Hydrocarbons

A hydrocarbon is an organic compound composed of two elements, hydrogen and carbon.  A large part of the composition of petroleum is made up of hydrocarbons of varying lengths.  The smallest hydrocarbon, methane, is composed of a single carbon atom and four hydrogen atoms.  However, hydrocarbons can literally consist of hundreds or thousands of individual atoms that are linked together in any number of ways, including chains, circles, and other complex shapes.

Properties

Because the structure of different hydrocarbons can vary so drastically, the properties of each class of molecule vary greatly as well. In order to help categorize the properties of hydrocarbons, they are broken up into several basic types.

• Alkanes – These are referred to as saturated hydrocarbons. Saturated has a specific definition in terms of carbon-based molecules. Carbon can form up to four separate bonds with four separate other atoms. However, it is also possible for carbon to form multiple bonds with a single atom, even another carbon atom. When two carbon atoms in a hydrocarbon are linked together by two or more bonds rather than one, the molecule is termed unsaturated. All alkanes are saturated, which means they only contain single bonds between all carbon atoms. Alkanes are the basis of petroleum fuels and are found in linear and branched forms.

• Unsaturated Hydrocarbons – Those hydrocarbons that have one or more double bonds between carbon atoms are called alkenes. Those with one or more triple bonds between carbon atoms are called alkynes. These are mixed with alkanes in petroleum and contribute more carbon dioxide per pound than do saturated hydrocarbons.

• Cycloalkanes – Any hydrocarbon containing one or more ring structures. These are generally used for the same functions as the non-cyclic alkanes, though they have additional uses in creating certain plastics and in pharmaceutical bases.

• Aromatic Hydrocarbons – This class of molecules has specialized ring structures where bonds between carbon atoms are an intermediate between single and double bonds. Molecules in this class include the industrial solvent benzene.

The structure, hydrogen to carbon ratio, and the length of a particular hydrocarbon determine its properties. In general, small linear hydrocarbons will be gases while medium sized linear hydrocarbons will be liquids. Branched hydrocarbons of intermediate size tend to be waxes with low melting points. Long hydrocarbons tend to be semi-solid or solid. Unsaturated hydrocarbons are more likely to be solid than their saturated counterparts as are cyclic hydrocarbons.

Hydrocarbons in Fuel

Hydrocarbons containing between six and 10 carbon molecules are the top components of most fuels, regardless of whether they are alkanes, alkenes, or cyclic. In general, these molecules are burned to produce energy.

Burning hydrocarbons requires oxygen. The hydrocarbon and oxygen combine, in a process called combustion, to produce water, carbon dioxide, and energy. Of course, these molecules are not the only products of the combustion of hydrocarbon. Hydrocarbons that are contaminated with atoms such as sulfur and nitrogen will also produce nitrogen dioxide and sulfur dioxide.

Because hydrocarbons are composed purely of carbon and hydrogen, their combustion with oxygen can only produce water as a result of the combination between hydrogen and oxygen and carbon dioxide as a result of the combination of carbon and oxygen. The energy produced by burning a hydrocarbon comes from breaking both carbon-hydrogen and carbon-carbon bonds and recombining them into carbon-oxygen and hydrogen-oxygen bonds. Because an unsaturated hydrocarbon has fewer hydrogen carbon bonds, it has less hydrogen per molecule than a similar unsaturated hydrocarbon and will produce more carbon dioxide. This also means unsaturated hydrocarbons produce less energy when burned than do saturated hydrocarbons. In order to gain the same amount of energy, a greater quantity of unsaturated hydrocarbon must be burned and as a result more carbon dioxide is created in the process. Thus, unsaturated hydrocarbons are less environmentally friendly than saturated hydrocarbons.

Beyond the release of carbon dioxide, burning hydrocarbons also releases other contaminants into the atmosphere. Because refining hydrocarbons is not perfect process, all fuels will contain some level of contaminants. During combustion, sulfur combines with oxygen to produce sulfur dioxide. Sulfur dioxide later combines with hydrogen in the atmosphere to produce the weak sulfurous acid as well as the strong sulfuric acid. Both of these contribute to acid rain. In addition to sulfur, nitrogen is also a common contaminant in hydrocarbons. Nitrogen dioxide can react with hydrogen in the atmosphere to produce nitric acid, which also contributes to acid rain.

Common Hydrocarbons and Their Uses

 
Hydrocarbons longer than 10 carbon atoms in length are generally broken down through the process known as “cracking” to yield molecules with lengths of 10 atoms or less.