Carbon Compounds as Fuels and Feedstock

1. What is "crude oil" ?

Answer: Crude oil is a finite resource found in rocks. It is a mixture of hydrocarbons.

2. What is a "hydrocarbon" ?

Answer: A compound containing only hydrogen and carbon atoms.

3. What is a "homologous series" ?

Answer: A family of (organic) similar chemicals. Each successive member differs by  - CH2 -  and they have similar chemical properties as they have the same functional group.

4. What is the general formula of an alkane?

Answer: CnH2n+2

5. List the names of the first four members of the alkanes.

Answer: methane, ethane, propane and butane

6. Name the process used to separate crude oil.

Answer: fractional distillation

7. Write down two things that happen during the combustion of hydrocarbon fuels.

Answer: release of energy (exothermic) ; the carbon and hydrogen are oxidised

8. What are the products of the complete combustion of a hydrocarbon fuel?

Answer: carbon dioxide and water

9. What is "cracking" ?

Answer: Breaking down of long chain hydrocarbons to form smaller (more useful) shorter chain molecules.

10. Name the two types of cracking.

Answer: catalytic cracking and steam cracking

11. What are "alkenes" ?

Answer: A homologous series of hydrocarbons containing at least one C=C (unsaturated).

12. Crude oil is a mixture of hydrocarbons. Methane (CH4 ) is a hydrocarbon. Write down the number of atoms in methane, and describe the bonding in it.

Answer: There are five atoms in methane, and it has covalent bonding made from a shared pair of electrons.

13. Alkanes are a homologous series. The figure below shows a diagram of two alkanes, ethane and butane.

a) Give the molecular formula of ethane.

Answer: C2H6

b) Name the technique that could be used to separate ethane and butane.

Answer: fractional distillation

c) Suggest which of these alkanes would have the highest boiling point.

Answer: Butane as it’s a large molecule (high relative formula mass). This means larger forces of attraction between molecules and so more energy is needed to overcome these forces.

d) Write down the molecular formula for an alkane with 10 carbon atoms.

Answer: C10H22

14. Crude oil is a mixture of hydrocarbons. It is separated using fractional distillation. Many of the products are used as fuels.

a) Decane (C10H22 ) is used in petrol. Write the word equation for the combustion of decane.

Answer: decane + oxygen  →  carbon dioxide + water

b) Is this reaction exothermic or endothermic?

Answer: Exothermic, as energy is released in the form of heat and light.

c) Pentadecane (C15H32 ) is a different alkane found in petrol. Compare the physical properties of decane and pentadecane.

Answer: Pentadecane has a higher boiling point, more viscous and ignites less easily.

15. Crude oil is a mixture of hydrocarbons. A teacher demonstrated how these can be separated in a laboratory using fractional distillation. The figure below shows the equipment used.

a) Explain why the bulb of the thermometer must be in line with the side arm.

Answer: To measure the temperature of the vapour and use this to help identify the fraction being collected.

b) Suggest a safer alternative to using a Bunsen burner.

Answer: Use another source of heat that is not a naked flame e.g. a heating mantle/water or oil bath. This is safer because fractions of crude oil are flammable and exposure to naked flames can cause a fire.

16. A student is breaking down long chain hydrocarbons in a laboratory using catalytic cracking. The figure below shows the equipment used.

a) Identify a hazard in this experiment. Describe how the risk could be reduced.

Answer: Hazards in this experiment are naked flame and suck back. The risk can be reduced by usind a heating mantle, adding a safety valve on the delivery tube, etc.

b) Describe a chemical test to show that an alkene has been made. Include any observations with your answer.

Answer: Take a sample of the reactant and the product. Add bromine (water) to each and shake. It will decolourise with the product showing it was an alkene, but there will be no colour change with the paraffin.

c) The porcelain chips act as a catalyst. Explain the purpose of the catalyst in this reaction.

Answer: Does not get used up, creates an alternative pathway with lower activation energy, speeds up the reaction and allows the reaction to happen at lower temperatures.

17. Alkanes are a family of hydrocarbons. The table below shows information about the first four members of the alkane homologous series.

a) Write down the missing data from boxes X, Y and Z in the table.

Answer: X = CH4  ;  Y = gas  ;  Z = 3

b) Describe the relationship between number of carbon atoms and boiling point.

Answer: As the number of carbon atoms increases, so does the boiling point.

c) Explain the effect of changing the number of carbon atoms on the melting point of an alkane.

Answer: Overall, the greater the number of carbon atoms, the higher the melting point. There is a larger forces of attraction between molecules, and so more energy is needed to overcome these forces (to melt the compound).

