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Chemistry - Hydrocarbons and Fuels (AQA Syllabus A)

In GCSE Science one topic studied is crude oil and other types of fuel. This is the second of six quizzes on that subject and it looks specifically at hydrocarbons - the compounds of carbon and hydrogen in crude oils.

Most of the compounds found in crude oil are hydrocarbons. They are so-called because they only contain hydrogen and carbon atoms, which are joined together by covalent chemical bonds. There are different families of hydrocarbon, each containing a variety of related chemicals. Most of the ones in crude oil are from the alkane family.

Alkanes are saturated hydrocarbons with one or more carbon atoms. Where there is more than one carbon atom, they are joined in chains, for example, butane. Butane has four carbon atoms in a chain. The carbon atoms at the end of each chain have three hydrogen atoms attached, the ones in the middle of the chain have only two attached. Saturated means that their carbon atoms are joined to each other only by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.

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Hydrocarbon compounds have different boiling points, and can be either solid, liquid or gas at room temperature:

  • Small hydrocarbons with only a few carbon atoms have low melting and boiling points and are therefore gases at room temperature.
  • Hydrocarbons that have five to about 18 carbon atoms have moderate melting and boiling points are usually liquids at room temperature.
  • Large hydrocarbons with many carbon atoms have high melting and boiling points and are solids.

Because they have different boiling points, the hydrocarbons in crude oil can be separated using fractional distillation. The crude oil is evaporated and injected into a fractionating column. The fractionating column is actually just a condenser which allows the vapourised crude oil to condense at different temperatures at different heights, thus enabling the fractions to be collected. Each fraction contains hydrocarbon molecules with a similar, but not identical, number of carbon atoms.

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  1. During the combustion of hydrocarbons, what happens to the carbon and hydrogen atoms?
    All combustion reactions involve oxidation of the atoms of the fuel
  2. Soot is mainly particles of what?
    Soot is a product of the incomplete combustion of hydrocarbons
  3. Some properties of hydrocarbons depend on the size of their molecules. These properties influence how hydrocarbons are used as fuels. A hydrocarbon with 30 carbon atoms is unlikely to be used as a fuel in vehicles. Why not?
    Liquids are the only practical fuels for vehicles
  4. 16g of methane required 64g of oxygen to burn completely producing 44g of carbon dioxide. How many grams of water were produced?
    The examiners could throw something like this at you in any question in order to test your knowledge of the law of conservation of mass in chemical reactions. To get the right answer, you only need to be able to add and subtract! Work out how many grams of chemical that you started with then look how many are missing at the end. That is your answer
  5. Which of these is the correct equation for the complete combustion of propane?
    You should have immediately dismissed the first two as they are not propane. The third and fourth answers are correctly balanced equations for propane but the fourth one shows the incomplete combustion of propane
  6. Which of the following is an alkane?
    Testing to see if you know that the general formula for alkanes is CnH2n+2 where n is the number of carbon atoms in the hydrocarbon
  7. There are environmental costs when using hydrocarbons as fuels. Using them in this way produces a number of pollutants. Which one of the following contributes to acid rain?
    Whilst carbon dioxide does make rainwater slightly acidic, it is the sulfur dioxide (and nitrogen oxides) that takes the pH below 6, creating the acid rain
  8. When we use hydrocarbons as fuels they produce particulates (small particles of solids e.g. soot). What environmental problem is caused by the release of these solids?
    They block some of the Sun's rays from reaching the surface of the Earth
  9. What is a hydrocarbon?
    It's all in the name! Although hydrocarbons are probably the most important fuel we use, they are by no means the only one
  10. Which of the following is not a product of burning hydrocarbons as fuels?
    Oxygen is required for burning hydrocarbons as fuels

Author: Kev Woodward

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