Is ammonia (NH3) an acid or base?

Ammonia, often known as NH3, is a gaseous chemical composed of 1 atom of Nitrogen and 3 atoms of Hydrogen. The boiling point of ammonia is -33 degrees Celsius, and it is lighter than air. Students frequently question whether NH3 is an acid or a basic. This article provides comprehensive information on the acidity and basicity of NH3.

So, is NH3 a basic or an acid? NH3 is a weak base with a pH of 11 (at typical conditions), but it is also amphoteric, meaning it can act as both an acid and a base depending on the circumstances. Under favourable conditions, NH3 acts as a weak base and takes H+ to generate its conjugate acid NH4+. However, under different conditions, NH3 acts as a weak acid and gives up H+ ion to make its conjugate base NH2-. However, according to Lewis’s theory of acids and bases, NH3 is regarded a Lewis base due to the presence of a lone pair of electrons.

What is ammonia (NH3)?

Ammonia (NH3) is a noncombustible, odourless gas that is lighter than air. Produced by the bacterial breakdown of urea, it has a very strong foul stench and is regarded a gas with a pungent odour. Ammonia is an extremely hazardous gas that, if extended exposure, can cause lung damage or even death.

Ammonia is typically utilised in industrial settings for the production of fertilisers, disinfection chemicals, refrigerants, and a wide variety of other nitrogen-based organic and inorganic compounds.

The chemical formula for ammonia is NH3 and it has a trigonal pyramidal shape with a nitrogen atom at the top and three hydrogen atoms at the pyramid’s three base corners. Nitrogen has 7 protons, 7 neutrons, and 5 electrons in its valence shell. Thus, after the establishment of three bonds with Hydrogen, Nitrogen carries a pair of electrons by itself.

NH3 has a molar mass of approximately 17,03 g and a bond angle of 107.5 degrees, which is marginally less than that of tetrahedral nitrogen (109.5 degrees). Due to the lone pair’s repulsion of bonds, the angle is slightly less than that of the tetrahedral.

Is NH3 an acid or a base?

As previously explained, although NH3 is a weak base and a normal solution of ammonia has a pH of 11, it is amphoteric, meaning that it may act as both an acid and a base depending on the circumstances.

When NH3 works as a base, it donates its lone pair to a proton H+ and creates the conjugate acid NH4+. However, when NH3 acts as an acid, it can release H+ ion and form a conjugate base as NH2-.

Listed below are responses:

(Performing as Lewis Base) NH3 + H+ ——-> NH4+

(Exercising the properties of a Lewis acid) NH3 ——–> NH2- + H+

Why does NH3 behave like a Lewis Base?

Any chemical substance that can give lone pairs to other chemical species can function as a Lewis base, according Lewis’s definition. Nitrogen(N) in NH3 contains 5 electrons in its valence shell with the configuration (1s2, 2s2, sp3), but Hydrogen has just 1 electron with the configuration (1s2, 2s2, sp3) (1s1).

NH3 is sp3 hybridised, and it shares 3 (sp3) hybridised electrons to form bonds with the 1s electrons of all three hydrogen atoms, leaving one (sp3) electron pair on the nitrogen unshared.

It indicates that the Nitrogen atom is left with a single pair of electrons, which it can transfer to a proton in an appropriate media, allowing NH3 to function as a Lewis base.

Consider an instance of NH3 interacting with H+ ion. When NH3 interacts with H+, it gives its lone pair to H+ and creates the conjugate acid NH4+.

NH3 Chemical Properties (Ammonia)

Structure

According to the VSEPR theory, ammonia has a trigonal pyramidal chemical structure with bond angles of 107.5 degrees. Nitrogen has one pair of electrons remaining after sharing three electrons with three hydrogen atoms to create bonds.

This trigonal pyramidal shape imparts a dipole potential to the molecule, transforming it into a polar molecule. The presence of a lone pair enables the formation of hydrogen bonds in water.

Amphoteric Character

As previously explained, NH3 is a weak base that forms salts when combined with acids. Although NH3 is a weak base, under some conditions it also works as a weak acid and interacts with bases. It is able to lose H+ and produce Amides (NH2-). One example of such a reaction is the formation of lithium amide when lithium combines with NH3.

(NH3 functioning as a mild acid) Li + NH3 ——-> LiNH2 + H2

Redox Response (Self Dissociation)

NH3 is capable of self-dissociation and redox reactions under specific conditions. Below is the reaction in which NH3 simultaneously creates its conjugate acid and conjugate base.

NH3 —–> NH4+ + NH2-

Combustion

NH3 is combusted exothermically to produce Nitrogen gas and water vapour. The NH3 combustion reaction is as follows.

The enthalpy change for the reaction between NH3 and O2 is 1267.20 kJ per mole.

Despite the fact that Nitrogen oxides are unstable in comparison to N2, they can be produced with the aid of catalysts. Exemplification of such a response:

NH3 + O2 ——–> NO + H2O

Due to the high heat of vaporisation and ignition temperature, NH3 combustion is always conducted in the presence of a catalyst. One of the catalysts used in the NH3 combustion reaction is platinum gauze.

In water, NH3 acts as a Weak Base.

Due to its polarity and capacity to create hydrogen bonds in water, NH3 quickly mixes with water when introduced to it. It assists in the breakdown of H2O molecules into H+ and OH- ions and bonds with H+ ions.

NH3 combines with H+ ions to produce NH4+ and OH- ions in a solution. Since the concentration of OH- ions increases, the pH rises, which imparts basicity to the solution.

In addition, the generated ammonium ion (NH4+) is constantly decomposing into NH3 and H+ ion; hence, not all ammonia results in the creation of OH- ions, and NH3 is considered a weak base.

Read more: Structure, Molecular Geometry, and Hybridization of PF3 Lewis

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.

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