Editing Hydrogen peroxide (section)

==Properties==
The boiling point of {{chem2|H2O2}} has been extrapolated as being {{Convert|150.2|C||abbr=}}, approximately {{Convert|50|C-change||abbr=}} higher than water. In practice, hydrogen peroxide will undergo potentially explosive [[thermal decomposition]] if heated to this temperature. It may be safely distilled at lower temperatures under reduced pressure.<ref>{{cite book |veditors = Brauer G |others=Translation editing by Reed F. |date=1963 |title=Handbook of preparative inorganic chemistry |edition=2nd |volume=1 |publisher=Academic Press |location=New York|isbn=978-0-12-126601-1 |page=140}}</ref>

Hydrogen peroxide forms stable [[adduct]]s with [[urea]] ([[hydrogen peroxide–urea]]), sodium carbonate ([[sodium percarbonate]]) and other compounds.<ref>{{Cite journal| vauthors = Chernyshov IY, Vener MV, Prikhodchenko PV, Medvedev AG, Lev O, Churakov AV |date=2017-01-04|title=Peroxosolvates: Formation Criteria, H2O2 Hydrogen Bonding, and Isomorphism with the Corresponding Hydrates|journal=Crystal Growth & Design|volume=17|issue=1|pages=214–220|doi=10.1021/acs.cgd.6b01449|bibcode=2017CrGrD..17..214C |issn=1528-7483}}</ref> An acid-base adduct with [[triphenylphosphine oxide]] is a useful "carrier" for {{chem2|H2O2}} in some reactions.

===Structure===
<div class="skin-invert-image">
{{multiple image
| direction = hoizontal
| align = left
| header =
| width = 210
| image1 = H2O2 gas structure.svg
| alt1 = O−O bond length = 147.4 pm<br/>O−H bond length = 95.0 pm
| caption1 = Structure and dimensions of {{chem2|H2O2}} in the gas phase
| image2 = H2O2 solid structure.svg
| alt2 = O−O bond length = 145.8 pm<br/>O−H bond length = 98.8 pm
| caption2 = Structure and dimensions of {{chem2|H2O2}} in the solid (crystalline) phase
}}</div>

Hydrogen peroxide ({{chem2|H2O2}}) is a nonplanar molecule with (twisted) C<sub>2</sub> [[Molecular symmetry#Common point groups|symmetry]]; this was first shown by [[Paul-Antoine Giguère]] in 1950 using [[infrared spectroscopy]].<ref>{{cite journal |vauthors = Giguère PA |date=1950 |title=The Infra-Red Spectrum of Hydrogen Peroxide |journal=Journal of Chemical Physics |volume=18 |issue=1 |page=88 |doi=10.1063/1.1747464 |url=https://authors.library.caltech.edu/11457/1/GIGjcp50.pdf |access-date=31 December 2018 |archive-url=https://web.archive.org/web/20171202122026/https://authors.library.caltech.edu/11457/1/GIGjcp50.pdf |archive-date=2 December 2017 |url-status=live |bibcode=1950JChPh..18...88G}}</ref><ref>{{cite journal |vauthors = Giguère PA |date=1983 |title=Molecular association and structure of hydrogen peroxide |journal=[[Journal of Chemical Education]] |volume=60 |issue=5 |pages=399–401 |doi=10.1021/ed060p399 |bibcode=1983JChEd..60..399G}}</ref> Although the O−O bond is a [[single bond]], the molecule has a relatively high [[rotational barrier]] of 386&nbsp;[[inverse centimeter|cm<sup>−1</sup>]] (4.62&nbsp;[[Kilojoule|kJ]]/[[Mole (chemistry)|mol]]) for rotation between [[enantiomer]]s via the [[Cis–trans isomerism|''trans'']] configuration, and 2460&nbsp;cm<sup>−1</sup> (29.4&nbsp;kJ/mol) via the [[Cis–trans isomerism|''cis'']] configuration.<ref name="Hunt1965">{{cite journal |vauthors = Hunt RH, Leacock RA, Peters CW, Hecht KT |date=1965 |title=Internal-Rotation in Hydrogen Peroxide: The Far-Infrared Spectrum and the Determination of the Hindering Potential |journal=The Journal of Chemical Physics |bibcode=1965JChPh..42.1931H |doi=10.1063/1.1696228 |volume=42 |issue=6 |page=1931 |url=https://deepblue.lib.umich.edu/bitstream/handle/2027.42/71115/JCPSA6-42-6-1931-1.pdf?sequence=2 |hdl=2027.42/71115 |access-date=9 April 2014 |archive-url=https://web.archive.org/web/20140409085232/https://deepblue.lib.umich.edu/bitstream/handle/2027.42/71115/JCPSA6-42-6-1931-1.pdf?sequence=2 |archive-date=9 April 2014 |url-status=live |hdl-access=free}}</ref> These barriers are proposed to be due to [[Repulsion (chemistry)|repulsion]] between the [[lone pair]]s of the adjacent oxygen atoms and dipolar effects between the two O–H bonds. For comparison, the rotational barrier for [[ethane]] is 1040&nbsp;cm<sup>−1</sup> (12.4&nbsp;kJ/mol).

