Chemistry - Crude Oil (AQA Syllabus A)

One of the topics covered in GCSE Science is the chemistry of crude oil and other fuels. This is the first if six quizzes on that subject and it looks in particular at the hydrocarbons and alkanes which are found in crude oil.

Crude oil formed over a long period of time - millions of years in fact. It formed from the remains of dead sea creatures and plants that rotted down anaerobically (with little or no oxygen) as they became buried with sediments on ancient sea beds. The oil is trapped underground by layers of impermeable rocks and is extracted from the ground by drilling oil wells.

But what is the chemistry of crude oil? Well, it is a mixture of thousands of different chemicals which are mainly hydrocarbons. These are covalently bonded molecules made mainly from the elements hydrogen and carbon, in fact, most are only made from those two elements and are that is why they are called hydrocarbons.

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The main types of hydrocarbon in crude oil come from the family of chemicals known as the alkanes - for example, octane, an important component of the petrol that we use as fuel for cars. The larger alkanes with carbon chain lengths of over 35 atoms are soft solids and are used in the construction industry for roofing and roads.

Crude oil is also always associated with another fossil fuel, gas, because some of the molecules of the alkane family are very small. Small covalently bonded molecules are often gases at room temperature. Natural gas is methane, the simplest alkane, and it is piped from the oil fields of the north sea to homes and businesses all over Britain.

But chemistry allows us to use crude oil for more than just fuel. There is more bitumen than can be used. To save throwing it away, it can be 'cracked'. This involves using heat and catalysts to break down the longer carbon chains into smaller molecules, including petroleum and molecules like ethene, which can be used to make plastics.

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  1. Crude oil is a mixture of what?
    The chemical properties of each substance in the mixture are unchanged. It is possible to separate the substances in a mixture by physical methods
  2. What are hydrocarbons?
    The hydrogen and carbon are covalently bonded. The carbon can be a single atom (for example, methane) or can be arranged as a chain (for example, octane has eight carbon atoms joined together in a chain)
  3. Which of the following best describes most of the hydrocarbons in crude oil?
    Saturated means that there are no double bonds in the molecule
  4. Which of the following best describes the saturated hydrocarbons in oil?
    You should be able to recognise alkanes from their formulae (drawn or written) but you only need to know the names of the first four - methane, ethane, propane and butane - for the exam
  5. The formula of any alkane can be worked out from which of the following?
    The letter 'n' represents the number of carbon atoms so an alkane with 20 carbon atoms would have 42 hydrogen atoms
  6. How are the different hydrocarbons separated from crude oil?
    This is so-called because the crude oil is separated into fractions
  7. Which of the following lists contains the words missing from the following passage in the correct order?

    The many __________ in crude oil may be separated into fractions by ___________ the oil and allowing it to condense at a number of different ___________. This process is ___________ distillation.
    This is the first step to converting crude oil into useful products
  8. Which of the following best describes the smaller hydrocarbons when compared to large hydrocarbons?
    The smallest molecules from fractional distillation of crude oil are so volatile that they are gases at ordinary temperatures
  9. The process of breaking down of the larger hydrocarbons to smaller, more useful chemicals is called what?
    Cracking is carried out using heat and a catalyst
  10. Crude oil is made from alkanes. During cracking what other type of hydrocarbon is produced and how is it different from the alkanes?
    Cracking of a long chain alkane produces a shorter chain alkane and an alkene. The characteristic of an alkene is that it has at least one double bond between two adjacent carbon atoms

Author: Kev Woodward

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