Nitrous acid (\(\text{HNO}_2\)), also known as HONO, is a chemical compound frequently encountered in atmospheric and aquatic chemistry. Determining whether the molecule is polar or nonpolar requires a detailed examination of its internal bonding structure and its three-dimensional molecular shape. This analysis confirms that nitrous acid is, in fact, a polar molecule due to the unequal distribution of electron density across its atoms.
What Makes a Molecule Polar?
Molecular polarity is determined by two factors: the polarity of its individual chemical bonds and the overall shape of the molecule. Bond polarity arises from a difference in electronegativity, which measures an atom’s ability to attract electrons within a bond. When two atoms form a covalent bond and have significantly different electronegativities, the electron pair is pulled closer to the more electronegative atom, creating a bond dipole. This unequal sharing results in one end of the bond having a slight negative charge and the other having a slight positive charge. A molecule consisting of polar bonds is not automatically polar, however. The overall molecular polarity, represented by the net dipole moment, depends on the spatial arrangement of these individual bond dipoles. If the dipoles are oriented symmetrically, they can effectively cancel each other out, resulting in a nonpolar molecule. If the arrangement is asymmetrical, a net dipole moment remains, making the molecule polar.
The Molecular Geometry of Nitrous Acid (\(\text{HNO}_2\))
To determine the overall molecular polarity, the specific three-dimensional arrangement of atoms in nitrous acid must be established. The structure of \(\text{HNO}_2\) is represented as \(\text{H-O-N=O}\), with the nitrogen atom sitting at the center. Nitrogen has three areas of electron density surrounding it: a single bond to one oxygen, a double bond to the other oxygen, and one non-bonding lone pair of electrons. This arrangement dictates the electron geometry around the central nitrogen atom as trigonal planar. However, the presence of the lone pair significantly influences the final molecular shape. The lone pair exerts a stronger repulsive force on the bonding electron pairs, pushing the bonded atoms away. This results in a bent or angular molecular geometry for both the \(\text{H-O-N}\) and \(\text{O-N=O}\) portions of the molecule.
Determining the Net Dipole Moment
The bent structure of nitrous acid is the key factor in determining its polarity because it prevents molecular symmetry. Analyzing the bonds confirms that Oxygen (3.44) is significantly more electronegative than both Nitrogen (3.04) and Hydrogen (2.20), creating strong individual bond dipoles. The \(\text{O-H}\) bond is highly polar (electronegativity difference of \(1.24\)), pulling electrons toward the oxygen atom. The \(\text{N-O}\) and \(\text{N=O}\) bonds are also polar (difference of \(0.40\)), pulling electrons toward their respective oxygen atoms. Because of the bent, asymmetric shape of the \(\text{HNO}_2\) molecule, these multiple bond dipoles do not point in opposite directions where they could cancel out. Instead, they sum together, creating a measurable net dipole moment for the entire molecule. This confirms that nitrous acid is a polar compound with an unequal distribution of electron density.
Practical Implications of Nitrous Acid Polarity
The polarity of nitrous acid has direct consequences for its chemical behavior, particularly regarding solubility. The principle of “like dissolves like” dictates that polar substances readily mix with other polar substances. Because \(\text{HNO}_2\) is polar, it exhibits high solubility in polar solvents, most notably water. In an aqueous solution, polar \(\text{HNO}_2\) molecules form strong attractive forces, specifically hydrogen bonds, with polar water molecules (\(\text{H}_2\text{O}\)). The polarity also contributes to its function as a weak acid in water, where it partially dissociates into the nitrite ion (\(\text{NO}_2^-\)) and a hydrogen ion (\(\text{H}^+\)). Conversely, nitrous acid is poorly soluble in nonpolar solvents, such as hexane or carbon tetrachloride, because the strong intermolecular forces cannot be established.