The compound hydrogen chloride has the chemical formula HCl and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride gas and hydrochloric acid are important in technology and industry. Hydrochloric acid, the aqueous solution of hydrogen chloride, is also commonly given the formula HCl.
Reactions
Hydrogen chloride is a diatomic molecule, consisting of a hydrogen atom H and a chlorine atom Cl connected by a polar covalent bond. The chlorine atom is much more electronegative than the hydrogen atom, which makes this bond polar. Consequently, the molecule has a large dipole moment with a negative partial charge (δ−) at the chlorine atom and a positive partial charge (δ+) at the hydrogen atom. In part because of its high polarity, HCl is very soluble in water (and in other polar solvents). Upon contact, H2O and HCl combine to form hydronium cations [H3O]+ and chloride anions Cl−:
HCl + H2O → [H3O]+ + Cl− The resulting solution is called hydrochloric acid and is a strong acid. The acid dissociation or ionization constant, Ka, is large, which means HCl dissociates or ionizes completely in water. Hydrogen chloride also ionizes in other solvents such as methanol:
HCl + CH3OH → [CH3OH2]+ + Cl− In the absence of such polar solvents, hydrogen chloride functions as a weak acid.
Structure and properties
Frozen HCl undergoes a phase transition at 98.4 K (−174.8 °C; −282.5 °F). X-ray powder diffraction of the frozen material shows that the material changes from an orthorhombic structure to a cubic one during this transition. In both structures the chlorine atoms are in a face-centered array. However, the hydrogen atoms could not be located. Analysis of spectroscopic and dielectric data, and determination of the structure of DCl (deuterium chloride) indicates that HCl forms zigzag chains in the solid, as does HF (see figure on right).
The infrared spectrum of gaseous hydrogen chloride, shown on the left, consists of a number of sharp absorption lines grouped around 2886 cm−1 (wavelength ~3.47 μm). At room temperature, almost all molecules are in the ground vibrational state v = 0. Including anharmonicity the vibrational energy can be written as:
E v = h ν e ( v + 1 2 ) + h x e ν e ( v + 1 2 ) 2 {\displaystyle E_{\mathrm {v} }=h\nu _{e}\left(v+{\tfrac {1}{2}}\right)+hx_{e}\nu _{e}\left(v+{\tfrac {1}{2}}\right)^{2}}
To promote an HCl molecule from the v = 0 to the v = 1 state, we would expect to see an infrared absorption about νo = νe + 2xeνe = 2880 cm−1. However, this absorption corresponding to the Q-branch is not observed due to it being forbidden by symmetry. Instead, two sets of signals (P- and R-branches) are seen owing to a simultaneous change in the rotational state of the molecules. Because of quantum mechanical selection rules, only certain rotational transitions are permitted. The states are characterized by the rotational quantum number J = 0, 1, 2, 3, ... selection rules state that ΔJ is only able to take values of ±1.
E ( J ) r o t = h ⋅ B ⋅ J ( J + 1 ) {\displaystyle E(J)_{\mathrm {rot} }=h\cdot B\cdot J(J+1)}
The value of the rotational constant B is much smaller than the vibrational one νo, such that a much smaller amount of energy is required to rotate the molecule; for a typical molecule, this lies within the microwave region. However, the vibrational energy of HCl molecule places its absorptions within the infrared region, allowing a spectrum showing the rovibrational transitions of this molecule to be easily collected using an infrared spectrometer with a gas cell. The latter can even be made of quartz as the HCl absorption lies in a window of transparency for this material. Naturally abundant chlorine consists of two isotopes, 35Cl and 37Cl, in a ratio of approximately 3:1. While the spring constants are nearly identical, the disparate reduced masses of H35Cl and H37Cl cause measurable differences in the rotational energy, thus doublets are observed on close inspection of each absorption line, weighted in the same ratio of 3:1.
Production
Historical routes In the 17th century, Johann Rudolf Glauber combined sodium chloride salt and sulfuric acid for the preparation of hydrogen chloride. This route is the basis of the Mannheim process. Joseph Priestley prepared hydrogen chloride in 1772, and by 1808 Humphry Davy had proved that the chemical composition included hydrogen and chlorine.
Industrial routes Most hydrogen chloride is produced as a byproduct of chlorination processes. Routes to tetrafluoroethylene, chlorobenzene, CFCs, chloroacetic acid, and vinyl chloride are illustrative. In these routes, formally speaking, hydrogen atoms on the hydrocarbon are replaced by chlorine atoms, whereupon the released hydrogen atom recombines with the spare atom from the chlorine molecule, forming hydrogen chloride:
RH + Cl2 → RCl + HCl Often the resulting hydrogen chloride is integrated with captive use of it on-site, e.g. oxychlorination. Hydrogen chloride is also produced in some routes to organofluorine compounds:
RCl + HF → RF + HCl
Hydrogen chloride can be produced by combining chlorine and hydrogen:
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