In most natural systems available or "fixed" nitrogen is usually the limiting factor in plant growth. This realization led to the invention and massive use of nitrogen fertilizers during the 20th century and ever increasing crop yields per acre of farmed land. Without this use of nitrogenous fertilizers, the Earth could not support its current population of six billion people (Smil, 1997).
At the same time, the widespread use of fossil fuels releases not only carbon dioxide, but nitrogen oxides as well. These nitrogen oxides contribute to urban photochemical smog and acid precipitation. The combined effect of these two anthropogenic processes, agriculture and fossil fuel combustion, is similar in magnitude to natural nitrogen fixation. These substantial modifications to the global nitrogen cycle have important implications in a number of areas including photochemical smog, climate, stratospheric ozone, regional eutrophication, and ecosystem diversity, which will be discussed in this chapter.
Nitrogen has five valence electrons and can take on oxidation states between +5 and — 3. Most of the nitrogen compounds we will discuss either have nitrogen bonded to carbon and hydrogen, in which case the oxidation state of the nitrogen is negative (N is more electronegative than either C or H); or have nitrogen bonded to O, in which case the nitrogen has a positive oxidation state.
Table 12-1 lists the most common nitrogen compounds that exist in the natural world, by oxidation state. In addition it also lists the boiling point for each compound as well as its heat of formation (AH°(/)) and free energy of formation (AG°(/)). For comparison, the data on H20 are also included.
To fully understand some of the major players in the nitrogen cycle, we should also consider some of the industrial and social implications of these compounds.
1. HN03. Nitric acid is a very strong acid; about 6.8 million metric tons per year are manufactured for industrial purposes in the US. Most of it is produced from ammonia by the catalytic oxidation to NO, which is then further oxidized to N02. Addition of water forms HN03. Most of the nitric acid produced is used in the manufacture of fertilizers, and a lesser amount is used to make explosives.
In the troposphere, nitrogen oxides react to also produce HN03. The oxidants are free radicals produced photochemically, such as H02, ROz, and OH. The HN03 produced in this manner is an important contributor to "acid rain."
In its pure form, nitric acid is a liquid with a high vapor pressure (47.6 torr at 20°C), so that in the lower atmosphere HN03 exists as a gas, in an aerosol or in a cloud droplet. When nitric acid reacts with a base a nitrate salt is produced, ff
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