18. The petrochemical industry uses some of the heavier fractions to make petrol.

a) Justify the classification of cracking as thermal decomposition.

Answer: Heat is used (endothermic reaction) to break down a chemical into simpler substances.

b) Explain why steam cracking uses less energy than catalytic cracking. In your answer, you should outline the conditions used for both types of cracking.

Answer: In steam cracking the alkane is heated to high temperatures to vaporise it, and then mixed it with steam. This requires a lot of energy. In catalytic cracking, the alkane is heated to lower temperatures and passed over a hot catalyst. It therefore requires more energy to make steam and get the high temperatures needed for steam cracking than for catalytic cracking.

c) Write the equation that shows the cracking of decane (C10H22 ).

Answer: C10H22  →  2C2H4 + C6H14

19. Crude oil is an important source of organic chemicals.

a) Describe how crude oil is formed.

Answer: Plankton and other organisms died in ancient times. These were buried in mud and over time (thousands of years) became crude oil.

b) Justify why crude oil cannot be described as pure.

Answer: It is more than one chemical.

c) Explain why it is necessary to separate crude oil.

Answer: Crude oil is a mixture of hydrocarbons. The mixture is more useful when it is separated into smaller ranges or hydrocarbons.

d) Explain why petrol is a formulation but the petrol fraction from a fractionating column is not.

Answer: A petrol fraction is a mixture of hydrocarbons, but the ratios of the mixture may be different for each fractionating column. Petrol is a mixture that has been designed to be a useful product and so is a formulation.

20. Alkanes are a homologous series of chemicals.

a) Draw the displayed formula of propane.


b) Write a balanced symbol equation for the complete combustion of propane.

Answer: C3H8 + 5O2  →  3CO2 + 4H2O

c) Is this reaction reduction or oxidation?

Answer: Oxidation, as propane is a hydrocarbon, containing only hydrogen and carbon atoms. The hydrogen gains oxygen to make water, while carbon atoms gain oxygen to make carbon dioxide.

21. Paraffin (C10H22 ) can undergo cracking to make ethene (C2H4 ) and one other product.

a) Write a balanced symbol equation for this reaction.

Answer: C10H22  →  C2H4 + C8H18

b) Draw a dot and cross diagram to show the bonding in ethene.


c) Describe the similarities and differences between ethene and ethane. Give your answer in terms of structure and bonding.

Answer: Both ethane and ethene have covalent bonds, are hydrocarbons, have the same number of atoms, and they have a simple molecular structure. Ethane only contains single bonds but ethene has one double bond. Ethane has two more hydrogen atoms than ethene.

d) Suggest a use for alkenes.

Answer: polymers, medicine and starting point for many other chemicals

22. In industry, crude oil is separated using fractional distillation. The figure below shows an industrial fractionating column. Explain how crude oil is separated.

Answer: Crude oil is heated and turned to gas. The fractionating column is hot at the bottom and colder at the top. The gases rise and when they reach their boiling point they condense and are tapped off at different levels. The smaller (lighter) hydrocarbons are collected at the top and the larger (heavier) hydrocarbons are collected at the bottom. Each fraction is still a mixture but has a smaller range of carbon chain length.

23. Petrol and lubricating oil are both fractions of crude oil. A student has two unnamed samples which they want to identify. Suggest how the student could determine which sample is petrol and which is lubricating oil by using colour, viscosity and flammability. Your answers should describe how the student should carry out each test and the results they would obtain.

Answer: Using colour: Take a sample of each, ensure they are the same depth and compare the colour of the two samples. Petrol would be lighter in colour than lubricating oil, as the carbon chain length is less.
Using viscosity: Take a sample of each and drop it onto a slope. The one that runs down the slope the quickest is petrol.
Using flammability: Take a small amount of each and try to ignite them. Petrol would be more flammable than lubricating oil.

24. Long chain hydrocarbons may undergo cracking to make smaller molecules. Cracking is often random, and a mixture of products is made.

a) Write down the molecular formula for an alkane with 25 carbon atoms.

Answer: C25H52

b) Write a balanced symbol equation for the thermal cracking of octane (C8H18 ) to make two ethene (C2H4 ) molecules and one other product.

Answer: C8H18  →  2C2H4 + C4H10

c) A different cracking reaction makes a colourless, odourless gas which ignites with a squeaky pop. Identify the gas.

Answer: hydrogen

d) Suggest how the products of cracking could be separated.

Answer: fractional distillation