In the laboratory, another method involves hydrolysis of PI 3, the iodine analog of PBr 3. This supports a mechanism whereby I 2 first dissociates into 2 iodine atoms, which each attach themselves to a side of an H 2 molecule and break the H−H bond: H 2 + I 2 + ( 578 nm radiation) → H 2 + 2I → I However, when a mixture of the gases is irradiated with the wavelength of light equal to the dissociation energy of I 2, about 578 nm, the rate increases significantly. This method is usually employed to generate high-purity samples.įor many years, this reaction was considered to involve a simple bimolecular reaction between molecules of H 2 and I 2. HI can also be distilled from a solution of NaI or other alkali iodide in concentrated phosphoric acid (note that concentrated sulfuric acid will not work for acidifying iodides, as it will oxidize the iodide to elemental iodine).Īnother way HI may be prepared is by bubbling hydrogen sulfide steam through an aqueous solution of iodine, forming hydroiodic acid (which is distilled) and elemental sulfur (this is filtered): H 2S + I 2 → 2 HI + SĪdditionally, HI can be prepared by simply combining H 2 and I 2: When performed in water, the HI must be distilled. The industrial preparation of HI involves the reaction of I 2 with hydrazine, which also yields nitrogen gas: 2 I 2 + N 2H 4 → 4 HI + N 2 I − interaction in HI facilitates dissociation of the proton from the anion and is the reason HI is the strongest acid of the hydrohalides. ![]() By contrast, a chloride ion is much smaller, meaning its negative charge is more concentrated, leading to a stronger interaction between the proton and the chloride ion. The iodide ion radius is much larger than the other common halides, which results in the negative charge being dispersed over a large space. The high acidity is caused by the dispersal of the ionic charge over the anion. The solution forms an azeotrope boiling at 127 ☌ with 57% HI, 43% water. Commercial "concentrated" hydroiodic acid usually contains 48–57% HI by mass. Hydroiodic acid is not pure hydrogen iodide, but a mixture containing it. One liter of water will dissolve 425 liters of HI gas, the most concentrated solution having only four water molecules per molecule of HI. It is exceptionally soluble in water, giving hydroiodic acid. With moist air, HI gives a mist (or fumes) of hydroiodic acid. ![]() HI is a colorless gas that reacts with oxygen to give water and iodine. HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under standard conditions, whereas the other is an aqueous solution of the gas. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide ( H I) is a diatomic molecule and hydrogen halide.
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