HI is subject to the same Markovnikov and anti-Markovnikov guidelines as HCl and HBr. HI 3 is dark brown in color, which makes aged solutions of HI often appear dark brown. HI will undergo oxidation if left open to air according to the following pathway:'Ĥ HI + O 2 → 2H 2O + 2 I 2 HI + I 2 → HI 3.In this method, I 2 reacts with phosphorus to create phosphorus triiodide, which then reacts with water to form HI and phosphorous acid.ģ I 2 + 2 P + 6 H 2O → 2 PI 3 + 6 H 2O → 6 HI + 2 H 3PO 3 Key reactions and applications Iodine forms all three possible diatomic interhalogens, a trifluoride and trichloride, as well as a pentafluoride and, exceptionally among the halogens, a heptafluoride. In the laboratory, another method involves hydrolysis of PI 3, the iodine equivalent of PBr 3. Only one I-I bond exists in the Lewis structure of an iodine molecule, and each iodine atom. 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 + 2 I → I - H - H - I → 2 HI Iodine is a diatomic molecule, meaning it only has two atoms. 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 sulfuric acid will not work for acidifying iodides as it will oxidize the iodide to elemental iodine).Īdditionally 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. This weaker H +-I − interaction in HI facilitates dissociation of the proton from the anion. 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 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. Hydroiodidic acid is one of the strongest of all the common halide acids because the electronegativity of iodine is weaker than the rest of the other common halides. The solution forms an azeotrope boiling at 127 ☌ with 57% HI, 43% water. Commercial "concentrated" hydroiodic acid usually contains 48% - 57% HI by mass. Once again, although chemically related, hydroiodic acid is not pure HI but a mixture containing it. One liter of water will dissolve 425 liters of HI, the final 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.
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