Edexcel GCSE Combined Science: Chemistry: Fuels, Earth and Atmospheric Science
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Crude oil is a mixture of many hydrocarbons with different chain lengths. It is separated into useful fractions by fractional distillation in a tall column that is hot at the bottom and cooler at the top.
Explain how fractional distillation separates crude oil into different fractions. In your answer, refer to boiling points and to the temperature gradient in the column. (6 marks)
The table gives some information about three hydrocarbon fractions obtained from crude oil.
| Fraction | Typical number of carbon atoms | Boiling-point range / °C |
|---|---|---|
| Refinery gases | 1–4 | below 25 |
| Kerosene | 11–15 | 150–250 |
| Bitumen | over 35 | above 350 |
(a) Using the data, describe how the boiling-point range changes as the number of carbon atoms in the fraction increases. (1 mark)
(b) Explain, in terms of the forces between molecules, why bitumen has a much higher boiling-point range than the refinery gases. (3 marks)
Methane (CH₄) is a hydrocarbon fuel. When it burns in a plentiful supply of air, it undergoes complete combustion to form carbon dioxide and water.
(a) Write a balanced symbol equation, including state symbols, for the complete combustion of methane. (Assume the water is formed as a gas.) (2 marks)
(b) When methane burns in a limited supply of air, incomplete combustion can occur, producing the toxic gas carbon monoxide. State why carbon monoxide is dangerous to humans. (1 mark)
Long-chain hydrocarbons from crude oil are often broken down into smaller, more useful molecules by cracking.
(a) State why oil companies carry out cracking — that is, why smaller hydrocarbons are more useful. (1 mark)
(b) Cracking a molecule of decane (C₁₀H₂₂) can produce octane (C₈H₁₈) and one other product. Use the equation below to identify the other product and explain how you worked it out.
C₁₀H₂₂ → C₈H₁₈ + ?
(2 marks)
Carbon dioxide is a greenhouse gas. Human activities, such as burning fossil fuels, have increased the amount of carbon dioxide in the atmosphere.
Explain how an increase in greenhouse gases such as carbon dioxide leads to an increase in the average temperature of the Earth. (2 marks)
The Earth's early atmosphere is thought to have been very different from the atmosphere today. Scientists believe it was formed largely by intense volcanic activity and contained very little oxygen.
Name the gas that is thought to have made up the largest proportion of the Earth's early atmosphere. (1 mark)