Chemistry - Processing Crude Oil (AQA Syllabus A)

Crude oil and other fuels is one area studied in GCSE Science. This is the fifth of six quizzes on that topic and it looks specifically at two methods of processing crude oil - 'cracking' and 'fractioning'.

Crude oil is formed from the remains of dead sea plants and creatures that became buried in the sediments of the sea floor millions of years ago. Ancient humans used crude oil that they found as 'pitch'. This gathered as pools where oil bearing rocks were exposed at the surface of the Earth. They used this thick and gooey substance for waterproofing their huts and perhaps even on the planks they used to make their boats. Come the modern era things changed. After the discovery was made of how to drill for it, crude oil become one of the most useful resources that we take from the Earth's crust.

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Various methods of processing are required before any use can be made of crude oil. Oil straight from the ground is a complex mixture of many thousands of chemical compounds which, when separated, can be used in many ways - as fuels, for making plastics, for making medicines, and much more besides. The pitch used by early humans was processed by natural means - the volatile chemicals evaporated into the air leaving the less volatile chemicals behind as pitch. We still use pitch for making waterproof building materials like roofing felt and for glueing together small stones to make road surfaces.

Mixtures of liquids can be separated by distillation. This works only if the liquids have different boiling points. The chemicals in crude oil have a wide range of boiling points so the process of fractional distillation (or fractioning) is used. This process allows complex mixtures of liquids to be separated because of their different boiling points. To obtain even more useful products from the 'fractions' of oil, some of the chemicals are 'cracked'. Cracking involves using heat and a catalyst, or steam and very high temperatures, to break longer molecules into shorter ones that are more reactive and can be used as polymers.

Take this quiz to test your knowledge of fractioning and cracking, two methods used in the processing of crude oil.

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  1. The chemicals from crude oil are mainly alkanes. Which of the following statements best describes alkanes?
    Hydrocarbons with only single bonds between the carbon atoms are called saturated
  2. The demand for certain fractions of oil is greater or smaller than the output, for example, there is not enough of the fraction containing the chemicals used as fuel for cars, and there is more of the bitumen fraction than can be used. How is this problem solved?
    Cracking of larger molecules breaks them down into smaller and more useful molecules which saves having to waste them
  3. What happens to the chemicals in the crude oil vapour as they rise?
    When they condense, the chemicals form droplets of liquid that fall back down the column onto a collecting tray and can be removed from the fractionating column
  4. Most of the fractions of crude oil are used as what?
    About the only fraction not used as a fuel is the bitumen fraction
  5. Crude oil is put into the fractionating column in what form?
    The crude oil is heated to a temperature where all of the chemicals have boiled and then injected into the fractional distillation column to rise and cool, allowing the different hydrocarbons to condense at different levels
  6. Why does fractional distillation of crude oil work?
    This allows them to be condensed from hot, vapourised crude oil at different places in a fractional distillation column, thus separating the mixture
  7. Which of the following is not required for cracking?
    Cracking of the long carbon chain alkanes from crude oil produces a shorter carbon chain alkane and an alkene. As well as this 'catalytic' cracking, other methods such as steam cracking are also used
  8. Where are the coolest temperatures in the fractionating column?
    The column is cooler the higher up you go
  9. At an oil refinery, how often is oil put into the fractional distillation column?
    You are expected to know that the process is continuous. The fractionating column at an oil refinery is only 'switched off' if there is a breakdown or if maintenance is required
  10. Which of the following statements best describes the molecules in crude oil?
    There is a wide range of sizes of molecules in crude oil, none of which are chemically combined with each other - crude oil is a mixture

Author: Kev Woodward

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