Nitric acid is a colorless, highly corrosive mineral acid. It is widely used in the manufacture of fertilizers and explosives. Determining whether a molecule is polar or nonpolar requires a detailed look into its internal structure and the distribution of electrical charge. This analysis explains the molecular polarity of nitric acid.
Understanding Chemical Polarity
A molecule’s overall polarity is determined by two main factors: the polarity of its individual bonds and the geometric arrangement of those bonds. Bond polarity arises from a difference in electronegativity. When two atoms share electrons unequally, the electrons spend more time near the more electronegative atom, creating a partial negative charge (delta-) and leaving the less electronegative atom with a partial positive charge (delta+).
This charge separation creates a bond dipole moment. For a molecule to be polar overall, it must possess these polar bonds, and its three-dimensional structure must be asymmetrical. Even if a molecule contains highly polar bonds, the bond dipoles can cancel each other out if the molecular geometry is perfectly symmetrical, resulting in a net nonpolar molecule, such as carbon dioxide (\(\text{CO}_2\)).
Mapping the Nitric Acid Molecule (\(\text{HNO}_3\))
Analyzing nitric acid’s polarity requires establishing its molecular structure. In the \(\text{HNO}_3\) molecule, the central atom is Nitrogen (N), which is bonded to three separate Oxygen (O) atoms. One of the oxygen atoms is also bonded to the single Hydrogen (H) atom, forming a hydroxyl (\(\text{O-H}\)) group.
The nitrogen atom is surrounded by three regions of electron density, consisting of a single bond to the \(\text{O-H}\) group, a double bond to one oxygen atom, and a single bond to the third oxygen atom. This arrangement causes the nitrogen atom to adopt \(\text{sp}^2\) hybridization, leading to an electron domain geometry that is approximately trigonal planar. The molecule’s atoms lie in a single plane, but the presence of three different types of bonds around the central nitrogen introduces structural asymmetry.
Due to resonance, where electron density is delocalized between the nitrogen and the two oxygen atoms that are not part of the \(\text{O-H}\) group, the three \(\text{N-O}\) bonds are not equivalent in length or character. Furthermore, the presence of the hydrogen atom on only one side of the molecule disrupts any potential symmetry. The bond angles around the nitrogen atom deviate from the ideal \(120^\circ\) of a perfect trigonal planar shape. This distortion of the geometry is a direct indicator of structural asymmetry, which is a prerequisite for molecular polarity.
Determining the Net Molecular Dipole
To determine the net molecular dipole, the polarity of each bond must be examined using the difference in electronegativity between the bonded atoms. Oxygen is significantly more electronegative than Hydrogen, creating a highly polar \(\text{O-H}\) bond. Similarly, the \(\text{N-O}\) bonds are also polar, as Oxygen is more electronegative than Nitrogen.
Each of these polar bonds possesses a dipole moment, which is a vector pointing from the less electronegative atom toward the more electronegative atom. In the \(\text{HNO}_3\) structure, the dipoles from the three \(\text{N-O}\) bonds and the single \(\text{O-H}\) bond all point toward the highly electronegative oxygen atoms. Because the molecule is structurally asymmetrical, these individual bond dipoles do not cancel each other out.
The combination of the different bond types, the unequal sharing of electrons, and the non-symmetrical planar structure results in a significant net dipole moment for the entire molecule. This net dipole has been experimentally measured to be approximately \(2.17\) Debye (D). Since the net dipole moment is non-zero and substantial, nitric acid (\(\text{HNO}_3\)) is a highly polar molecule.
How Polarity Affects Nitric Acid’s Behavior
The high polarity of nitric acid is responsible for many of its chemical and physical behaviors. The principle of “like dissolves like” dictates that polar substances readily dissolve in other polar solvents. Consequently, \(\text{HNO}_3\) is completely miscible with water, which is itself a highly polar solvent.
Furthermore, the strong polarity of the \(\text{O-H}\) bond is what makes nitric acid a strong acid. The two oxygen atoms bonded directly to the nitrogen atom are highly electronegative and pull electron density away from the \(\text{O-H}\) bond.
This strong electron-withdrawing effect makes the hydrogen atom’s proton (\(\text{H}^+\)) susceptible to being cleaved by water molecules. The result is the rapid and complete dissociation of \(\text{HNO}_3\) in an aqueous solution to form the hydronium ion (\(\text{H}_3\text{O}^+\)) and the nitrate ion (\(\text{NO}_3^-\)).