18.7 Occurrence, Preparation, and Properties of Nitrogen

Occurrence, Preparation, and Properties of Nitrogen

Nitrogen is the most abundant element in Earth's atmosphere and a building block of proteins, nucleic acids, and countless industrial chemicals. Understanding how nitrogen occurs in nature, how it's converted into useful compounds, and how its oxides and oxyacids behave gives you a foundation for topics ranging from biochemistry to environmental chemistry.

Occurrence and Properties of Nitrogen

Nitrogen makes up about 78% of Earth's atmosphere by volume, making it by far the most abundant atmospheric gas. It exists as a diatomic molecule ($N_2$), where the two nitrogen atoms are held together by a very strong triple covalent bond. That triple bond is one of the strongest in all of chemistry, which is why $N_2$ is so unreactive under normal conditions.

At room temperature and pressure, nitrogen is a colorless, odorless, and tasteless gas. A few other physical properties worth knowing:

• Low solubility in water: only about 20 mg/L at 20°C
• Boiling point: $-196°C$ (liquid nitrogen is widely used as a coolant)
• Freezing point: $-210°C$

Despite being relatively inert as $N_2$, nitrogen is essential for life. It's a key element in amino acids (the building blocks of proteins) and in nucleic acids like DNA and RNA. The challenge is that most organisms can't use $N_2$ directly from the air, so it has to be "fixed" into other compounds first.

Nitrogen Fixation Processes

Nitrogen fixation is the conversion of atmospheric $N_2$ into nitrogen-containing compounds that plants and animals can actually use, such as ammonia ($NH_3$) and nitrates. There are two main pathways:

Biological nitrogen fixation:

1. Certain bacteria called nitrogen-fixing bacteria (most notably Rhizobium) live in the root nodules of legumes like soybeans and alfalfa.
2. These bacteria use an enzyme called nitrogenase to break the triple bond in $N_2$ and convert it into ammonia ($NH_3$).
3. The ammonia is then incorporated into organic molecules that plants can use for growth.

Industrial nitrogen fixation (the Haber-Bosch process):

1. $N_2$ and $H_2$ are combined at high temperatures (400–500°C) and high pressures (200–300 atm) in the presence of an iron catalyst.
2. The balanced equation is: $N_2 + 3H_2 \rightleftharpoons 2NH_3$
3. The double arrow indicates this is a reversible, equilibrium reaction, so conditions must be carefully controlled to maximize ammonia yield.

The Haber-Bosch process is one of the most important industrial reactions ever developed. The ammonia it produces is the starting material for nitrogen-based fertilizers, which support a huge portion of global agriculture. It's also the gateway to other industrial chemicals, including nitric acid and nylon.

Both biological and industrial fixation feed into the nitrogen cycle, the biogeochemical process that moves nitrogen between the atmosphere, soil, water, and living organisms.

Nitrogen Oxides vs. Oxyacids

Nitrogen forms a variety of compounds with oxygen. These fall into two categories: nitrogen oxides (binary compounds of N and O) and oxyacids (acids containing N, O, and H).

Nitrogen oxides:

• Nitrous oxide ($N_2O$): A linear molecule commonly known as "laughing gas." It's used as an anesthetic in dentistry and as a propellant in whipped cream cans.
• Nitric oxide ($NO$): An odd-electron molecule (it has an unpaired electron, making it a free radical). In biology, $NO$ acts as a signaling molecule that causes vasodilation (widening of blood vessels).
• Nitrogen dioxide ($NO_2$): A bent molecule and a reddish-brown, toxic gas. It's a major air pollutant and contributes to smog and acid rain.

Oxyacids of nitrogen:

• Nitrous acid ($HNO_2$): A weak acid that forms nitrite salts. It's unstable and decomposes: $3HNO_2 \rightarrow HNO_3 + 2NO + H_2O$
• Nitric acid ($HNO_3$): A strong acid that forms nitrate salts. It's produced industrially by the Ostwald process, which oxidizes ammonia. Nitric acid is a powerful oxidizing agent; when it reacts with metals, it typically produces nitrogen oxides (like $NO_2$) rather than hydrogen gas, which sets it apart from most other acids.

Key reactions to know:

• Nitrogen dioxide can dimerize (two molecules join together) to form dinitrogen tetroxide: $2NO_2 \rightleftharpoons N_2O_4$
• Nitric oxide reacts with oxygen to form nitrogen dioxide: $2NO + O_2 \rightarrow 2NO_2$

Bonding and Molecular Structure of Nitrogen Compounds

Nitrogen has five valence electrons, so it needs three more to complete an octet. This means nitrogen commonly forms three covalent bonds and retains one lone pair of electrons.

The triple bond in $N_2$ has a bond energy of about 946 kJ/mol, making it one of the strongest bonds in chemistry. This is exactly why nitrogen fixation requires either specialized enzymes (nitrogenase) or extreme industrial conditions (high temperature and pressure) to break that bond apart.

Nitrogen's ability to form single, double, and triple bonds with other elements gives rise to the wide variety of nitrogen compounds you've seen in this section, from ammonia (single bonds) to nitric acid (a mix of single and double bonds) to $N_2$ itself (triple bond). That versatility is what makes nitrogen so central to both biology and industry.