The approximately 100° [[dihedral angle]] between the two O–H bonds makes the molecule [[chiral]]. It is the smallest and simplest molecule to exhibit [[enantiomer]]ism. It has been proposed that the [[stereospecific|enantiospecific]] interactions of one rather than the other may have led to amplification of one enantiomeric form of [[ribonucleic acid]]s and therefore an origin of [[homochirality]] in an [[RNA world]].<ref>{{cite journal |vauthors = Ball R, Brindley J |title = The Life Story of Hydrogen Peroxide III: Chirality and Physical Effects at the Dawn of Life |journal = Origins of Life and Evolution of the Biosphere |volume = 46 |issue = 1 |pages = 81–93 |date = March 2016 |pmid = 26399407 |doi = 10.1007/s11084-015-9465-y |s2cid = 9564774 |bibcode = 2016OLEB...46...81B}}</ref>

The molecular structures of gaseous and [[crystalline]] {{chem2|H2O2}} are significantly different. This difference is attributed to the effects of [[hydrogen bonding]], which is absent in the gaseous state.<ref>{{cite book |vauthors = Dougherty DA, Anslyn EV |date=2005 |title=Modern Physical Organic Chemistry |publisher=University Science |isbn=978-1-891389-31-3 |page=122}}</ref> Crystals of {{chem2|H2O2}} are [[tetragonal]] with the [[space group]] ''D''{{su|b=4|p=4}} or ''P''4<sub>1</sub>2<sub>1</sub>2.<ref>{{cite journal |vauthors = Abrahams SC, Collin RL, Lipscomb WN |date=1 January 1951|title=The crystal structure of hydrogen peroxide |journal=Acta Crystallographica |doi=10.1107/S0365110X51000039 |volume=4 |issue=1 |pages=15–20|bibcode=1951AcCry...4...15A |doi-access=free}}</ref>

===Aqueous solutions===
In [[aqueous solution]]s, hydrogen peroxide forms a [[eutectic]] mixture, exhibiting [[freezing-point depression]] down as low as −56&nbsp;°C; pure water has a freezing point of 0&nbsp;°C and pure hydrogen peroxide of −0.43&nbsp;°C. The boiling point of the same mixtures is also depressed in relation with the mean of both boiling points (125.1&nbsp;°C). It occurs at 114&nbsp;°C. This boiling point is 14&nbsp;°C greater than that of pure water and 36.2&nbsp;°C less than that of pure hydrogen peroxide.<ref>{{cite web|url=https://www.h2o2.com/intro/FMC_MSDS_40_to_60.pdf|title=Hydrogen Peroxide Technical Library|access-date=3 March 2016|archive-url=https://web.archive.org/web/20091229052623/https://www.h2o2.com/intro/FMC_MSDS_40_to_60.pdf|archive-date=29 December 2009|url-status=dead}}</ref>

<li style="display: inline-table; vertical-align:top;>
[[File:Phase diagram hydrogen peroxide water.svg|class=skin-invert-image|423px|thumb|[[Phase diagram]] of {{chem2|H2O2}} and water: Area above blue line is liquid. Dotted lines separate solid–liquid phases from solid–solid phases.]]
</li>

<li style="display: inline-table; vertical-align:top;>
{| class="wikitable"
|+ Density of aqueous solution of {{chem2|H2O2}}
|-
! {{chem2|H2O2}} ([[w/w]]) !! Density<br>(g/cm<sup>3</sup>) !! Temp.<br>(°C)
|-
| 3% || 1.0095 || 15
|-
| 27% || 1.10 || 20
|-
| 35% || 1.13 || 20
|-
| 50% || 1.20 || 20
|-
| 70% || 1.29 || 20
|-
| 75% || 1.33 || 20
|-
| 96% || 1.42 || 20
|-
| 98% || 1.43 || 20
|-
| 100% || 1.45 || 20
|}
</li>

Hydrogen peroxide is most commonly available as a solution in water. For consumers, it is usually available from pharmacies at 3 and 6 [[wt%]] concentrations. The concentrations are sometimes described in terms of the volume of oxygen gas generated; one milliliter of a 20-volume solution generates twenty milliliters of oxygen gas when completely decomposed. For laboratory use, 30 wt% solutions are most common. Commercial grades from 70% to 98% are also available, but due to the potential of solutions of more than 68% hydrogen peroxide to be converted entirely to steam and oxygen (with the temperature of the steam increasing as the concentration increases above 68%) these grades are potentially far more hazardous and require special care in dedicated storage areas. Buyers must typically allow inspection by commercial manufacturers.

===Comparison with analogues===
Hydrogen peroxide has several structural analogues with {{chem2|H_{''m''}X\sXH_{''n''}|}} bonding arrangements (water also shown for comparison). It has the highest (theoretical) boiling point of this series (X = O, S, N, P). Its melting point is also fairly high, being comparable to that of [[hydrazine]] and water, with only [[hydroxylamine]] crystallising significantly more readily, indicative of particularly strong hydrogen bonding. [[Diphosphane]] and [[hydrogen disulfide]] exhibit only weak hydrogen bonding and have little chemical similarity to hydrogen peroxide. Structurally, the analogues all adopt similar skewed structures, due to repulsion between adjacent [[lone pair]]s.

{| class="wikitable sortable"
|+ Properties of {{chem2|H2O2}} and its analogues<br/>Values marked * are extrapolated
|-
! Name !! Formula !! [[Molar mass]]<br>(g/mol) !! Melting<br>point (°C) !! Boiling<br>point (°C)
|-
| [[Properties of water|Water]] || HOH || 18.02 || 0.00 || 99.98
|-
| Hydrogen peroxide || HOOH || 34.01 || −0.43 || 150.2*
|-
| [[Hydrogen disulfide]] || HSSH || 66.15 || −89.6 || 70.7
|-
| [[Hydrazine]] || H<sub>2</sub>NNH<sub>2</sub> || 32.05 || 2 || 114
|-
| [[Hydroxylamine]] || NH<sub>2</sub>OH || 33.03 || 33 || 58*
|-
| [[Diphosphane]] || H<sub>2</sub>PPH<sub>2</sub> || 65.98 || −99 || 63.5*
|}