This news item was created by students Christie Comm, Brandi Decker, Hillaree Haberle, Kaley Todd and Michael Wunsch as part of their Chemistry 210 Semester Project in WS99 under the guidance of Prof. Rainer Glaser.

Glaser's "Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 4. The Study of Chemical Reactions



For each of the following questions, please refer to the following article:

NO LAUGHING MATTER: RIVER EMITTING NNO
by John Roach (Environmental News Network, January 12, 1999)


Editorial Comments

A mention of global warming and ozone depletion generally brings to mind two compounds: carbon dioxide (CO2) and chlorofluorocarbons (CFC's). Largely overlooked by the popular press, however, is the role of nitrous oxide. Nitrous oxide, or N2O, is a more powerful greenhouse gas than CO2 and a stronger contributor to ozone depletion than CFC's. Like CO2 and CFC's, its global emissions have been rising sharply. Industrial processes are partly responsible for this rise, but, in what is likely a surprise to many people, farming is the most significant contributor to the increased N2O emissions.

Modern agriculture makes heavy use of commercial nitrogen-based fertilizers. Though the fertilizers increase crop yields, they also lead to an increase in nitrous oxide emissions. Bacterial microorganisms convert nitrogen in the fertilizers into nitrous oxide through two chemical processes: nitrification and denitrification. When the amount of nitrogen present in the soil increases these processes accelerate. Thus, increased fertilizer use by farmers has resulted in an increased release of nitrous oxide into the environment. The nitrous oxide produced has been noted to reach rivers two ways. Nitrous oxide can either leach into the ground water that drains into the river or enter through agricultural runoff. In agricultural areas in which high quantities of fertilizers are applied, the amounts of nitrous oxide that reach the rivers can be significant. The Platte River system described in the article is one such example.

From the Platte River and other bodies of water, nitrous oxide is released into the atmosphere. Because nitrous oxide is a relatively stable compound, it is able to reach the stratosphere intact. Once it reaches this upper level of the atmosphere, nitrous oxide does not directly destroy ozone, but a molecule derived from it does. Although N2O is stable, it is susceptible to photolysis. Ultraviolet radiation can degrade the molecule, yielding a combination of N and O or NO and N. The formation of NO leads to a radical chain reaction in which the NO reacts with ozone to eventually produce two O2 molecules and a new copy of NO. In effect, N2O acts as a catalyst that converts O3 to O2. The radical chain reaction only ends when the NO radical is degraded into N and O and the N and O react with other free oxygen and nitrogen molecules to form N2 and O2.

Nitrous oxide, however, does more than just destroy ozone in the stratosphere; it is a key contributor to global warming. Photolysis does not typically occur as soon as N2O arrives in the upper atmosphere. The average nitrous oxide molecule has a life span of 140 to 190 years before it is degraded. During this period before photolysis these molecules contribute to global warming. Like other greenhouse gases, nitrous oxide has a tremendous heat trapping ability. In fact, nitrous oxide is 200 to 300 times more effective than carbon dioxide.

In addition, one should not overlook the other sources of nitrous oxide including waste water treatment plants, combustion engines, and biomass burning. All of these sources contribute to the unacknowledged N2O as a primary factor in ozone depletion and global warming. Considering the impact that N2O plays in our environment, it should be a major concern, especially since nitrous oxide emissions have risen sharply in the past century.

Questions

1. What are the main sources of N2O in the Platte River?

A: Wastewater treatment plants and agriculture.

2. What are the principle effects of N
2O in the upper atmosphere?

A: N
20 contributes to ozone depletion and global warming.

3. What two chemical processes are responsible for the N
2O accumulation in the Platte River System? Identify the nitrogen reactant, nitrogen intermediates and nitrogen products of the reactions involved, and note where N2O appears. Hint: check the links provided in the editorial comments.

A: The two chemical processes responsible for the nitrous oxide accumulation in the Platte River are nitrification and denitrification.
I. Nitrification



General reaction: NH4+   ---->   NO2- + energy   ---->   NO3- + energy
Nitrogen reactant:   NH
4+
Nitrogen intermediate:   NO
2-
Nitrogen product:   NO
3-
II. Denitrification



General reaction: 2NO3- + 4 e-   -->   2NO2- + 4 e-   -->   N20 + 2 e-   -->   N2
Nitrogen reactant:   NO
3-
Nitrogen intermediates:   NO
2-, N2O
Nitrogen product: N
2
N
20 appears as an intermediate product during denitrification.


4. What is the radical chain reaction involving N2O that causes ozone depletion? Write initiation and propagation steps. Note: to answer this question, you should check the link to 1995 Nobel Prize in Chemistry.

A:
Initiation step:

photolysis of N2O
N20 + uv-light   ---->   NO + N
Propagation steps:


NO + O3   ---->   NO2 + O2
NO2 + O   ---->   NO + O2
O3 + uv-light   ---->   O2 + O
Net Reaction: 2O3     ---->     3O2

5. Should our knowledge of the contribution of agricultural fertilizers to ozone depletion and global warming change our current usage of fertilizers?

A: you decide.