Ammonia is an inorganic compound of nitrogen and hydrogen, with the chemical formula NH₃. At ordinary room temperature and atmospheric pressure, it is a colorless gas with a sharp, pungent odor. It occurs naturally and is manufactured on a large scale, principally as a raw material for nitrogen fertilizers. Its chemistry connects agricultural production, biological nitrogen transformations, industrial synthesis, and refrigeration. Pure ammonia is commonly called anhydrous ammonia, distinguishing it from solutions of ammonia in water. (cdc.gov)
Molecular structure and physical properties
An ammonia molecule contains one nitrogen atom joined to three hydrogen atoms by polar covalent bonds. Its Lewis structure includes one nonbonding pair of electrons on nitrogen. The four electron domains have an approximately tetrahedral arrangement, but the positions of the atoms define a trigonal pyramid rather than a flat triangle. The lone pair compresses the hydrogen–nitrogen–hydrogen bond angles below the ideal tetrahedral angle of 109.5°. Because the bond dipoles do not cancel, ammonia is polar. (openstax.org)
Ammonia has a molar mass of approximately 17.0 grams per mole. At atmospheric pressure, it boils at about −33 °C and freezes near −78 °C. It can therefore be maintained as a liquid either by refrigeration or by applying sufficient pressure, and is commonly transported as a liquefied compressed gas. It dissolves readily in water, producing solutions whose properties depend on concentration and temperature. (cdc.gov)
Acid–base and coordination chemistry
Ammonia is a weak base: it accepts a proton from water through an acid–base reaction, establishing the equilibrium
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).
The product NH₄⁺ is the ammonium ion, which is chemically distinct from neutral ammonia. Formation of hydroxide ions makes the solution alkaline. “Weak” describes the limited extent of reaction with water, not the absence of chemical hazards. The equation represents a reversible reaction rather than complete conversion of dissolved ammonia into ammonium. (assets.openstax.org)
The ammonia–ammonium equilibrium is important in both laboratory and environmental chemistry. Increasing pH increases the proportion present as uncharged NH₃. Mixtures containing ammonia and an ammonium salt can form a basic buffer: ammonia consumes added acid, while ammonium reacts with added hydroxide, limiting changes in pH. (epa.gov)
Nitrogen’s lone pair also enables ammonia to act as a Lewis base and a ligand. It donates an electron pair to a metal center, forming a coordination complex. For example, aqueous ammonia can promote the dissolution of silver chloride by forming a soluble silver–ammonia complex. Ammonia ligands are conventionally called ammine ligands, a spelling that distinguishes them from organic amines. (openstax.org)
Industrial synthesis
The principal industrial manufacturing route is the Haber–Bosch process, in which nitrogen and hydrogen react:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
Fritz Haber developed the laboratory synthesis, and Carl Bosch and industrial collaborators developed the high-pressure equipment required for large-scale production around 1913. The process combines elevated pressure, elevated temperature, catalysis, and circulation of unreacted gases. Iron was central to the development of industrial ammonia catalysts. (nobelprize.org)
Industrial hydrogen production has historically depended heavily on fossil resources. The International Energy Agency’s 2021 roadmap reported that just over 70% of ammonia production used natural gas-based steam reforming, with most of the remainder using coal gasification. These upstream processes account for substantial energy demand and carbon dioxide emissions. The emissions associated with ammonia therefore depend not only on the synthesis reaction but also on how its hydrogen and process energy are supplied. (iea.org)
Uses
Ammonia’s largest application is fertilizer production. The IEA’s 2021 assessment attributed approximately 70% of ammonia use to fertilizers. It may be applied directly as anhydrous ammonia or converted into products such as urea, ammonium sulfate, and ammonium phosphates. These products supply nitrogen in forms available to crops. Ammonia is also a feedstock for nitric acid and numerous other industrial chemicals, including materials used in plastics, synthetic fibers, and explosives. (iea.org)
Other established uses include industrial refrigeration, where ammonia is designated refrigerant R717, and cleaning products containing dilute aqueous solutions. Refrigeration applications include food processing, ice production, and cold storage. Its usefulness in these settings coexists with toxicity and material-compatibility constraints. (cdc.gov)
Natural occurrence and environmental effects
Ammonia and ammonium are intermediates in the nitrogen cycle. Nitrogen fixation introduces atmospheric nitrogen into biologically usable compounds, while microbial decomposition releases ammoniacal nitrogen from organic matter. In oxygenated environments, nitrification converts ammonia or ammonium first into nitrite and then nitrate. This oxidation consumes dissolved oxygen and can contribute to oxygen depletion in receiving waters. (epa.gov)
Aquatic ammonia can originate from animal wastes, municipal effluent, agricultural runoff, and atmospheric deposition. Its toxicity depends on concentration, exposure duration, pH, temperature, and organism sensitivity. Higher pH increases the un-ionized NH₃ fraction, an important determinant of toxicity. As a nitrogen nutrient, ammonia can also contribute to eutrophication, which is distinct from its direct toxic effects. (epa.gov)
Hazards and energy applications
Concentrated ammonia can damage the eyes, skin, and respiratory tract; severe inhalation exposure can injure the lungs. Liquefied ammonia also presents a frostbite hazard. Although not readily ignited under many ordinary conditions, ammonia can form flammable mixtures with air. (cdc.gov)
Ammonia is being investigated as a fuel and hydrogen carrier. Lower-emission production routes include hydrogen obtained through water electrolysis powered by renewable energy, and fossil-based production coupled with carbon capture and storage. Ammonia contains no carbon, but its overall environmental performance depends on production emissions, conversion efficiency, and emissions during use; combustion can generate nitrogen oxides. (iea.org)