Crude oil, fractional distillation and fuels

What you'll learn

This revision guide covers crude oil as a finite resource, the separation of petroleum fractions by fractional distillation, and the properties and uses of hydrocarbons as fuels. You'll understand how molecular size affects physical properties, complete and incomplete combustion reactions, and environmental concerns surrounding fossil fuel use.

Key terms and definitions

Crude oil — a finite fossil fuel consisting of a complex mixture of hydrocarbons of different chain lengths, formed from the remains of ancient marine organisms over millions of years

Hydrocarbon — a compound containing only hydrogen and carbon atoms

Fractional distillation — a physical separation method that separates crude oil into useful fractions based on differences in boiling points

Fraction — a mixture of hydrocarbons with similar boiling points, obtained from fractional distillation of crude oil

Viscosity — a measure of how easily a liquid flows; high viscosity liquids flow slowly (thick), low viscosity liquids flow quickly (runny)

Complete combustion — burning in sufficient oxygen to produce carbon dioxide and water only, releasing maximum energy

Incomplete combustion — burning in insufficient oxygen, producing carbon monoxide and/or carbon (soot) alongside carbon dioxide and water

Fossil fuel — a fuel formed from the remains of living organisms over millions of years, including coal, crude oil and natural gas

Core concepts

Formation and composition of crude oil

Crude oil formed over millions of years from the remains of tiny marine organisms (mainly plankton) that died and accumulated on sea beds. These organic remains were buried under layers of sediment and rock. High pressure and temperature, combined with the absence of oxygen, converted the organic material into crude oil through a slow chemical process.

Crude oil is a finite resource — it cannot be replaced once used because formation takes millions of years. Global reserves are being depleted faster than new reserves are being discovered.

Crude oil consists almost entirely of hydrocarbons. These molecules vary in:

• Chain length (number of carbon atoms)
• Structure (straight chains, branched chains, rings)
• Boiling point
• Properties

Most hydrocarbons in crude oil are alkanes — saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. Common examples include:

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)
• Butane (C₄H₁₀)
• Longer chain alkanes up to C₅₀ and beyond

Fractional distillation of crude oil

Crude oil must be separated into useful fractions because:

• The mixture itself has limited practical use
• Different hydrocarbons have different applications
• Each fraction contains molecules with similar properties

The fractional distillation process:

1. Heating: Crude oil is heated to approximately 350-400°C in a furnace, vaporising most hydrocarbons
2. Vaporisation: The hot vapour enters the fractionating column at the bottom
3. Temperature gradient: The column is hottest at the bottom (350°C) and coolest at the top (25°C)
4. Condensation: As vapours rise, they cool. Each hydrocarbon condenses when the temperature falls below its boiling point
5. Collection: Fractions are collected at different levels (trays) in the column

Major fractions from top to bottom:

Fraction	Approximate carbon chain length	Boiling point range (°C)	Uses
Refinery gases	C₁-C₄	Below 25	Camping gas, LPG, heating
Petrol (gasoline)	C₅-C₁₀	25-75	Fuel for cars
Naphtha	C₇-C₁₄	75-190	Chemical feedstock, petrol production
Kerosene (paraffin)	C₁₀-C₁₆	190-250	Aircraft fuel, heating
Diesel oil (gas oil)	C₁₅-C₂₀	250-350	Fuel for diesel engines, heating
Fuel oil	C₂₀-C₃₀	350-400	Fuel for ships, power stations
Bitumen	C₃₀+	Above 400	Road surfacing, roofing

Note: Fractional distillation is a physical process — no chemical bonds are broken. The molecules remain unchanged.

Properties of hydrocarbons and their relationship to chain length

As hydrocarbon chain length increases:

Boiling point increases

• Longer molecules have stronger intermolecular forces (van der Waals forces)
• More energy is needed to separate the molecules
• Therefore higher temperatures are required for boiling

Viscosity increases

• Longer molecules become more tangled
• They flow less easily past each other
• Fractions become thicker and flow more slowly

Flammability decreases

• Shorter chain hydrocarbons ignite more easily
• They are more volatile (evaporate more readily)
• This makes them better fuels for engines

Colour becomes darker

• Lighter fractions (short chains) are colourless or pale
• Heavier fractions (long chains) are darker yellow, brown or black

Volatility decreases

• Shorter chains evaporate more easily at room temperature
• Longer chains remain liquid or become solid (like bitumen)

These property trends explain why different fractions suit different purposes. Petrol (short chains) is volatile, runny, and ignites easily — ideal for car engines. Bitumen (very long chains) is thick, non-volatile and doesn't ignite easily — perfect for road surfaces.

