Editing Iodine (section)

===Hydrogen iodide===
The simplest compound of iodine is [[hydrogen iodide]], HI. It is a colourless gas that reacts with oxygen to give water and iodine. Although it is useful in [[Halogenation|iodination]] reactions in the laboratory, it does not have large-scale industrial uses, unlike the other hydrogen halides. Commercially, it is usually made by reacting iodine with [[hydrogen sulfide]] or [[hydrazine]]:<ref name="Greenwood809">Greenwood and Earnshaw, pp. 809–812</ref>

:2 I<sub>2</sub> + N<sub>2</sub>H<sub>4</sub> {{overset|H<sub>2</sub>O|⟶}} 4 HI + N<sub>2</sub>

At room temperature, it is a colourless gas, like all of the hydrogen halides except [[hydrogen fluoride]], since hydrogen cannot form strong [[hydrogen bond]]s to the large and only mildly electronegative iodine atom. It melts at {{convert|−51.0|°C}} and boils at {{convert|−35.1|°C}}. It is an [[Endothermic process|endothermic]] compound that can exothermically dissociate at room temperature, although the process is very slow unless a [[Catalysis|catalyst]] is present: the reaction between hydrogen and iodine at room temperature to give hydrogen iodide does not proceed to completion. The H–I [[Bond-dissociation energy|bond dissociation energy]] is likewise the smallest of the hydrogen halides, at 295&nbsp;kJ/mol.<ref name="Greenwood812">Greenwood and Earnshaw, pp. 812–819</ref>

Aqueous hydrogen iodide is known as [[hydroiodic acid]], which is a strong acid. Hydrogen iodide is exceptionally soluble in water: one litre of water will dissolve 425 litres of hydrogen iodide, and the saturated solution has only four water molecules per molecule of hydrogen iodide.<ref>{{Cite book |last1=Holleman |first1=A. F. |title=Inorganic Chemistry |last2=Wiberg |first2=E. |publisher=Academic Press |year=2001 |isbn=0-12-352651-5 |location=San Diego}}</ref> Commercial so-called "concentrated" hydroiodic acid usually contains 48–57% HI by mass; the solution forms an [[azeotrope]] with boiling point {{convert|126.7|°C}} at 56.7&nbsp;g HI per 100&nbsp;g solution. Hence hydroiodic acid cannot be concentrated past this point by evaporation of water.<ref name="Greenwood812" /> Unlike gaseous hydrogen iodide, hydroiodic acid has major industrial use in the manufacture of [[acetic acid]] by the [[Cativa process]].<ref name="Cativa">{{Cite journal |last=Jones |first=J. H. |year=2000 |title=The Cativa Process for the Manufacture of Acetic Acid |url=http://www.platinummetalsreview.com/pdf/pmr-v44-i3-094-105.pdf |url-status=live |journal=[[Platinum Metals Review]] |volume=44 |issue=3 |pages=94–105 |doi=10.1595/003214000X44394105 |archive-url=https://web.archive.org/web/20150924074441/http://www.platinummetalsreview.com/pdf/pmr-v44-i3-094-105.pdf |archive-date=24 September 2015 |access-date=26 August 2023}}</ref><ref>{{Cite journal |last1=Sunley |first1=G. J. |last2=Watsonv |first2=D. J. |year=2000 |title=High productivity methanol carbonylation catalysis using iridium – The Cativa process for the manufacture of acetic acid |journal=Catalysis Today |volume=58 |issue=4 |pages=293–307 |doi=10.1016/S0920-5861(00)00263-7}}</ref>