Is NH3 (Ammonia) an Acid or a Base?

Is NH3 an Acid or Base

NH3 also known as Ammonia is a pungent-smelling gas compound which comprises of 1 atom of Nitrogen and 3 atoms of Hydrogen. Ammonia has a low boiling temperature at -33 degrees celsius and is lighter than air. Often students wonder about whether NH3 is an acid or base. In this article, you’ll get to know about the acidity and basicity of NH3 in detail.

So, is NH3 an acid or base? NH3 is a weak base with pH 11 ( at standard conditions) but it is also considered amphoteric which means it can act as both acid and base under different conditions. NH3 under suitable condition act as a weak base and accepts H+ and forms its conjugate acid NH4+ and under different condition NH3 will act as an extremely weak acid and give away H+ ion to form its conjugate base NH2-. However, according to Lewis’s theory of acids and bases, NH3 due to the presence of a lone pair of electrons is considered as a Lewis base.

What is NH3 (Ammonia)?

NH3

NH3 (Ammonia) is a non-flammable colorless gas that is lighter than the air. It has a very strong bad odor and considered a pungent-smelling gas due to its production by bacterial decomposition of urea. It is a highly toxic gas that can cause lung damage or even death due to prolonged exposure to Ammonia.

In industries, Ammonia is generally used for the preparation of fertilizers, disinfectant chemicals, refrigerants and many other nitrogen-based organic and inorganic compounds.

Ammonia’s chemical formula is NH3 and has a trigonal pyramidal shape with a Nitrogen atom on the pyramid top and 3 hydrogen atoms at the 3 base corners.  The atomic number of Nitrogen is 7 and 5 electrons in its valence shell. This means that after the formation of 3 bonds with Hydrogen, Nitrogen carries a lone pair of electron.

The molar mass of NH3 is around 17.03g and has a bond angle of 107.5 degrees which is slightly less than that in tetrahedral (109.5 degrees). The lone pair provides some repulsion to bonds due to which the angle is slightly less than the tetrahedral.

 

Is NH3 Acidic or Basic?

As explained earlier, although NH3 is a weak base and standard solution of ammonia has a pH 11 but still, it is amphoteric in nature which means it can act as both acid as well as a base under different conditions.

When NH3 acts as a base, it will donate its lone pair to a proton H+ and form its conjugate acid NH4+ whereas when NH3 acts as an acid, it can give out H+ ion and forms a conjugate base as NH2-.

Reactions are given below:

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

(Acting as a Lewis Acid)      NH3                      ——–>  NH2-     +    H+

 

Why NH3 acts as a Lewis Base?

Lewis’s definition of Base is that any chemical compound that has the ability to donate lone pairs to other chemical species can act as a lewis base. We know that, in NH3, Nitrogen(N) has 5 electrons in its valence shell with the configuration of (1s2, 2s2, sp3) and Hydrogen has only 1 electron with the configuration of (1s1).

NH3 is sp3 hybridized and it shares 3 (sp3) hybridized electrons to form bonds with 1s electrons of all 3 hydrogen atoms and result of which 1 (sp3) electron pair on N remain unshared.

It means the Nitrogen atom is left with one lone pair of electrons, which it can donate to a proton in a suitable medium and thus NH3 can act as a Lewis base.

Let us take an example of NH3 reacting with H+ ion. When NH3 reacts with H+ ion, it donates its lone pair to H+ ion and forms NH4+ as conjugate acid.

 

Chemical Properties of NH3 (Ammonia)

Structure

NH3 Structure

According to the VSEPR theory, the chemical structure of Ammonia is trigonal pyramidal with bond angles of 107.5 degrees. Nitrogen has one lone pair of electrons left after sharing 3 electrons to form bonds with 3 hydrogen atoms.

This trigonal pyramidal shape provides the molecule with a dipole moment and makes it a polar molecule. Due to the presence of a lone pair, it has the ability to form hydrogen bonds in water.

 

Amphoteric Nature

As explained earlier, NH3 is a weak base and reacts with acids to form salts. Although NH3 is a weak base, it also acts as a weak acid under certain conditions and reacts with bases. It can lose H+ ion and form Amides (NH2-). One of the examples of such reaction is when Lithium reacts with NH3 to form Lithium Amide.

(NH3 acting as a weak acid)      Li + NH3 ——-> LiNH2 + H2

 

Redox Reaction (Self Dissociation)

NH3 also perform self dissociation and carries out redox reaction under certain conditions. Below is the reaction where NH3 forms its conjugate acid and conjugate base simultaneously.

NH3 —–> NH4+   +   NH2-

 

Combustion

NH3 undergoes exothermic combustion to produce Nitrogen gas and water vapor. Following is the combustion reaction of NH3.

NH3 + O2    ——->  N2 + H2O    ( enthalpy change of this reaction is −1267.20 kJ/mol)

Although Nitrogen oxides are unstable with respect to N2 still we can form Nitrogen oxides with the help of some catalysts. Example of such a reaction is:

NH3  +   O2   ——–>    NO + H2O

The combustion of NH3 is always done in the presence of a catalyst due to the high heat of vaporization and ignition temperature. Platinum gauze is one of the catalysts used in the NH3 combustion reaction.

 

NH3 Acts as a Weak Base in Water

When put in water, NH3 readily mixes with water due to its polar nature and ability to form hydrogen bonds in water. It helps in the dissociation of H2O molecules in (Hydrogen ions) H+ and (Hydroxyl ions) OH- ions and forms bonding with H+ ions.

NH3 on combining with H+ ions form NH4+ and leaves OH- ions in the solution. Since OH- ion concentration increases, it results in the increase in pH and hence imparts basicity to the solution.

Also, the formed ammonium ion (NH4+) keeps breaking into NH3 and H+ ion and hence not all ammonia results in the formation of OH- ions and thus NH3 is known as a weak base.

 

References:

University of UTAH

California State University

University of Colorado Boulder

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