Combustion of hydrocarbon fuels

Hydrocarbons are excellent fuels because they release large amounts of energy when burned. The combustion type depends on oxygen availability.

Complete combustion

Occurs when sufficient oxygen is available. Products are carbon dioxide and water only:

General equation:

Hydrocarbon + oxygen → carbon dioxide + water

Example with methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

Example with propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Complete combustion produces:

• Maximum energy output
• A blue flame
• No smoke

Incomplete combustion

Occurs when oxygen supply is limited. Products may include carbon monoxide and/or carbon (soot):

Hydrocarbon + oxygen → carbon monoxide + water (+ carbon dioxide)

Example:

2CH₄ + 3O₂ → 2CO + 4H₂O

Or producing carbon:

CH₄ + O₂ → C + 2H₂O

Incomplete combustion produces:

• Less energy than complete combustion
• A yellow or orange flame
• Smoke (carbon particles)
• Carbon monoxide — a toxic, colourless, odourless gas

Carbon monoxide dangers:

Carbon monoxide is extremely dangerous because:

• It is absorbed by red blood cells in preference to oxygen
• It reduces the blood's oxygen-carrying capacity
• It causes headaches, dizziness, unconsciousness and death
• It has no smell, so victims are unaware of exposure
• Faulty gas appliances in enclosed spaces cause carbon monoxide poisoning deaths annually

Proper ventilation of gas appliances ensures complete combustion and prevents carbon monoxide build-up.

Environmental concerns with fossil fuels

Climate change — carbon dioxide

Burning fossil fuels releases carbon dioxide:

• CO₂ is a greenhouse gas
• It traps heat in Earth's atmosphere
• Increased atmospheric CO₂ concentrations contribute to global warming
• This causes climate change: rising sea levels, extreme weather, ecosystem disruption

The carbon in fossil fuels was removed from the atmosphere millions of years ago. Burning them now releases this "ancient carbon" rapidly, increasing atmospheric CO₂ levels.

Acid rain — sulfur dioxide

Crude oil contains sulfur impurities. When burned:

sulfur + oxygen → sulfur dioxide
S + O₂ → SO₂

Sulfur dioxide dissolves in atmospheric water forming sulfurous acid:

SO₂ + H₂O → H₂SO₃

This can oxidise further to sulfuric acid. Acid rain:

• Damages plant leaves and roots
• Kills aquatic life in lakes and rivers
• Erodes limestone buildings and statues
• Corrodes metal structures

Sulfur can be removed from fuels before combustion or sulfur dioxide can be removed from power station emissions using scrubbers.

Nitrogen oxides

At high temperatures in engines, nitrogen and oxygen from air react:

nitrogen + oxygen → nitrogen oxides (NOₓ)

Nitrogen oxides contribute to:

• Acid rain formation
• Photochemical smog
• Respiratory problems

Catalytic converters in vehicle exhausts reduce nitrogen oxide emissions by converting them to harmless nitrogen gas.

Particulates

Incomplete combustion produces tiny carbon particles (soot). These:

• Cause respiratory diseases when inhaled
• Reduce air quality, especially in urban areas
• Contribute to 'smog' formation
• Blacken buildings

Alternatives to crude oil

Due to environmental concerns and finite supplies, alternatives are being developed:

Biofuels

• Made from renewable plant materials (crops, waste)
• Examples: bioethanol, biodiesel
• Carbon neutral in principle (CO₂ released was recently absorbed by plants)
• However, growing crops for fuel competes with food production

Hydrogen fuel

• Produces only water when burned: 2H₂ + O₂ → 2H₂O
• Requires energy to produce (often from fossil fuels currently)
• Storage and transportation challenges

Electric vehicles

• No direct emissions
• Electricity generation may still involve fossil fuels
• Battery production has environmental impacts

Renewable energy

• Solar, wind, tidal, geothermal
• For electricity generation rather than direct fuel replacement
• Intermittent supply challenges

Worked examples

Example 1: Fractional distillation

Question: Crude oil is separated by fractional distillation. (a) Describe how fractional distillation separates crude oil into useful fractions. [3 marks] (b) Explain why diesel oil has a higher boiling point than petrol. [2 marks]

Answer:

(a)

• Crude oil is heated and vaporised [1]
• Vapours rise up the fractionating column which has a temperature gradient [1]
• Different fractions condense at different temperatures/heights depending on their boiling points [1]

Examiner note: Each marking point requires a distinct process step. Simply stating "boiling points differ" without explaining the temperature gradient and condensation would not gain full marks.

