Fuels and Crude Oil
Natural gas, crude oil and bioethanol: non-renewable and renewable fuels, fractional distillation, and carbon-dioxide sustainability.
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The core idea
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Learning objectives
- name natural gas, mainly methane, and crude oil as non-renewable sources of energy
- describe crude oil as a mixture of hydrocarbons and its separation by fractional distillation to yield fractions which have competing uses as fuels and as a source of chemicals (see also 1.2(a))
- describe biofuel (exemplified by bioethanol from sugarcane) as a renewable alternative to natural gas and crude oil
- describe how biofuel, when compared to fossil fuels, can be more environmentally sustainable in terms of carbon dioxide emission (see also 12(g)).
This lesson connects three ideas: where important fuels come from, how crude oil is separated, and how fuel choices affect the environment.
1. Definition
A. Fossil fuel
A fossil fuel is a fuel formed over millions of years from the remains of living organisms, such as crude oil and natural gas.
B. Non-renewable energy source
A non-renewable energy source is one that cannot be replaced quickly once used up (it takes millions of years to form).
C. Crude oil
Crude oil is a complex mixture of hydrocarbons (compounds made of only carbon and hydrogen).
D. Fractional distillation
Fractional distillation separates a mixture into fractions using differences in boiling points.
A fraction is a mixture of hydrocarbons with a similar range of boiling points.
E. Biofuel
A biofuel is a renewable fuel made from plant material (biomass), e.g. bioethanol.
2. Key Ideas
- Crude oil is a mixture, so it must be separated (it is not a single substance).
- Fractional distillation separates fractions by different boiling points (not by chemical reactions).
- Top of column: small molecules, low boiling point, volatile, more flammable.
- Bottom of column: large molecules, high boiling point, viscous, less flammable.
- Biofuels are renewable, but “carbon neutral” is not automatic (production can emit CO₂ and use land).
Combustion basics: Fossil fuels
Pollution (carbon monoxide): Air pollutants
3. Detailed Explanations
- Crude oil is a mixture of hydrocarbons; fractional distillation separates by different boiling points (physical separation).
- The fractionating column is hot at the bottom and cool at the top; vapours condense at different heights.
- Up the column: smaller molecules, lower boiling point, more volatile, more flammable. Down: larger molecules, higher boiling point, more viscous, less flammable.
- Crude-oil fractions have competing uses as fuels and as sources of chemicals. Use any names or property data supplied in the question to justify a use.
A. Non-renewable sources in this course
| Fuel | Main idea |
|---|---|
| natural gas | mainly methane, CH₄ |
| crude oil | mixture of hydrocarbons |
Natural gas is not pollution-free. Complete combustion still produces CO₂, while incomplete combustion can produce toxic CO.
B. Why crude oil must be separated
Crude oil contains many different hydrocarbons mixed together. Different hydrocarbons have different boiling points, so we separate them into fractions.
C. Fractional distillation (how it works)
- Crude oil is heated so most of it vaporises.
- Vapour enters a fractionating column (hot at the bottom, cooler at the top).
- As vapour rises, it cools and condenses at the level where the temperature is just below its boiling point.
Swipe or scroll sideways to inspect the complete overview.
D. Trend down the column (what properties change)
| Moving up the column | What happens | Keyword reason |
|---|---|---|
| fewer carbon atoms per molecule | molecules are smaller | lower molecular mass |
| boiling point decreases | condense higher up | weaker intermolecular forces |
| viscosity decreases | more runny | smaller molecules |
| flammability increases | ignites more easily | more volatile |
Data table
| Fraction (top → bottom) | Boiling point | Viscosity | Flammability |
|---|---|---|---|
| Petroleum gas | 1 | 1 | 7 |
| Petrol | 2 | 2 | 6 |
| Naphtha | 3 | 3 | 5 |
| Kerosene | 4 | 4 | 4 |
| Diesel | 5 | 5 | 3 |
| Fuel oil | 6 | 6 | 2 |
| Bitumen | 7 | 7 | 1 |
E. Illustrative fractions and competing uses
These examples show the two competing demands: fuels and chemical feedstocks. Use the information supplied in a question; the important reasoning is why smaller and larger hydrocarbons suit different jobs, not memorising every row.
| Fraction (top → bottom) | Typical uses (exam-safe) |
|---|---|
| petroleum gas | cooking and heating |
| petrol (gasoline) | fuel for cars |
| naphtha | feedstock for making chemicals (e.g. plastics) |
| kerosene (paraffin) | aircraft fuel; heating |
| diesel | fuel for buses/lorries |
| fuel oil | ships; power stations |
| bitumen | road surfaces; roofing |
F. Biofuels (bioethanol) and “carbon neutrality”
In this syllabus, bioethanol (from sugarcane) is a key example of a renewable biofuel.
