Crude Oil
Crude oil formed by the action of heat and pressure on the remains of microscopic plants in the ocean over millions of years. These plants absorbed energy from the sun and stored it as carbon compounds in their bodies. When they died their bodies sank to the bottom of the oceans and the remains were buried by sediments which caused anaerobic conditions which did not allow them to decay. As sedimentation continued these remains were put under more and more pressure and heat that eventually turned them into crude oil. The sedimentary rocks crude oil formed under are usually porous and allow fluids and gases to pass through. Since oil is less dense than water, the crude oil passed through the rock and rose up until it naturally seeped onto the land or into the ocean, or it was stopped by a layer of impermeable rock which trapped the crude oil underground.
This crude is a mixture of a number of substances made up of predominately hydrocarbons. Hydrocarbons are compounds which contain primarily hydrogen and carbon atoms. The different hydrocarbons have different boiling points. This range of boiling points allows the separation of the mixture by fractional distillation with a fractionating column. Within this structure, crude oil is heated with high pressure steam to 600 oC and hydrocarbon gases are released. The smaller the hydrocarbon molecule, the further the gas rises up the column before condensing back into a liquid. The larger the molecule is (the more carbons it contains), the higher the condensing temperature will be, the more viscous the fluid will be, and the less volatile and less flammable it is.
To illustrate why crude is so important to our society, you should discuss the different fractions that are separated in the column. The petroleum gas fraction is made up of small compounds with four or less carbons with a boiling point up to 60 oC and is used for heating, cooking, and making plastics. The naphtha fraction is made up of compounds that have between five to nine carbons with a boiling point of 60-100 oC and is the fraction that is further processed to make gasoline. The kerosene fraction is made up of compounds with 10 to 18 carbons and aromatics with a boiling point of 175-325 oC and is used for fuel for jet engines and tractors and as starting materials for a number of products. The gas oil fraction is made of compounds containing 12 or more carbon atoms with a boiling point of 250-350 oC and is used for diesel fuel and heating oil and as starting materials for a number of products. The lubricating oil fraction contains long chain compounds with 20 to 50 carbon atoms with a boiling point of 300-370 oC and is used for motor oil, grease, and other lubricants. The heavy gas fraction is made of long chain compounds with 20 to 70 carbon atoms with a boiling point of 370-600 oC and is used for industrial fuel and as starting materials for a number of products. The residuals from the fractioning column are multiple-ringed compounds with 70 or more carbon atoms with a boiling point of greater than 600 oC and are used for asphalt, tar, waxes, and as starting materials for a number of products [2].
Approximately 90% of the crude oil is converted into some type of fuel. Of the remaining 10%, about 5% is used to produce plastics. The other 5% is used to make dyes, inks, household detergents, pharmaceuticals, and a wide range of compounds suitable for a variety of applications [3]. The distillation process is illustrated in the lesson Distillation of Simulated Crude Oil at the end of this unit. The materials to run this lab can be bought as part of the science and sustainability kit from the science education for public understanding program (SEPUP).
The field of chemistry that studies these hydrocarbons is called organic chemistry. You may think this is an interesting name since anytime when you hear the term "organic" now, you automatically think of the most pure food or substance. These substances derived from hydrocarbons are called organic since early chemists thought that plants and animals were needed to produce them. However, chemists now know how to make organic compounds without any assistance. Chemists can take advantage of the properties of these compounds to derive all sorts of products used in our society. This is made possible by the building block of life, carbon.
Carbon is such a special case in chemistry as a result of the combination of its properties, including the number of valence electrons on a neutral atom, its electronegativity, and its atomic radius. Carbon has four valence electrons, and it must either gain four electrons or lose four electrons. The electronegativity of carbon is too small for carbon to gain electrons from most elements to form C 4 - ions, and too large for it to lose electrons to for C 4 + ions. Carbon forms covalent bonds with a number of other elements, including big players like hydrogen (H), nitrogen (N), oxygen (O), phosphorous (P), and sulfur (S) found in the environment. Because carbon atoms are relatively small, they can come close enough together to form strong C=C double bonds or even triple bonds. Carbon can form strong multiple bonds to nonmetals such as N, O, P, and S. No other element can provide the variety of combinations for life to exist.
Hydrocarbons are named according to the number of carbon and hydrogen atoms they contain. Compounds that contain as many hydrogen atoms as possible are said to be saturated. This group of saturated hydrocarbons is known as the alkanes. The simplest of these is methane (CH 4) this molecule combines the four valance electrons in a neutral carbon atom with four hydrogen atoms. Looking at methane, you can explain to students the general rule that carbon is tetravalent, in other words, it forms a total of four bonds in almost all of its compounds. This structure minimizes the repulsion of the pairs of electrons in the four C—H bonds. If your classroom is equipped with ball and stick molecular models kits, having students build this will illustrate the tetrahedral geometry of the carbon atom.
Have students predict what longer carbon chains would look like and what their chemical formulas would be, using the fact that carbon is tetravalent. Compounds that contain more than one carbon usually are held together by single or double C—C bonds and since they are tetravalent, the formula of the compounds have a pattern. The generic formula of the compounds can be better understood by the students by pointing out that the compounds contain chains of CH 2 groups with an additional hydrogen atom capping either end of the chain. You can have students plug and chug using the following: for every n carbon atoms there must be 2n+2 hydrogen atoms C nH 2 n + 2.
Students have no idea about the myriad of products in our lives that are made from petrochemicals. There are many products used everyday yet they do not fully understand their origin. Using the very complete list of products found on the web [4], have students determine the raw material from crude oil that was used to produce the particular product. Students should research that material and determine the structure of the hydrocarbon compound that it is derived from. Have students research the properties of the material and the other applications and uses for this compound in our society. Students should also research the behavior of the compound once it is released into the environment and any potential ills associated with it. Finally, students should postulate what would happen if we did not have this material to use to make products and what that would mean for our society.
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