(b)

• Diesel oil contains longer hydrocarbon chains/molecules than petrol [1]
• Longer chains have stronger intermolecular forces, requiring more energy to separate them [1]

Examiner note: Both the structural difference AND the consequence must be explained for full marks.

Example 2: Combustion reactions

Question: Natural gas is mainly methane, CH₄. (a) Write a balanced symbol equation for the complete combustion of methane. [2 marks] (b) State two differences between complete and incomplete combustion. [2 marks] (c) Explain why incomplete combustion of methane is dangerous in homes. [2 marks]

Answer:

(a) CH₄ + 2O₂ → CO₂ + 2H₂O [2] (Correct formulae [1], balanced equation [1])

(b) Any two from:

• Complete combustion produces more energy/heat than incomplete combustion
• Complete combustion produces only carbon dioxide and water; incomplete combustion produces carbon monoxide and/or carbon
• Complete combustion occurs with sufficient oxygen; incomplete combustion occurs with insufficient oxygen
• Complete combustion produces a blue flame; incomplete combustion produces a yellow/orange flame [2]

(c)

• Incomplete combustion produces carbon monoxide [1]
• Carbon monoxide is toxic/poisonous/prevents blood carrying oxygen [1]

Examiner note: The toxicity explanation must be specific — "dangerous" or "harmful" alone would not gain the second mark.

Example 3: Properties and uses

Question: The table shows three fractions from crude oil.

Fraction	Carbon atoms per molecule
Bitumen	40
Kerosene	14
Petrol	7

(a) Which fraction has the highest boiling point? [1 mark] (b) Which fraction is the most viscous? [1 mark] (c) Explain why petrol is more suitable than bitumen as a fuel for cars. [3 marks]

Answer:

(a) Bitumen [1]

(b) Bitumen [1]

(c)

• Petrol has shorter chain molecules/lower molecular mass [1]
• Petrol is more volatile/evaporates more easily [1]
• Petrol ignites more easily/is more flammable, so it burns readily in car engines [1]

Examiner note: Three distinct points needed. Simply stating "petrol is a better fuel" without explaining the property that makes it suitable would not gain marks.

Common mistakes and how to avoid them

• Confusing physical and chemical processes: Fractional distillation is physical separation — no chemical bonds break and molecules remain unchanged. Do not describe it as a chemical reaction.

• Incomplete combustion equations: When writing equations for incomplete combustion, remember that carbon monoxide (CO), carbon (C), carbon dioxide (CO₂) and water can all be products. The question will usually specify which products to include.

• Vague property descriptions: Use precise terms: "viscous/viscosity" not just "thick", "volatile/volatility" not "evaporates", "flammable/flammability" not "burns easily". The scientific terminology gains marks.

• Reversing boiling point trends: Remember — as chain length increases, boiling point increases. Shorter chains have lower boiling points, so they are collected higher up the fractionating column (where it's cooler).

• Stating "crude oil is a compound": Crude oil is a mixture of many different hydrocarbon compounds, not a single compound itself. Each fraction is also a mixture of similar hydrocarbons.

• Overlooking balancing in combustion equations: Always check your combustion equations are balanced. Count atoms systematically: carbons first, then hydrogens, then oxygens. Adjust oxygen last.

Exam technique for "Crude oil, fractional distillation and fuels"

• "Describe" versus "Explain": Describe questions require stating what happens (observations, processes). Explain questions require reasons why something happens, often using scientific principles like intermolecular forces or molecular structure. Use "because" or "therefore" in explanations.

• Use the mark allocation: A 3-mark question needs three distinct points. Don't write one point three different ways. List separate facts, stages or reasons. If asked to "explain", each mark usually requires a statement plus a linked consequence.

• Name the hazards specifically: Don't just say "pollution" or "harmful gases". Name the actual substance (carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen oxides) and state the specific environmental problem (acid rain, global warming, respiratory disease).

• Apply knowledge to unfamiliar fractions: You might see fractions you haven't encountered (like "naphtha" or "lubricating oil"). Apply the rules: position in the column tells you chain length, which determines all other properties.

Quick revision summary

Crude oil is a finite fossil fuel — a mixture of hydrocarbons with different chain lengths. Fractional distillation separates it by heating and condensing vapours at different temperatures in a fractionating column. Shorter chain hydrocarbons have lower boiling points, lower viscosity and higher flammability. Complete combustion in sufficient oxygen produces only CO₂ and H₂O. Incomplete combustion produces toxic carbon monoxide. Burning fossil fuels causes environmental problems: CO₂ drives climate change and SO₂ causes acid rain.