Determining the color of nitrogen dioxide through inquiry: a demonstration of LeChatlier's principle.

Lisa Hines, Tech High School, St. Cloud, MN, based on the original activity provided by Major Dan Brannan, USAFA Department of Chemistry.
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Summary

In this inquiry activity, students observe the effects of temperature change on the color of nitrogen dioxide and dinitrogen tetroxide by manipulating glass tubes containing the gases at equilibrium. Students are challenged to figure out how to impose the stresses of temperature change on their gas tubes using a limited set of provided equipment. Given only that two nitrogen dioxide molecules can combine to make one dinitrogen tetroxide molecule and the reaction is exothermic, students are asked to write a balanced equation for the reaction and correctly determine the color of nitrogen dioxide gas.

Learning Goals

1. Correctly identify the color of nitrogen dioxide gas through an inquiry approach to problem solving.
2. Write a balanced equation for the reaction observed.
3. Introduce the concepts of equilibrium reactions, Le Chatlier's principle, endothermic and exothermic reactions.

Context for Use

This activity could be used as a demonstration with younger students. It would work well as an anticipatory set to get students thinking about the chemistry behind the reactions observed. As an inquiry lab activity, it would work well with older, upper-level students who have had at least one semester of lab experience.
The availability of NO2/N2O4 gas tubes will dictate the logistics of the lab and how many groups you have working at one time. The activity can easily be completed in 20-25 minutes, leaving ample time for reflection and analysis.
This activity fits nicely with a discussion of gases, air quality, equilibrium reactions, Le Chatlier's principle, or concepts of endothermic and exothermic reactions.
I would use it as an example of Le Chatlier's principle in chemistry and as an extension in a discussion of air pollution in Environmental science.

Description and Teaching Materials

Inquiry Lab: What Color is Nitrogen Dioxide?

Purpose: In this lab, you will use the lab skills you have acquired to manipulate the temperature variable in a chemical reaction for the purpose of determining the color of nitrogen dioxide gas, a common air pollutant.

Background: Dinitrogen tetroxide and nitrogen dioxide are common nitrogen compounds found in polluted air. Dinitrogen tetroxide can come apart producing two molecules of nitrogen dioxide. The reaction is endothermic. One of these gases is reddish-brown, and one is colorless.

Safety: Use caution around the hot plate. Remember that heated glass looks just like cool glass. Nitrogen dioxide can be converted to nitric acid in the presence of water. Therefore, if a bulb (test tube) containing NO2 should break, it is important to avoid inhaling the gas. In the event of an accident, let the instructor know immediately. Work under proper ventilation and wear your goggles.

Materials:
3 test tube or sealed bulbs containing NO2/N2O4
Wooden test tube rack
2 beakers large enough to submerge bulb/test tube
Ice
Hot plate

Procedure:
1. Clearly state your hypothesis, including a brief explanation to support your statement.
2. Write a balanced equation for the reaction described.
3. Observe and record the color of the gas tubes at room temperature.
4. Observe and record the color of the gas tubes at a higher temperature.
5. Observe and record the color of the gas tubes at a lower temperature

Analysis
Correctly identify the color of nitrogen dioxide. Read the section in your text on Le Chatlier's principle. Use the information from your text and from lecture to explain the chemistry behind your observations and answer.

Closure:
How would temperature affect air pollution? Find a related article to discuss in class.

Reference:
"Mini Lab 10: What Color is Nitrogen Dioxide," Major Dan Brannan, USAFA Department of Chemistry. Background Information and bibliography (Microsoft Word 31kB Aug5 08)

Teaching Notes and Tips

Nitrogen dioxide gas tubes may be available from scientific supply companies, but it may be just as easy to generate the Nitrogen dioxide gas in a test tube by reacting concentrated nitric acid and copper powder. This reaction should be done under a vent hood or other well ventilated area. Add copper powder to the nitric acid in a test tube. Evolution of nitrogen dioxide is evident by the production of brown gas. Collect the gas in a second test tube using a single whole stopper apparatus and rubber tubing. Since nitrogen dioxide is denser than air, it fills the test tube from the bottom up. Place a solid stopper on the collection test tube when the NO2 gas approaches the top.
I would recommend heating the test tube in a water bath. Angle the test tube away from others in case the stopper should pop off as the gases expand and pressure increases during heating. Protective eyewear should be worn. Nitrogen dioxide can be converted to nitric acid when it comes in contact with water. If a test tube breaks or NO2 gas is released, inform students that every effort should be made to prevent inhaling the gas.
It may be possible to collect the gas in a ziploc plastic bag, which could then be placed on ice for testing the effects of cooler temperatures. Collection in a plastic bag would also reduce the chances of students breaking test tubes.
The manufactured glass, gas tubes are reusable; therefore, no disposal is required. If producing and collecting your own NO2 gas, the gas would need to be converted to nitric acid by adding water to the test tube (wear gloves) and then neutralizing the acid with a base. Once the nitric acid is neutralized, it can be safely washed down the sink.
In the past, I have done an acid rain lab in a plastic ziploc bag, where NO2 is produced and then combines with distilled water to produce nitric acid. The acidic solution is then tested to determine its characteristics. This lab would be a nice addition to create a pair of labs dealing with the chemistry of air pollution. This lab would also provide the opportunity for more of an inquiry approach.

Assessment

Once students have been taught the principles of equilibrium reactions, Le Chatlier's principle, and the concepts of endothermic and exothermic reactions, they would be asked to make a journal entry in their lab notebooks explaining the chemistry behind the color change observed in this activity. The journal entry would be scored by peers using a provided rubric. Notebooks would then be collected and scores recorded. I highly recommend spot checking a few of the notebooks to ensure accuracy in peer grading.

Standards

IIB1, IIB4

References and Resources