At normal temperature and pressure, hydrogen halide is a colorless and strongly irritating gas, which is easy to combine with water vapor in the air to form a white acid mist.
Hydrogen halide is a molecule with strong polarity, so it has great solubility in water. The aqueous solution of hydrogen halide is called hydrohalide acid, so HF, HCl, HBr and HI are respectively called hydrofluoric acid, hydrochloric acid, hydrobromic acid and hydroiodic acid. Liquid hydrogen halides do not conduct electricity, which indicates that they are covalent compounds rather than ionic compounds.
The polarity of hydrogen halide decreases in the order of hydrogen fluoride, hydrogen chloride, hydrogen bromide and hydrogen iodide, and the intermolecular force increases in the order of HCl, HBr and HI. Therefore, their melting and boiling points increase in turn.
HF is the only exception in many properties, because there is a strong hydrogen bond between HF molecules, so in hydrogen halide, it has the largest melting heat, vaporization heat and highest boiling point, and the melting point is also greater than HCl and HBr. In terms of chemical properties, hydrogen halide and hydrohalide acid also show regular changes, and HF also shows some particularity. [1]
HF>HCl>HBr>HI。
HF: There is no evidence of decomposition above 2000 ℃ (it is generally believed that hydrogen fluoride will not be thermally decomposed).
HCl: slowly decomposes when the temperature is above 1000 ℃.
HBr: decomposition starts at 500 ℃.
HI: Decomposition starts at 300 ℃. [1]
Hydrochloric acid (hydrochloric acid), hydrobromic acid and hydroiodic acid are all strong acids in the hydrohalide acid, and the acidity increases in turn. Hydrofluoric acid is a weak acid, but it will become a strong acid when its concentration is greater than 5 mol/L. The reason for this abnormal phenomenon is that when the concentration of HF increases, associated ions are formed to further dissociate HF. [1]
Reducibility of hydrogen halide and hydrohalide acid
The reduction capacity of hydrogen halide increases in order of HF, HCl, HBr and HI, and HF cannot be oxidized by ordinary oxidants; HCl can only be oxidized with strong oxidant; HBr and HI have strong reducibility, and the oxygen in the air can oxidize HBr and HI into simple substances. HBr solution can turn brown under the action of oxygen in sunlight and air; The HI solution will gradually turn brown even in the dark. [1]
Both HF gas and hydrofluoric acid solution have strong corrosion effect on glass. In analytical chemistry, hydrofluoric acid is widely used to determine mineral or steel samples silicon dioxide Content of. HF gas can be used to etch marks and patterns on glassware. Smooth nicks can be obtained by etching glass with hydrofluoric acid solution, and the "frosting" of ground glass and bulb is corroded with hydrofluoric acid. Both HF gas and hydrofluoric acid solution must be stored in plastic containers or containers coated with paraffin. Hydrogen fluoride is called as the source of fluorine. It can be used to produce simple fluorine and many fluorides. Hydrogen fluoride will cause burns to the skin that are difficult to treat, so pay attention to safety when using it. [1] Hydrofluoric acid and hydrochloric acid in halogen acid have great practical value. Hydrochloric acid is the most important acid except sulfuric acid in industry, and is an important chemical raw material and Chemical Reagents 。 In human stomach, the mass fraction of hydrochloric acid is 0.5%, which can promote food digestion and kill bacteria. [2] Displacement reaction of metal halides with concentrated sulfuric acid
The industrial and laboratory preparation of hydrogen fluoride is based on the reaction of calcium fluoride (fluorite) with concentrated sulfuric acid. When a small amount of hydrogen chloride is prepared in the laboratory, NaCl can be used to react with concentrated sulfuric acid. To prepare hydrogen bromide and HI, non oxidizing acids (such as phosphoric acid) should be used instead of concentrated sulfuric acid. This is because the products obtained when using concentrated sulfuric acid are not hydrogen bromide and HI, but bromine and iodine. [2]
Direct combination of halogen and hydrogen
Because the direct combination of fluorine and hydrogen is too intense to control, it cannot be used for the preparation of HF. Bromine reacts slowly with hydrogen, and the reaction needs to be carried out at high temperature, but HX decomposes at high temperature, affecting its yield. In industry, only chlorine and hydrogen are directly combined to produce HCl. The hydrogen chloride gas generated from the combustion of hydrogen and chlorine in the synthesis furnace is led out from the furnace, cooled, and absorbed with water or dilute hydrochloric acid to obtain concentrated hydrochloric acid with a mass fraction of 29%~31%. [2]
Hydrogen bromide and hydrogen iodide are usually prepared by hydrolysis of non-metallic halides in the laboratory. If water drops are added to the surface of phosphorus tribromide and phosphorus triiodide, hydrogen bromide and hydrogen iodide can be produced. In actual use, it is not necessary to make non-metallic halides first, but to add water drop by drop on the mixture of phosphorus and a little water, which can continuously produce hydrogen bromide or hydrogen iodide. [2]
