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Encyclopedia > Hydrogen iodide


Hydrogen iodide
Hydrogen iodide Hydrogen iodide
Systematic name Hydrogen iodide
Other names Only aqueous HI is hydroiodic acid
Molecular formula HI
Molar mass 127.904 g/mol
Appearance Colorless gas.
CAS number [10034-85-2]
Density and phase 2.85 g/mL (-47 °C)
Melting point –50.80 °C (184.55 K)
Boiling point –35.36 °C (237.79 K)
Acidity (pKa) –10
Molecular shape Terminus
Dipole moment 0.38 D
MSDS hydrogen iodide
hydroiodic acid
Main hazards Toxic, corrosive.
NFPA 704
Flash point Non-flammable.
R/S statement R: R20, R21, R22, R35
S: S7, S9, S26, S45
RTECS number MW3760000
Thermodynamic Data
Standard enthalpy
of formation
ΔfH0 (crystal) = –88.3 kJ/mol
ΔfH0 (gas) = 27.6 kJ/mol
Heat of vaporization ΔHvap = 141.9 J/g
Heat of fusion ΔHfus = 22.6 J/g
Heat of solution ΔHsol (gas) = –82.0 kJ/mol
Critical Point Tcrit = 151 °C
Pcrit = 8.3 MPa
Supplementary data page
Structure and
n, εr, etc.
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)
Infobox disclaimer and references

Hydrogen iodide (HI) is a diatomic molecule. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different chemical compounds: one is a gas at room temperature, the other is an aqueous solution. They are interconvertible. HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent. Image File history File links Size of this preview: 800 × 428 pixelsFull resolution (1003 × 536 pixel, file size: 10 KB, MIME type: image/png) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Hydrogen-iodide-3D-vdW.svg‎ Vector conversion from Image:Hydrogen-iodide-3D-vdW.png File historyClick on a date/time to view the file as it appeared at that time. ... IUPAC nomenclature is a system of naming chemical compounds and of describing the science of chemistry in general. ... This article or section does not cite any references or sources. ... This article does not cite any references or sources. ... CAS registry numbers are unique numerical identifiers for chemical compounds, polymers, biological sequences, mixtures and alloys. ... In physics, density is mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as: where, in SI units: ρ (rho) is the density of the substance, measured in kg·m-3 m is the mass of the substance, measured in kg V is... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... The melting point of a crystalline solid is the temperature at which it changes state from solid to liquid. ... Italic text This article is about the boiling point of liquids. ... The acid dissociation constant (Ka), also known as the acidity constant or the acid-ionization constant, is a specific equilibrium constant for the reaction of an acid with its conjugate base in aqueous solution [1]. // When an acid dissolves in water, it partly dissociates forming hydronium ions and its conjugate... four sp³ orbitals three sp² orbitals In chemistry, hybridisation or hybridization (see also spelling differences) is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. ... The Earths magnetic field, which is approximately a dipole. ... The debye (symbol: D) is a non-SI and non-CGS unit of electrical dipole moment. ... An example MSDS in a US format provides guidance for handling a hazardous substance and information on its composition and properties. ... The examples and perspective in this article or section may not represent a worldwide view. ... NFPA 704 is a standard maintained by the U.S. National Fire Protection Association. ... Image File history File links NFPA_704. ... For other uses, see Flash point (disambiguation). ... Risk and Safety Statements, also known as R/S statements, R/S numbers, R/S phrases, and R/S sentences, is a system of hazard codes and phrases for labeling dangerous chemicals and compounds. ... R-phrases are defined in Annex III of European Union Directive 67/548/EEC: Nature of special risks attributed to dangerous substances and preparations. ... S-phrases are defined in Annex IV of European Union Directive 67/548/EEC: Safety advice concerning dangerous substances and preparations. ... RTECS, also known as Registry of Toxic Effects of Chemical Substances, is a database of toxicity information compiled from the open scientific literature that is available for charge. ... The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 100 kPa... The heat of vaporization is a physical property of substances. ... Heat of fusion is the amount of heat energy which must be absorbed or lost for 1 gram of a substance to change states from a solid to a liquid or vice versa. ... Heat of solution is the quantity of heat evolved or absorbed when one mole of a solute is dissolved in a large volume of a solvent. ... In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions (temperature, pressure) at which the liquid state of the matter ceases to exist. ... The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ... This page provides supplementary chemical data on hydrogen iodide. ... This page provides supplementary chemical data on hydrogen iodide. ... The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. ... The relative dielectric constant of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. ... This page provides supplementary chemical data on hydrogen iodide. ... Ultraviolet-Visible Spectroscopy or Ultraviolet-Visible Spectrophotometry (UV/ VIS) involves the spectroscopy of photons (spectrophotometry). ... Infrared spectroscopy (IR Spectroscopy) is the subset of spectroscopy that deals with the IR region of the EM spectrum. ... Nuclear Magnetic Resonance Spectroscopy most commonly known as NMR Spectroscopy is the name given to the technique which exploits the magnetic properties of nuclei. ... Mass spectrometry (also known as mass spectroscopy (deprecated)[1] or informally, mass-spec and MS) is an analytical technique used to measure the mass-to-charge ratio of ions. ... This article is about the electrically charged particle. ... Hydrogen fluoride is a chemical compound with the formula HF. Together with hydrofluoric acid, it is the principal industrial source of fluorine and hence the precursor to many important compounds including pharmaceuticals and polymers (e. ... R-phrases , S-phrases , , , , Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ... Flash point Non flammable Supplementary data page Structure and properties n, εr, etc. ... In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals) and 25 degrees Celsius (298. ... A computer rendering of the Nitrogen Molecule, which is a diatomic molecule. ... Hydroiodic acid (sometimes also spelled hydriodic acid) is a highly acidic aqueous solution of hydrogen iodide (HI) (Concentrated solution is usually 48 - 57% HI). ... Hydroiodic acid (sometimes also spelt hydriodic acid) is a highly acidic aqueous solution of hydrogen iodide (HI) (Concentrated solution is usually 48 - 57% HI). ... Acids and bases: Acid-base reaction theories pH Self-ionization of water Buffer solutions Systematic naming Electrochemistry Acids: Strong acids Weak acids Bases: Strong bases Weak bases edit A strong acid is an acid that dissociates completely in an aqueous solution, or in other terms, with a pKa < −1. ... Organic synthesis is the construction of organic molecules via chemical processes. ... Inorganic chemistry is the branch of chemistry concerned with the properties and reactions of inorganic compounds. ... General Name, Symbol, Number iodine, I, 53 Chemical series halogens Group, Period, Block 17, 5, p Appearance violet-dark gray, lustrous Standard atomic weight 126. ... A reducing agent (also called a reductant or reducer) is the element or a compound in a redox (reduction-oxidation) reaction (see electrochemistry) that reduces another species. ...


