Chapter 13: Nitrogen

Nitrogen gas (N2) is highly unreactive, making up about 78% of the Earth's atmosphere, due to the presence of a very strong triple covalent bond between the two nitrogen atoms, requiring an exceptionally high activation energy to break. This inert nature is overcome naturally during thunderstorms, where lightning provides the energy to oxidize nitrogen gas into gaseous nitrogen(II) oxide and subsequently nitrogen(IV) oxide, which ultimately dissolve to form dilute nitric acid, serving as a soluble source of nitrate ions (NO3 minus) for plant proteins. A crucial nitrogen compound is ammonia (NH3), an alkaline gas with a triangular pyramidal molecular shape stemming from a lone pair of electrons on the nitrogen atom. Ammonia functions as a Brønsted-Lowry base because it can donate its lone pair to accept a proton (H plus ion), resulting in the formation of the ammonium ion (NH4 plus). Since ammonia is a weak base, an equilibrium mixture is established in aqueous solutions, producing a lesser concentration of hydroxide ions compared to strong bases. Ammonium compounds, such as ammonium nitrate and ammonium sulfate, are widely used as water-soluble, essential fertilizers; however, excess use leads to environmental issues, including nitrates leaching into watercourses, causing eutrophication—a process where excessive plant and algae growth depletes dissolved oxygen, leading to the death of aquatic life. Nitrogen oxides (NOx) are also primary atmospheric pollutants generated by human activity, notably through the high-temperature combustion within internal combustion engines, and they contribute significantly to acid rain by forming nitric acid directly, and by catalytically oxidizing sulfur dioxide into sulfur trioxide, which then forms sulfuric acid. Furthermore, these oxides participate in photochemical reactions driven by sunlight, combining with volatile organic compounds (VOCs, such as unburnt hydrocarbons) to produce photochemical smog, characterized by harmful secondary pollutants like peroxyacetyl nitrate (PAN). To mitigate this pollution, modern vehicle exhaust systems are fitted with catalytic converters that reduce harmful nitrogen oxides into harmless nitrogen gas and simultaneously oxidize other pollutants like carbon monoxide.