The Propane Molecule
A three-carbon alkane, propane is sometimes derived from other petroleum products during oil or natural gas processing. Propane has the Chemical Formula: C3H8
When commonly sold as fuel, it is also known as liquified petroleum gas (LPG or LP gas) and is a mixture of propane with smaller amounts of propylene, butane and butylene, plus an ethyl mercaptan odorant to allow the normally odorless propane to be smelled. It is used as fuel in cooking on many barbecues and portable stoves and in motor vehicles. Propane powers some buses, forklifts, and taxies and is used for heat and cooking in recreational vehicles and campers. In many rural areas of the US, propane is also used in furnaces, water heaters, laundry driers, and other heat-producing appliances. Delivery trucks fill up large tanks that are permanently installed on the property (sometimes called pigs) or exchange bottles of propane.
Properties and reactions
Propane undergoes combustion reactions in a similar fashion to other alkanes. In the presence of excess oxygen, propane burns to form water and carbon dioxide.
- C3H8 + 5 O2 → 3 CO2 + 4 H2O + heat
When not enough oxygen is present for complete combustion, propane burns to form water and carbon monoxide.
- 2 C3H8 + 7 O2 → 6C O + 8 H2O + heat
Unlike natural gas, propane is heavier than air (1.5 times denser). In its raw state, propane sinks and pools at the floor. Liquid propane will flash to a vapor at atmospheric pressure and appears white due to moisture condensing from the air.
When properly combusted, propane produces about 2,500 BTU per cubic foot of gas (91,600 BTU per liquid gallon). The gross heat of combustion of one normal cubic meter of propane is around 50 megajoules (≈13.8 kWh) or 101 MJ/m3 in SI units.
Propane is nontoxic; however, when abused as an inhalant it poses a mild asphyxiation risk through oxygen deprivation. It must also be noted that commercial product contains hydrocarbons beyond propane, which may increase risk. Propane and its mixtures may cause mild frostbite during rapid expansion.
Propane combustion is much cleaner than gasoline, though not as clean as natural gas. The presence of C-C bonds, plus the multiple bonds of propylene and butylene, create organic exhausts besides carbon dioxide and water vapor during typical combustion. These bonds also cause propane to burn with a visible flame.
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