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| Hydrogen iodide | |
|---|---|
| General | |
| Systematic name | Hydrogen iodide |
| Other names | Hydroiodic acid Hydriodic acid |
| Molecular formula | HI |
| Molar mass | 127.904 g/mol |
| Appearance | Colorless gas. |
| CAS number | * |
| Properties | |
| Density and phase | 1.701 g/L, gas. |
| Melting point | -88.6 °C (184.55 K) |
| Boiling point | -67 °C (206.15 K) |
| Acidity (pKa) | -10 |
| Structure | |
| Molecular shape | Terminus |
| Dipole moment | 0.38 D |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | Toxic, corrosive. |
| NFPA 704 | |
| Flash point | Non-flammable. |
| R/S statement | R: , , , S: , , , |
| RTECS number | MW3760000 |
| Supplementary data page | |
| Structure and properties | n, εr, etc. |
| Thermodynamic data | Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Other anions | Hydrogen fluoride Hydrogen chloride Hydrogen bromide |
| Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox | |
Properties
Hydrogen iodide is one of the strongest of all the common halide acids, despite the fact that the electronegativity of iodine is weaker than the rest of the other common halides. The high acidity is caused by the dispersal of the ionic charge over the anion. The iodide ion is much larger than the other common halides which results in the negative charge being dispersed over a large space. 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. This weaker H+---I- interaction in HI facilitates dissociation of the proton from the anion .
- HI → H+ + I− : Ka = ~1010
- HBr → H+ + Br− : Ka = ~109
- HCl → H+ + Cl− : Ka = ~108
Preparation
The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas.1
- 2 I2 + N2H4 → 4 HI + N2
HI can also be prepared by simply combining H2 and I2. This method is usually employed to generate high purity samples.
- H2 + I2 → 2 HI
For many years, this reaction was considered to involve a simple bimolecular reaction between molecules of H2 and I2. However, when a mixture of the gases is irradiated with the wavelength of light equal to the dissociation energy of I2, about 578 nm, the rate increases significantly. This supports a mechanism whereby I2 first dissociates into 2 iodine atoms, which each attach themselves to a side of an H2 molecule and break the H -- H bond3:
- H2 + I2 + 578 nm radiation → H2 + 2 I → I - - - H - - - H - - - I → 2 HI
In the laboratory, another method involves hydrolysis of PI3, the iodine equivalent of PBr3. In this method, I2 reacts with phosphorus to create phosphorus triiodide, which then reacts with water to form HI and phosphoric acid1.
- 3 I2 + 2 P + 6 H2O → 2 PI3 + 6 H2O → 6 HI + 2 H3PO3
Key reactions & Applications
- HI will undergo oxidation if left open to air according to the following pathway:3
- 4 HI + O2 → 2H2O + 2 I2
- HI + I2 → HI3
- Like HBr and HCl, HI add to alkenes4:
- HI + H2C=CH2 → H3CCH2I
- HI reduces certain α-substituted ketones replacing the α substituant with a hydrogen atom.4.
References
1 Greenwood, N.N. and A. Earnshaw. The Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heineman. p 809-815. 1997. 2 Nishikata, E., T.; Ishll, and T. Ohta. “Viscosities of Aqueous Hydrochloric Acid Solutions, and Densities and Viscosities of Aqueous Hydroiodic Acid Solutions”. J. Chem. Eng. Data. 26. 254-256. 1981. 3 Holleman, A.F. Wiberg, e. Inorganic Chemistry. San Diego: Academic Press. p 371, 432-433. 2001. 4 Breton, Gary W., P. J. Kropp, and Ronald G. Harvey. “Hydrogen Iodide”. Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons, Ltd. 2001. 28 Feb. 2006 http://www.mrw.interscience.wiley.com/eros/articles/rh039/sect0-fs.html .
External links
Hydrogen compounds | Iodides | Acids | Nonmetal halides
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"Hydrogen iodide"