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Chemistry - Other Uses of Crude Oil (AQA Syllabus A)

One of the major topics in GCSE Science is crude oil and other sources of fuel. This is the fourth of six quizzes on that topic and it looks specifically at some of the many different uses of crude oil, other than as a source of fuel.

When crude oil is separated into its fractions many of the hydrocarbons can be used as fuels. These range from gases, like propane and butane, to the heavy fuel oil that is used to power ships. The greater the number of carbon atoms in the hydrocarbon, the more energy it releases when it is burnt. But, despite being a valuable source of energy, there is a downside to using hydrocarbons as fuel. Burning them releases carbon dioxide and other pollutants into the air, causing global warming and other types of environmental damage.

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When crude oil became widely available in the middle of the nineteenth century, it wasn't long before the process of cracking was developed. This is used to break down the longer molecules into smaller and more reactive ones. There are many different ways of cracking crude oil chemicals but the one thing they all have in common is they require high temperatures. In schools, the only method of cracking you are likely to have seen (or done for yourself) is catalytic cracking. In this, a catalyst of broken pot is heated up to a high temperature and when a long molecule (for example those in liquid paraffin) passes over it is broken down to ethene and a smaller alkane. Other methods of cracking use high temperatures, high pressures and steam.

Cracking makes crude oil the source of other things besides fuel. Once cracking had been perfected, scientists found uses for the alkenes that were produced. They could be used to manufacture many new materials. You will probably be most familair with crude oil as a raw material for plastics but it is also used in a wide range of other products like antiseptics, drugs, inks, lubricating oils, fertilisers and many more. The uses of crude oil are expanded greatly by cracking, making it a source of many different products. Cracking is an important process as it enables us to use virtually everything in crude oil.

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  1. Why is the residue fraction often used for cracking?
    It also has large molecules which can be cracked in different ways to provide various different chemicals for industry
  2. What is the key difference between the alkanes and the alkenes that are produced by cracking, and what is the cause?
    It is their increased reactivitiy that makes the alkenes particularly useful. Being unsaturated means there is a double bond between at least one pair of carbon atoms. This is easier to break than the single bonds in the alkanes
  3. Ethanol can be added to petrol used in car engines. Why is this?
    Another reason is that petrol reserves are limited and diluting it with ethanol will make it last longer
  4. Ethanol can be made by fermenting plants or from the ethene obtained by cracking crude oil. What would be a disadvantage of making it from oil?
    So although pollution is less, because it too is made from oil, it will no longer be available when oil reserves are exhausted
  5. Which of the following represents a carbon to carbon double bond in an alkene?
    When drawing the structure of alkenes, the symbol for the double bond resembles an equals sign. You only need to know the names of the first two alkenes - ethene and propene
  6. What is the general formula for any alkene?
    Having a double bond means that there will always be 2 hydrogen atoms less in an alkene when compared to the equivalent alkane
  7. During the cracking of alkanes from crude oil, one of the products is:
    Alkenes can be used to make any of the other 3 chemcals mentioned in this question, which is why they are a great starting point for making other products from crude oil
  8. What is the test used to detect an alkene and what is the result?
    The orange colour in bromine water is from the bromine molecules. When you add an alkene, the bromine molecules react with the double bond and become attached to the alkene molecules. This is why the colour disappears
  9. Which of the fractions of oil is most often used to manufacture chemicals instead of being used as a fuel?
    The naptha fraction comprises molecules that have 6 - 11 carbon atoms in their chain
  10. Read the following passage. Which answer gives the missing words in the correct order?

    There are two good reasons for cracking oil __________. Firstly, there just aren't enough fuels like petrol or _________ in the original crude oil to meet demand. Secondly, _______ that are needed to manufacture other chemicals are not found in oil but can be made from it. Cracking means the vast majority of crude oil can be turned into useful products and not just burnt as ______.
    Cracking is an important process as it enables us to use virtually everything in crude oil

Author: Kev Woodward

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