Hydrogen chloride

Hydrogen chloride, or HCl, is a hydrogen halide compound. At room temperature, it exists as a colorless gas. When it meets water vapor in the air, it produces white fumes of hydrochloric acid. Both the gas and its water-based solution, hydrochloric acid, play key roles in technology and industry. Hydrochloric acid is often written simply as HCl as well.

The HCl molecule is diatomic, with a hydrogen and a chlorine atom joined by a polar covalent bond. Because chlorine is much more electronegative than hydrogen, the bond is polar, giving the molecule a large dipole moment—a partial negative charge on the chlorine and a partial positive charge on the hydrogen. This high polarity makes HCl very soluble in water and other polar solvents. When HCl meets water, it forms hydronium cations and chloride anions, creating hydrochloric acid, a strong acid. Its acid dissociation constant is large, meaning it fully ionizes in water. HCl also ionizes in solvents like methanol, forming a similar cation and chloride. Without such polar solvents, hydrogen chloride acts as a weak acid.

When frozen, HCl undergoes a phase transition at 98.4 K, shifting from an orthorhombic crystal structure to a cubic one. In both structures, chlorine atoms form a face-centered array, though hydrogen atoms could not be located directly. Studies of spectroscopic and dielectric data, along with the structure of deuterium chloride, show that solid HCl forms zigzag chains, similar to hydrogen fluoride.

The infrared spectrum of gaseous HCl shows sharp absorption lines around 2886 cm⁻¹. At room temperature, nearly all molecules are in the ground vibrational state. The vibrational energy includes an anharmonic term, but the expected absorption from the v=0 to v=1 transition (around 2880 cm⁻¹) is forbidden by symmetry, so the Q-branch is not observed. Instead, P- and R-branches appear due to simultaneous changes in rotational states. Only certain rotational transitions are allowed, with the rotational quantum number J changing by ±1. The rotational constant B is much smaller than the vibrational frequency, so rotational energy changes are small, but the vibrational energy of HCl places its absorptions in the infrared region.