Editing Propane (section)

== Properties and reactions ==
[[File:Propane flame contours-en.svg|thumb|[[Pyrometry]] of a propane flame using thin-filament velocimetry. The hottest parts of the flame are in a hollow cone-shaped area near its base and pointing upward.
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Propane is a colorless, odorless gas. [[Ethyl mercaptan]] is added as a safety precaution as an [[odorizer]],<ref>NIOSH [2021]. Odor fade in natural gas and propane. Morgantown, WV: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention,  National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2021-106 (revised 01/2022), https://doi.org/10.26616/NIOSHPUB2021106revised012022external icon.</ref> and is commonly called a "rotten egg" smell.<ref>{{cite web | url=https://portal.ct.gov/PURA/Gas-Pipeline-Safety/What-To-Do-If--You-Smell-Propane-Gas#:~:text=Propane%20gas%20has%20no%20odor,distinctive%20%E2%80%9Crotten%20egg%E2%80%9D%20smell. | title=What to do if You Smell Propane Gas }}</ref> At normal pressure it liquifies below its [[boiling point]] at −42&nbsp;°C and solidifies below its [[melting point]] at −187.7&nbsp;°C. Propane crystallizes in the [[space group]] P2<sub>1</sub>/n.<ref>{{cite web| url = https://log-web.de/chemie/Start.htm?name=propaneCryst&lang=en| title = geometry of crystalline propane}}</ref><ref>{{cite journal |vauthors=Boese R, Weiss HC, Blaser D | title= The melting point alternation in the short-chain ''n''-alkanes: Single-crystal X-ray analyses of propane at 30&nbsp;K and of ''n''-butane to ''n''-nonane at 90&nbsp;K | journal= Angew Chem Int Ed | year=1999 | volume=38 | pages=988–992 |doi=10.1002/(SICI)1521-3773(19990401)38:7<988::AID-ANIE988>3.3.CO;2-S| doi-broken-date= 25 January 2025 }}</ref> The low space-filling of 58.5% (at 90 K), due to the bad stacking properties of the molecule, is the reason for the particularly low melting point.

Propane undergoes [[combustion]] reactions in a similar fashion to other [[alkane]]s. In the presence of excess oxygen, propane burns to form water and [[carbon dioxide]].
<chem display="block">C3H8 + 5 O2 -> 3 CO2 + 4 H2O + heat </chem>
When insufficient oxygen is present for complete combustion, [[carbon monoxide]], soot ([[carbon]]), or both, are formed as well:
<chem display="block">C3H8 + 9/2 O2 -> 2 CO2 + CO + 4 H2O + heat </chem>
<chem display="block">C3H8 + 2 O2 -> 3 C + 4 H2O + heat </chem>
The complete combustion of propane produces about 50 MJ/kg of heat.<ref name="nist" />

Propane combustion is much cleaner than that of coal or unleaded gasoline. Propane's per-BTU production of CO<sub>2</sub> is almost as low as that of natural gas.<ref name="eiagov">{{cite web |title=How much carbon dioxide is produced when different fuels are burned | url=https://www.eia.gov/tools/faqs/faq.php?id=73&t=11 | author=United States Energy Information Association|access-date=2019-03-25}}</ref> Propane burns hotter than home heating oil or diesel fuel because of the very high hydrogen content. The presence of [[Carbon–carbon bond|C–C bonds]], plus the multiple bonds of [[propylene]] and [[butene|butylene]], produce organic exhausts besides carbon dioxide and water vapor during typical combustion. These bonds also cause propane to burn with a visible flame.

=== Energy content ===
The [[enthalpy]] of combustion of propane gas where all products return to standard state, for example where water returns to its liquid state at standard temperature (known as [[Heat of combustion#Higher heating value|higher heating value]]), is (2,219.2 ± 0.5) kJ/mol, or (50.33 ± 0.01) MJ/kg.<ref name="nist">[http://webbook.nist.gov/cgi/cbook.cgi?ID=C74986&Units=SI&Mask=1#Thermo-Gas Propane]. NIST Standard Reference Data referring to {{Cite journal | last1 = Pittam | first1 = D. A. | last2 = Pilcher | first2 = G. | doi = 10.1039/f19726802224 | title = Measurements of heats of combustion by flame calorimetry. Part 8.—Methane, ethane, propane, n-butane and 2-methylpropane | journal = Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases | volume = 68 | pages = 2224 | year = 1972 }} and {{cite journal|author=Rossini, F.D.|title= Calorimetric determination of the heats of combustion of ethane, propane, normal butane, and normal pentane|journal=  Bureau of Standards Journal of Research|year= 1934|volume= 12|issue= 6|pages= [https://archive.org/details/calorimetricdete12673ross/page/735 735]–750|url=https://archive.org/details/calorimetricdete12673ross|doi=10.6028/jres.012.059|doi-access= free}}</ref>

The enthalpy of combustion of propane gas where products do not return to standard state, for example where the hot gases including water vapor exit a chimney, (known as [[Heat of combustion#Lower heating value|lower heating value]]) is −2043.455 kJ/mol.<ref>{{cite book|last1=Ҫengel|first1=Yunus A.|last2=Boles|first2=Michael A.|title=Thermodynamics: An Engineering Approach|date=2006|publisher=McGraw Hill|isbn=978-0-07-288495-1|page=925|edition=Fifth}}</ref>  The lower heat value is the amount of heat available from burning the substance where the combustion products are vented to the atmosphere; for example, the heat from a fireplace when the flue is open.

=== Density ===
The density of propane gas at 25&nbsp;°C (77&nbsp;°F) is 1.808&nbsp;kg/m<sup>3</sup>, about 1.5× the density of air at the same temperature. The density of liquid propane at 25&nbsp;°C (77&nbsp;°F) is 0.493&nbsp;g/cm<sup>3</sup>, which is equivalent to 4.11 pounds per U.S. liquid gallon or 493&nbsp;g/L. Propane expands at 1.5% per 10&nbsp;°F. Thus, liquid propane has a density of approximately 4.2 pounds per gallon (504&nbsp;g/L) at 60&nbsp;°F (15.6&nbsp;°C).<ref>{{Cite journal|last=Razmi|first=Amir|date=May 2019|title=Propylene Production by Propane Dehydrogenation (PDH)|url=https://www.slideshare.net/AmirRazmi/propylene-production-by-propane-dehydrogenation-pdh|journal=Engineering|pages=3}}</ref>

As the density of propane changes with temperature, this fact must be considered every time when the application is connected with safety or custody transfer operations.<ref>{{Cite journal|last=Zivenko|first=Oleksiy|title=LPG Accounting Specificity During ITS Storage and Transportation|date=2019|journal=Measuring Equipment and Metrology|language=en|volume=80|issue=3|pages=21–27|doi=10.23939/istcmtm2019.03.021|s2cid=211776025|issn=0368-6418|doi-access=free}}</ref>
[[File:Propane butane liquid vapor density zivenko.png|alt=Temperature-Density Curve for Propane|thumb|Temperature–density curve for liquid/vapor propane]]