📋 Structured Questions
These questions test key concepts from Topic 7. Attempt each question on paper, then click "Show Mark Scheme" to check your answer.
Question 1: Combustion of an Alkane
2 marksPetrol contains a hydrocarbon with the formula C₉H₂₀.
(a) Complete and balance the equation for the complete combustion of C₉H₂₀. [2]
C₉H₂₀ + _____ → _____ + _____
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- C₉H₂₀ + 14O₂ → 9CO₂ + 10H₂O [2]
- Allow 1 mark for correct products (CO₂ and H₂O) but incorrect balancing
Question 2: Testing for Alkenes
2 marksCompound A has an alkene functional group.
(a) Describe what will be seen when compound A is shaken with bromine water. [2]
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- Orange bromine water (allow yellow/brown) [1]
- Turns colourless (allow "is decolourised") [1]
Question 3: Properties of Hydrocarbon Fractions
2 marksHeavy fuel oil consists of molecules with 20–40 carbon atoms, while kerosene consists of molecules with 11–15 carbon atoms. A student predicted that heavy fuel oil is more viscous than kerosene. The prediction was correct.
(a) Justify the student's prediction. [2]
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- As molecular size increases, viscosity increases [1]
- Heavy fuel oil has larger molecules than kerosene [1]
Question 4: Fractional Distillation
3 marksCentral heating boilers can burn kerosene. Kerosene is produced from crude oil in a fractionating column. In the first step, crude oil is heated and hydrocarbon vapours are formed.
(a) Explain how kerosene is produced from these hydrocarbon vapours. [3]
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- There is a temperature gradient in the column (it gets cooler going up) [1]
- The kerosene vapour condenses [1]
- At the level in the column that matches its boiling point range [1]
Question 5: Cracking Conditions
3 marksEthene is an alkene produced from large hydrocarbon molecules.
(a) Name the process used to produce ethene and describe the conditions used. [3]
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- Cracking (accept steam cracking or catalytic cracking) [1]
- High temperature (allow a value in the range 300–900 °C) [1]
- Steam or a catalyst (e.g. aluminium oxide / zeolite) [1]
Question 6: Production of Ethanol
3 marksMethylated spirit contains ethanol.
(a) Describe how ethanol is produced from a sugar solution. Give the name of this process. [3]
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- The process is fermentation [1]
- Add yeast (as the catalyst/biological agent) [1]
- Anaerobic conditions (absence of oxygen) and warm temperatures (allow 5–45 °C) [1]
Question 7: Fractional Distillation of Crude Oil ⭐ Extended Response
6 marksCrude oil is a complex finite resource made mostly of hydrocarbons.
(a) Describe and explain how crude oil is separated into its different fractions using fractional distillation. [6]
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Level 3 (5–6 marks): Clear, logically sequenced explanation of how fractions are separated, correctly using concepts of vaporisation, temperature gradient, and condensation linked to different boiling points.
Level 2 (3–4 marks): Descriptive account including heating and condensing, with some reference to boiling points, but steps may not be perfectly sequenced or the temperature gradient may be omitted.
Level 1 (1–2 marks): Simple statements identifying that crude oil is heated and separates, but lacking scientific explanation of state changes.
Indicative content:
- Crude oil is heated until most hydrocarbons evaporate/vaporise
- Vapours are fed into a fractionating column
- There is a temperature gradient — hot at the bottom, cooler at the top
- As vapours rise and cool, different hydrocarbons condense at different temperatures because they have different boiling points
- Long-chain molecules (high boiling points) condense near the bottom; short-chain molecules (low boiling points) condense near the top
Question 8: Required Practical — Fractional Distillation 🔬
4 marksA student carries out a laboratory fractional distillation to separate a mixture of liquids with different boiling points.
(a) Explain how the process of fractional distillation separates the different fractions. [4]
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- Fractions evaporate / vaporise [1]
- At different boiling points [1]
- These vapours then condense [1]
- At different points / heights in the fractionating column [1]
Question 9: Required Practical — Molecule Size & Distillation ⭐🔬 Extended Response
6 marksCrude oil is a complex mixture of hydrocarbons. In industry, it is continuously fed into a fractionating column to be separated into useful fractions.
(a) Describe the process of fractional distillation. In your answer, explain how the size of the hydrocarbon molecules affects their properties and dictates where they are collected from the fractionating column. [6]
Show Mark Scheme
Level 3 (5–6 marks): Detailed, logically sequenced description of fractional distillation. Student clearly links heating and condensing steps to properties of hydrocarbons (molecular size and boiling point) and explains where they are collected.
Level 2 (3–4 marks): Mostly correct description with some explanation of how molecular size relates to boiling points or collection points, but may lack some detail or clarity.
Level 1 (1–2 marks): Simple, fragmented statements about heating, condensing, or hydrocarbon sizes, lacking a clear logical structure.
Indicative content:
- Process: crude oil is heated until it evaporates/vaporises
- The fractionating column has a temperature gradient — hotter at the bottom, cooler at the top
- Hydrocarbon vapours rise up the column and condense when they reach a region at their boiling point
- Molecule size link: smaller/shorter hydrocarbons have lower boiling points → rise further → collected at the top (e.g. petroleum gases)
- Larger/longer hydrocarbons have higher boiling points → condense lower down → collected at the bottom (e.g. heavy fuel oil)