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An overview of alkanes, a family of hydrocarbons found in crude oil. It explains their chemical structure, the process of distillation used to extract them, and their uses as fuels. Displayed formulas, molecular models, and a diagram of the fractional distillation process.
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Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels. Crude oil forms naturally over millions of years from the remains of living things. Most of the compounds in crude oil are hydrocarbons. These are compounds that contain hydrogen and carbon atoms only, joined together by chemical bonds called covalent bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes. The alkanes are a family of hydrocarbons that share the same general formula. This is: CnH2n+ The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH 4 and ethane is C 2 H 6. Alkane molecules can be represented by displayed formulas. In a displayed formula, each atom is shown as its symbol (C or H) and each covalent bond by a straight line. This table shows four different alkanes.
Notice that the molecular models show that the bonds are not really at 90°C, but this makes displayed formulas easier to draw. Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from burning or combustion, which is their reaction with oxygen in the air. Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water. Distillation process to separate ethanol from water
Step 1 - water and ethanol solution are heated The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid. The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates. This is the sequence of events in distillation: heating → evaporating → cooling → condensing Fractional distillation is different from distillation in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top. The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
The diagram below summarises the main fractions from crude oil and their uses, and the trends in properties. Note that the gases leave at the top of the column, the liquids condense in the middle and the solids stay at the bottom.