Crude oil, coal, and natural gas supply about 85% of the energy used in the world. These fuels are called fossil fuels because they were formed hundreds of millions of years ago by the transformation of dead organic matter. Fossil fuels are valuable as sources of energy because they contain hydrocarbons and other carbon-based materials.

World energy use in successively increasing detail. Credit: Wikipedia

In chemistry, hydrocarbons are any of a class of organic compounds composed only of carbon and hydrogen. The carbon atoms form the framework or “skeleton’” and the hydrogen atoms attach to them. Hydrocarbons are the principal constituents of petroleum (crude oil) and natural gas that serve as fuels, lubricants, and raw materials for production of plastics, rubbers, solvents, explosives, and industrial chemicals.  In the process of combustion, hydrocarbons burn to carbon dioxide (CO₂) and water (H₂O) with enough oxygen (O₂), or to carbon monoxide (CO) without it. 

Hydrocarbons are the simplest organic compounds and act as “parent” substances from which other organic compounds are derived. Hydrocarbons are classified into two major groups, open-chain and cyclic. Open-chain compounds contain more than one carbon atom that are attached to each other to form an open chain. Cyclic compounds have carbon atoms that one or more closed rings. The two major groups are subdivided according to chemical behavior into alkanes (saturated compounds) and alkenes (unsaturated compounds).

Some simple hydrocarbons. Credit: Chemistryland.com.

Hydrocarbons can be gases (e.g. methane and propane), liquids (e.g. hexane and benzene), waxes or low melting solids (e.g. paraffin wax and naphthalene) or polymers (e.g. polyethylene, polypropylene and polystyrene).

Natural gas extracted from the Earth is a mixture of lightweight alkanes.  A typical sample of natural gas when it is collected at its source contains 80% methane (CH₄), 7% ethane (C₂H₆), 6% propane (C₃H₈), 4% butane and isobutane (C₄H₁₀), and 3% pentanes (C₅H₁₂). The C3, C4, and C5 hydrocarbons are removed before the gas is sold. The commercial natural gas delivered to the customer is therefore primarily a mixture of methane and ethane. The propane and butanes removed from natural gas are usually liquefied under pressure and sold as liquefied petroleum gases (LPG).

Crude oil is a complex liquid mixture of hydrocarbons and lesser quantities of other organic molecules containing sulfur (S), oxygen (O), nitrogen (N) and some metals. Various crude oils range  between 50 and 95% hydrocarbon by weight. The major classes of hydrocarbons in crude oil include paraffins (methane, ethane, propane, butane); aromatics (benzene, naphthalene), and napthenes (cyclohexane, methyl cyclopentane).

Coal is readily combustible black or brownish-black rock whose composition, including inherent moisture, consists of more than 50 percent by weight and more than 70 percent by volume of carbonaceous (carbon-rich) material. Coal is rich in hydrocarbons, but technically coal is not a hydrocarbon (like methane) because it contains a variety other chemical elements such as oxygen,nitrogen, sulfur, mercury, cadmium, titanium, chlorine, among others.  However, like natural gas and crude oil, the carbon-rich nature of coal enables it to be burned in the process of combustion, making it a valuable source of energy for society.

Coal tar is viscous black liquid containing numerous hydrocarbons that is obtained by the destructive distillation of coal. It is used as a roofing, waterproofing, and insulating compound and as a raw material for many dyes, drugs, and paints.  Coal tar is used in medicated shampoo, soap and ointment, as a treatment for dandruff and psoriasis, and is used to kill and repel head lice.

Oil refineries convert crude oil into a variety of fuels and material feedstocks. Credit: USDOE

Natural gas, petroleum, and coal tar are important sources of many hydrocarbons, some of which are fuels such as motor gasoline, others of which are materials such as oils and waxes. For example, the process of petroleum refining converts complex mixtures in crude oil into simpler mixtures or pure substances by fractional distillation, the separation of a mixture of its component parts based on their different boiling points. Refineries also convert one kind of hydrocarbon to a more useful kind by a process known as cracking, the breaking up large hydrocarbon molecules into smaller and more useful forms using temperature, pressure and sometimes catalysts  The alkanes in crude oil from pentane (C₅H₁₂) to octane (C₈H₁₈) are refined into gasoline, the ones from nonane (C₉H₂₀) to hexadecane (C₁₆H₃₄) into diesel fuel and kerosene (primary component of many types of jet fuel), and the ones from hexadecane upwards into fuel oil and lubricating oil.

A typical set of petroleum fractions is given in the table below. Since there are a number of factors that influence the boiling point of a hydrocarbon, these petroleum fractions are complex mixtures. More than 500 different hydrocarbons have been identified in the gasoline fraction, for example.

Sources

• Bodner Group, Division of Chemistry Education, Purdue University, General Chemistry, Accessed 7 May 2009.
• Carnegie Mellon University, Environmental Decision Making, Science, and Technology web site, Accessed 7 May 2009.
• Wikipedia contributors, Hydrocarbon, Wikipedia The Free Encyclopedia, Accessed 7 May 2009.