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TEACHERS: Photochemical Reactions

Objectives
Students will be able to:
  • describe the process of ozone formation
  • use manipulatives to illustrate how VOCs and NOx combine to form ground level ozone

Materials

Background:

Atoms are always moving around, bumping into each other combining compounds, breaking apart and forming others. The bonding of some compounds is stronger than others. Take for instance, a glass of water. On a molecular level, at any given time, the majority of Oxygen and Hydrogen atoms in the liquid are combining to form H2O. However because the atoms are always on the move there are also random OH and H30 compounds in the glass as well as a few "free" Oxygen and Hydrogen atoms. Not to mention the other elements that are represented in the glass, like Chlorine and Sulfur that also like to combine with the O's and H's, but whose bonding might not be as
strong. So, as stated before, at any given time in the glass, the compound found in the majority will be H2O, but there are plenty of other compounds forming, breaking apart and forming into something else.

The air around us is also in a similar state. There are several compounds and "free" atoms bumping into each other, combining and breaking apart and recombining, with a frequency dependent on the bond strength. NO2 happens to be a compound that will react readily with other substances.

For this lesson, remember that the Nitrogen and Oxygen atoms are all "free" to start.

Explain to the students that although they can be "free" atoms, they would prefer to be in a more stable state, for instance in the gas forms, O2 and N2.

You could have the students physically pair up the Nitrogen and Oxygen atoms at this point to create O2 and N2 molecules.

But when heat is added to the system, it begins to break the bonds between the N2 and the O2 molecules. Because the atoms are searching for something to connect with to be more stable, some Nitrogen atoms will connect with some Oxygen atoms to create NO. Some Nitrogen and Oxygen atoms might remain "free" while some of the Nitrogen atoms might pair back up to become N2 and some of the Oxygen atoms might pair up to become O2.  So at this point the students should have NO, N2, O2 molecules and "free" Nitrogen and Oxygen atoms.

NO is not a particularly stable compound and would prefer to become NO2, so the NO molecules will combine with Oxygen atoms to create NO2

Reinforce the idea with students that in nature, these reactions can occur in split seconds.

Because we have to start somewhere, the students are challenged to create 5  2NO molecules. Once again, the remaining "free" Nitrogen and Oxygen atoms might pair back up to become N2 and O2.

Once again, because NO2 is a highly reactive compound, in the presence of lots of sun and heat, NO2 compounds can quickly break down into NO and Oxygen atoms.

The free Oxygen atoms are unstable and will look to become more stable by connecting up with O2, creating O3 (OZONE). Because O3 is also relatively unstable, it would prefer another state, so they tend to react with the NO to create NO2 and O2.

It is very important for the students to know that these reactions happen all the time and very quickly and at any given time there can be several different compounds around us and several "free" atoms searching for a more stable state as part of a compound.

The problem arises when VOCs are introduced, interrupting the balance of these natural chemical reactions, trapping the "free" oxygen atoms in ozone molecules.

VOCs + NOx + Sunlight = Ozone

Ground level ozone is formed by chemical reactions between volatile organic compounds (VOCs), also known as hydrocarbons and oxides of nitrogen (NOx) in the presence of sunlight.  NOx and VOCs are known as the chief "precursors" of ozone.  These compounds react in the presence of sunlight to produce ozone. These gaseous compounds mix like a thin soup in the atmosphere, and when they are exposed to sunlight, ozone is formed.  Due to the nature of these reactions, ozone concentrations can reach unhealthful levels when the weather is hot and sunny with little or no wind.  As a result, ground level ozone pollution, or smog, is mainly a daytime problem during summer months.

Students may have to work through the activity a few times before fully grasping the abstract concepts presented in the lesson.

 

Procedure

1.  Break up into groups of 2 students.   Obtain a set of materials (a large sheet of paper and set of Post-it tabs, 30 red, 30 blue, 20 yellow) for each group.

2.  Place the red and blue post-its randomly on one half of the large sheet of paper, representing the atmosphere.

3.  Air is mostly nitrogen molecules and oxygen molecules.   Oxygen atoms will be represented by red post-its and Nitrogen atoms will be represented by blue post-its.  It may be helpful to mark the red post-its with an "O", and the blue post-its with an "N".

4.  The temperature inside engines causes the nitrogen molecules and oxygen molecules in the air inside the chambers to combine and form nitric oxide which is emitted to the atmosphere.  With the post-its, create 5 nitric oxide molecules.

N2(g) + O2(g)  heat >  2 NO(g)

5.  The nitric oxide emitted to the atmosphere quickly combines with more oxygen in the air to form nitrogen dioxide.   Add red post-its to create 5 nitrogen dioxide molecules.

2 NO(g) + O2(g)  --> 2 NO2(g)

6.  In warm, sunny air, the nitrogen dioxide breaks down into a molecule of nitric oxide and an atom of oxygen.  Break down the post-it molecules.

NO2(g)  light >   NO(g) + O(g)

7.  The atom of oxygen combines with a molecule of oxygen to form ozone, the form of oxygen that has 3 oxygen atoms in its molecule.  Rearrange the post-its to create the ozone molecules.

O(g) + O2(g)  ----->   O3(g)

8.  The nitric oxide reacts with the ozone to form nitrogen dioxide and an ordinary oxygen molecule.  Rearrange the post-its to form.

NO + O3 -----> NO2 + O2

9.  We’re back where we started.  As long as there’s sunlight, this cycle can repeat, and the ozone concentration will never get very high, since the ozone reacts with nitric oxide almost as fast as it is formed.

Part 2
10.  The cycle can be broken by other substances we add to the atmosphere.  For example, fumes from fuels, solvents from drying paint and from glue—all add volatile organic compounds (VOC's) to the air.  VOC's will be represented by yellow post-its.  It may be helpful to mark the yellow post-its with "VOC".  Add the yellow VOC post-its to the "atmosphere" randomly among the red and blue post-its on the paper.

11.  For example, the temperature inside engines causes the nitrogen and oxygen in the air inside the engine's chambers to combine and form nitric oxide (NO) which is emitted to the atmosphere.  With the post-its, create 5 nitric oxide molecules (NO).  Write the chemical equation.

12.  The nitric oxide emitted from the engine to the atmosphere quickly combines with more oxygen in the air to form nitrogen dioxide (NO2).   Add red post-its to create 5 nitrogen dioxide molecules (NO2).  Write the chemical equation.

13.  In warm, sunny air, the nitrogen dioxide (NO2) breaks down into a molecule of nitric oxide (NO) plus an atom of oxygen.  The  nitric oxide (NO) molecule then combines with a VOC molecule, leaving the atom of oxygen (O) free in the atmosphere.  Rearrange the post-its to reflect this reaction.  Write the chemical reaction.

14.  The atom of oxygen (O) combines with a molecule of oxygen (O2) to form ozone, the form of oxygen that has 3 oxygen atoms (O3) in its molecule. 

15.  Repeat steps 11 - 14.  Do you see a pattern emerging?  Answer the questions on the Student Worksheet.

 


 
 
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