Chemical Science Lesson Plan

Title: Nitrogen Oxide Equilibrium

Grade Level: High School

Prepared by: Pat Shapley
Science standards

Science background for teachers


Activity procedure


Summary and discussion

Objectives: Students will explore the concept of chemical equilibrium. They will monitor the equilibrium between NO2 and N2O4 in the gas phase by monitoring the color of the mixture. They will learn about the relationship between automobile exhaust and the formation photochemical smog.

Science standards

11.A.5a Formulate hypotheses referencing prior research and knowledge.

11.A.5b Design procedures to test the selected hypotheses.

12.C.5a Analyze reactions (e.g., nuclear reactions, burning of fuel, decomposition of waste) in natural and man-made energy systems.

Science background

For information on smog and air quality, see the AirNow website.

The light, oxygen, volatile organic compounds, and nitrogen oxides in the troposphere produces ozone. This mixture is called photochemical smog (smog= smoke + fog).

Because the VOCs and NOx are both products of petroleum combustion, the irritating effects of smog are most common in cities.

The color of smog is due to the presence of NO2.

In the exhaust of automobiles, there are hydrocarbon vapors and partially combusted hydrocarbons, including alkyl peroxides RCH2-O-O, and nitrogen oxides.

Nitric oxide can react with oxygen atom donors, such as CH3-O-O and H-O-O. These peroxy radicals are intermediates in the aerobic oxidation of hydrocarbons in the atmosphere.

Nitrogen dioxide is photolyzed in daylight (420 nm) to NO and O. The oxygen atom can react with molecular oxygen to form ozone.

These reactions of NO and NO2 make NO a catalyst for the formation of ozone. Nitric oxide is both used and produced in this series of reactions so its concentration remains unchanged.

The net reaction in the catalytic cycle is:
2 O2 + RCH3 RCH2OH + O3

The brown, nitrogen-centered radical NO2 is in equilibrium with its colorless, diamagnetic dimer N2O2. The N-N bond in the dimer is weak.

Below is a picture of 2 tubes of the gas mixture at 25 deg. The same tubes were heated (100 deg) and cooled (0 deg). Aftetr the initial color change, there is no further change.

All reactions under equilibrium conditions are governed by thermodynamics. For the reaction as written below, the Grxn is temperature dependent. That means that the equilibrium constant is also temperature dependent.


As you drive from Champaign-Urbana towards Chicago you see a brown layer on the horizon. It is worse in the summer than in the winter. Once you arrive in the city, you might notice some odor or irritation to your lungs and it's harder to exercise. Plants don't seem to grow as well as they do back in Champaign-Urbana.

What is the cause of all this? It is photochemical smog.

Geography also play a role. In Chicago, the "windy city", the smog never gets as bad as it does in Los Angeles because the winds from across the plains dilute it. The Los Angeles basin holds the air above LA and prevents the smog from dissipating.

Photochemical smog is a mixtures of many organic and inorganic gas phase molecules. It contains ozone, nitrogen oxides, and organic molecules. Typically, we estimate the amount of smog by measuring the the concentration of one component, ozone.

Smog is:
  • Toxic to plants and animals
  • Decreases photosynthesis activity by ~50%s
  • Responsible for 10-40% yearly loss in the growth of crops outside the cities
  • Responsible for a 50% loss of trees in Los Angeles area

What is needed to form smog?
  1. volatile organic molecules from internal combustion engines or power plants
  2. nitrogen oxides from internal combustion engines or power plants
  3. sunlight
  4. oxygen

Ozone is toxic to humans and causes respiratory irritation and reduced lung function. It is especially dangerous to people with asthma or reduced lung function. It may also reduce the immune system's ability to fight infection. Because of the deleterious effects of ozone and because it is associated with other pollutants, the government measures air quality in terms of ozone concentration.

Ozone levels in Illinois last week were pretty good! You can check the level today on the ozone map. Note that the ozone level goes up through the day and is reduced at night. Metropolitan areas on the east and west coasts of the US typically have a higher ozone level than we do in central Illinois.

Today we're going to investigate a mixture of nitrogen oxides produced by cars. Two molecules of nitrogen dioxide, NO2, can come together to form dinitrogen tetroxide, N2O4, with a weak nitrogen-nitrogen bond.

Both the forward and the reverse reactions are very rapid under normal atmospheric conditions so that the ratio of the concentrations of products and reactants remains the same at a particular set of temperature/pressure conditions. Specifically, the concentration of N2O4 divided by the square of the concentration of NO2 is constant.

A reactants will go to products when energy is released in the process.

If the heat energy contained in chemical bonds of the reactants is greater than the heat energy contained in chemical bonds of the products, then heat energy is released as the reaction goes forward.

Another contribution to the total energy is entropy, or disorder. If the heat energy difference between the reactants and product is very small, the reaction can go forward if the system becomes more disordered.

Think about the reaction above. Which has more order, reactants or product?

If the pressure increases, the volume must decrease (PV=nRT). How do you think this will affect the equilibrium between NO2 and N2O4?


copper powder or wire
test tubes with stoppers
septum caps
1 mL syringe
2 large beakers
temperature probe
hot plate
pressure probe

Activity procedure

Download the labsheet here.

Research question: NO2 is brown and N2O4 is colorless. How do temperature and pressure changes affect the position of the equilibrium between these gases? Can you use the color of the mixture to obtain the information?

When preparing samples of NO2/N2O4, add a small amount of copper powder on the end of a spatula to a test tube. Cover the metal with a few drops of the nitric acid solution. A brown gas should evolve. You can cap the test tube with a stopper or septum cap. You can transfer the NO2/N2O4 gas mixture with a syringe between test tube with septum caps.

Summary and discussion

1. At what temperature are there more molecules of NO2 in the tube?
    (a) 0 deg.
    (b) 25 deg. (c) 100 deg.

2. What causes the N-N bond to break?
    (a) heat energy
    (b) instability of the bond
    (c) pressure

3. Do all N-N bonds break at the same temperature?
    (a) yes
    (b) no

4. Please explain what happens to the gas molecules when we heat the tube?

5. Can N-N bonds form at the same temperature where N-N bonds break?
    (a) yes
    (b) no

6. Are there any N-N bonds forming at 100 degrees C?
    (a) yes
    (b) no

7. What do you think would happen if I added more NO2 to the tube at 100 degrees C (constant pressure)?
    (a) darker color
    (b) lighter color
    (c) no change in color

8. What do you think would happen if I added more NO2 to the tube at 100 degrees C (constant volume)?
    (a) darker color
    (b) lighter color
    (c) no change in color
9. Please explain your answer to the question above.

10. What can you tell me about the rate of N-N bond formation and N-N bond breaking?
at 0 deg. at 25 deg. at 100 deg
(a) bond making > bond breaking

(b) bond making < bond breaking

(c) bond making = bond breaking
(a) bond making > bond breaking

(b) bond making < bond breaking

(c) bond making = bond breaking
(a) bond making > bond breaking

(b) bond making < bond breaking

(c) bond making = bond breaking

EnLIST Chemistry Workshop, University of Illinois, 2010