Properties of hydrogen iodide

HI is a colorless gas that reacts with oxygen to give water and iodine. With moist air, HI gives a mist (or fumes) of hydroiodic acid. It is exceptionally soluble in water, giving hydroiodic acid. One liter of water will dissolve 425 liters of HI, the final solution having only four water molecules per molecule of HI.[1]

Hydroiodic acid

Once again, although chemically related, hydroiodic acid is not HI but made from it. Commercial "concentrated" hydroiodic acid usually contains 48% - 57% HI by weight. The solution forms an azeotrope boiling at 127 °C at 57% HI, 43% water. Hydroiodidic acid 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. This article needs more context around or a better explanation of technical details to make it more accessible to general readers and technical readers outside the specialty, without removing technical details. ... A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. ... Electronegativity is a measure of the ability of an atom or molecule to attract electrons in the context of a chemical bond. ... General Name, Symbol, Number iodine, I, 53 Chemical series halogens Group, Period, Block 17, 5, p Appearance violet-dark gray, lustrous Standard atomic weight 126. ... An iodide ion is an iodine atom with a −1 (negative one) charge. ... In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... A dissociative is a drug which reduces (or blocks) signals to the conscious mind from other parts of the brain, typically (but not necessarily, or limited to) the physical senses. ...

HI(g) + H2O(l) H3O+(aq) + I(aq)        (Ka 1010)
HBr(g) + H2O(l) H3O+(aq) + Br(aq)        (Ka 109)
HCl(g) + H2O(l) H3O+(aq) + Cl(aq)        (Ka 108)

In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ... In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ... In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ...


The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas.[2] Hydrazine should not be confused with Hydergine (Ergoloid mesylates) an ergot alkaloid Hydrazine is the chemical compound with formula N2H4, widely used in chemical synthesis. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless gas Standard atomic weight 14. ...

2 I2 + N2H4 → 4 HI + N2

When performed in water, the HI must be distilled. יחכיטכיגיגיוגקאטגקעיגקDistillation is a method of separating chemical substances based on differences in their volatilities in a boiling liquid mixture. ...

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). Sodium iodide (NaI) is used in polymerase chain reactions (PCR) Categories: Chemistry stubs ... Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is an inorganic mineral acid having the chemical formula H3PO4. ... Sulfuric (or sulphuric) acid, H2SO4, is a strong mineral acid. ...

Additionally HI can 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 bond:[3] In chemistry, bond dissociation energy, D0, is one measure of the bond strength in a chemical bond. ...

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 phosphorous acid. Hydrolysis is a chemical reaction or process in which a chemical compound reacts with water. ... Phosphorus triiodide (PI3) is an unstable red solid which reacts violently with water. ... Phosphorus tribromide is a colourless liquid with the formula PBr3. ... General Name, Symbol, Number phosphorus, P, 15 Chemical series nonmetals Group, Period, Block 15, 3, p Appearance waxy white/ red/ black/ colorless Standard atomic weight 30. ... Phosphorus triiodide (PI3) is an unstable red solid which reacts violently with water. ... The compound phosphorous acid, with formula H3PO3, is an oxoacid of phosphorus. ...

3 I2 + 2 P + 6 H2O → 2 PI3 + 6 H2O → 6 HI + 2 H3PO3

Key reactions and applications

  • HI will undergo oxidation if left open to air according to the following pathway:'
4 HI + O2 → 2H2O + 2 I2
HI + I2 → HI3

HI3 is dark brown in color, which makes aged solutions of HI often appear dark brown.

  • Like HBr and HCl, HI add to alkenes4:
HI + H2C=CH2 → H3CCH2I

HI is subject to the same Markovnikov and anti-Markovnikov guidelines as HCl and HBr. An alkene is one of the three classes of unsaturated hydrocarbons that contain at least one carbon-carbon double bond and have the general molecular formula of CnH2n (the other two being alkynes and arenes). ... In chemistry, Markovnikovs rule is an observation based on Zaitsevs rule. ...

  • HI reduces certain α-substituted ketones and alcohols replacing the α substituent with a hydrogen atom.[4]


  1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  2. ^ Greenwood, N.N. and A. Earnshaw. The Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heineman. p 809-815. 1997.
  3. ^ Holleman, A.F. Wiberg, E. Inorganic Chemistry. San Diego: Academic Press. p 371, 432-433. 2001.
  4. ^ Breton, G. W., P. J. Kropp, P. J.; Harvey, R. G. “Hydrogen Iodide” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.

See also: Nishikata, E., T.; Ishii, 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.

External links

  • International Chemical Safety Card 1326



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