not the bridging oxygen

How does the N-O-N bond angle compare to the C-O-C angle (111o) found in ethers?  

Now if a nitrogen-oxygen bond was going to be broken during a reaction, which one in the molecule would be the easiest to break?  Why?

What is the N-N distance in the N2O5 molecule?  Just click on one nitrogen and then the other.

How does the N-N distance in the N2O5 molecule compare to the N-N bond distance in N2O4? (See the Oxides of Nitrogen activity.)

Let's consider the kinetics of the decomposition of dinitrogen pentoxide.  The reaction is given below.

2N2O5 (g)        2N2O4(g)   +    O2 (g)

2N2O4(g)     =    4 NO2(g)

2N2O5 (g)       4 NO2(g)     +    O2 (g)

We will examine the kinetics of the decomposition reaction using a STELLA model.  Click here to download the model.  Use the simulation of the kinetics of the decomposition reaction to address the following:

1.    Select a temperature and collect initial concentration and initial rate data to determine the rate law.  Find the order, n, of the reaction.        Rate = k (N2O5)n        Temperature ______    Explain your method.        

Experiment (N2O5)o Initial Rate
1    
2    
3    
4    

2.    For the temperature above, calculate the rate constant, k, for the reaction.

3.    Does the reaction come to equilibrium or go to completion?  Explain your choice.

4.    What is the effect on the reaction of changing the temperature?

5.    Why is the level of NO2 produced so high?

6.    The initial rate is for the N2O5 starting to decompose very near time zero, or shortly there after.  Why do we use initial rates?

Propose a reaction mechanism for the decomposition.  Consider a transition state that involves a structure intermediate between the N2O5 molecule and the N2O4 molecule.  Here is a comparison for dimethyl ether and epoxide.

dimethyl ether bending vibration along
 C-O-C
ring compound or epoxide

animated molecules from http://www.nicol.ac.jp/~honma/mva/indexE.html

Draw your proposed structure of the transition state.  Label any bonds forming and breaking.

transition state

 

 

 

 

 

The kinetics of the decomposition reaction are first order as this reaction is unimolecular.  Let's explore the temperature dependence of the rate constant by constructing an Arrhenius plot.  Click here to get an interactive Excel spreadsheet of an Arrhenius plot where ln k is plotted against 1/T, where T is Kelvin temperature.  The activation energy, Ea, is calculated from the slope of an Arrhenius plot.  Use the interactive Excel spreadsheet to address the following:

1.    How does the graph behave if the activation energy, Ea, is changed?  Compare two values.

2.    Which reaction, low Ea or high Ea, is influenced by temperature to a greater extent (has the larger change in k)?

3.    Polar stratospheric clouds form at temperatures below 190 K.  What is the value of the rate constant at 190 K?  What do you conclude about the rate of the decomposition reaction at 190 K?

In polar stratospheric clouds, the N2O5 starts to react with the ice such as the reaction given below:

N2O5   +   H2O        2HNO3

The nitric acid on the ice forms a variety of nitric acid hydrates, which removes the nitrogen from reacting with Cl in the atmosphere.  The nitric acid and its hydrates remain on the ice, which settles out of the atmosphere.  The Cl then participates in ozone destruction, which enhances the ozone hole.

Dinitrogen pentoxide is found to be a molecular compound in polar stratospheric clouds.  It can also exist as an ionic solid as NO2+ NO3-See figure 2 in this article in the Internet Journal of Vibrational Spectroscopy for the experimental bond distances and bond angles.  How do the literature values compare to your measured values?  Conversion: pm = 100 x

Measurement

Literature Value Chime Molecule Value
N-O terminal oxygen bond distance    
N-O bridging oxygen bond distance    
N-O-N bond angle    
O-N-O bond angle    

If the Chime measured N-O-N bond angle was closer to the literature value, would this help forming the N-N bond?  Why?

Does the longer N-O bridging oxygen bond distance help or hinder your transition state model?  Explain.

Since the reaction below is endothermic, which oxide is favored at the temperatures found in polar stratospheric clouds?

N2O4(g)    +   heat   =    2 NO2(g)

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