Bioethanol can be made by fermentation of glucose using yeast (warm conditions, no oxygen): C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)
Bioethanol burns to form CO₂ and H₂O: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
Exam-safe idea: the CO₂ released on burning can be balanced by CO₂ absorbed during plant growth (photosynthesis). But biofuels can still cause emissions (farming/transport) and environmental issues (land use, deforestation).
4. Common Mistakes
- Saying fractional distillation is “cracking” (wrong process; cracking is chemical change).
- Writing “fractions react” in the column (no reactions; it is physical separation).
- Mixing up the column: small molecules are at the top, large molecules at the bottom.
- Writing “biofuels produce no CO₂” (false; they do produce CO₂ when burnt).
5. Exam Tips
Write: “mixture”, “different boiling points”, “temperature gradient (hot bottom, cool top)”, “vapour condenses at different heights”.
Top: small molecules, low boiling point, volatile, flammable. Bottom: large molecules, high boiling point, viscous, less flammable.
6. Worked Examples
Modelled example 1
Fraction trends (top vs bottom)
Problem
Study the worked solution
Relate height to molecular size
Method
Recognise that fractions collected higher in the column contain smaller molecules.Reason
Smaller hydrocarbons have weaker intermolecular forces.Working
Moving up: fewer carbon atoms per molecule and lower molecular mass.Infer boiling point and viscosity
Method
State that boiling point and viscosity decrease.Reason
Weaker intermolecular forces require less energy to overcome, and smaller molecules flow more easily.Working
Up the column: lower boiling point and lower viscosity.Infer flammability
Method
State that flammability increases.Reason
Lower-boiling fractions are more volatile and ignite more readily.Working
Up the column: boiling point ↓, viscosity ↓, flammability ↑.
Guided practice 2
Identify the fraction by properties
Problem
Link properties, molecular size and position
Hints
Hint 1: size
Hint 2: temperature gradient
View solution step by step
Infer molecular size
Method
Identify the fraction as containing large hydrocarbon molecules.Reason
Large molecules have stronger intermolecular forces, giving higher boiling points, and are more viscous.Working
High boiling point + high viscosity → large molecules.Locate the fraction
Method
Place the fraction near the bottom.Reason
Its vapour condenses only in the hotter lower region of the column.Working
Collected near the bottom of the fractionating column.
Common misconception 3
Incomplete combustion equation
Learner equation
Count atoms before choosing coefficients
View solution step by step
Balance carbon and hydrogen
Method
Keep one carbon monoxide and use two water molecules per methane molecule.Reason
One methane contains one C atom and four H atoms.Working
CH₄ + 3/2O₂ → CO + 2H₂O.Use whole-number coefficients
Method
Multiply every coefficient by two.Reason
A balanced chemical equation is conventionally written using the smallest whole-number ratio.Working
2CH₄ + 3O₂ → 2CO + 4H₂O.
Examiner practice 4
Explain the separation (keywords)
Examination question
Write the complete causal explanation
View solution step by step
Describe crude oil
1 markMethod
State that crude oil is a mixture of hydrocarbons.Reason
A mixture can be physically separated because its substances are not chemically bonded together.Working
Crude oil contains hydrocarbons with different boiling points.Use the temperature gradient
1 markMethod
Describe the column as hot at the bottom and cooler towards the top.Reason
Different heights provide the temperatures needed for different vapours to condense.Working
Hydrocarbon vapours cool as they rise.Explain separate collection
1 markMethod
State that hydrocarbons condense at different heights.Reason
Each condenses when the local temperature falls below its boiling point.Working
Different boiling points + temperature gradient → fractions collected at different levels.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the mixture, temperature-gradient and condensation points.
Challenge 5
Bioethanol “carbon neutral” explanation
Evaluation transfer
Consider uptake, release and production
Hints
Hint 1: carbon cycle
Hint 2: wider boundary
View solution step by step
Explain the carbon-cycle basis
Method
Balance combustion emissions against photosynthetic uptake.Reason
Sugarcane absorbs CO₂ while growing, while burning its ethanol releases CO₂.Working
If the crop is replanted, those amounts can be approximately balanced.Test the real-life boundary
Method
Identify an additional source of net emissions.Reason
Farming machinery, processing and transport may burn fossil fuels; land clearance can also release carbon and reduce uptake.Working
Bioethanol can be described as carbon neutral in principle, but its full production life cycle may add net emissions.
7. Mind Stretchers
Mind stretcher 1: Fix the overclaimExtension
Question: A student writes: “Biofuels are always carbon neutral.” Explain why “always” is unsafe and rewrite an exam-safe sentence.
Show Answer
“Always” is unsafe because carbon neutrality depends on how the fuel is produced (energy input, transport emissions, land use).
Exam-safe: “Biofuels can be described as carbon neutral because plants absorb CO₂ as they grow, but production may still cause extra emissions.”
Final: Avoid “always”; use “can be described as”.
8. Quiz
Test fraction trends (top vs bottom), fraction uses, combustion products, and biofuel sustainability logic.
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