Crude Oil

Chemistry· 4.7–4.18 (Section 4(b))· 18 min read

1. Composition of Crude Oil★☆☆☆☆⏱ 3 min

Crude oil is a non-renewable fossil fuel formed over millions of years from the remains of dead marine organisms, buried under sediment at high pressure and temperature.

📘 Definition

Crude oil

A complex mixture of mostly alkane hydrocarbons, with chain lengths ranging from 1 carbon atom to over 70 carbon atoms.

📐 Worked Example

A student claims crude oil is a pure compound. Explain why this statement is incorrect.

1
Pure compounds contain only one type of molecule, with a fixed boiling point.
2
Crude oil contains hundreds of different hydrocarbon molecules of varying chain lengths, each with a unique boiling point, so it is a mixture, not a pure substance.

Exam tip:

You will be asked to state that crude oil is a mixture of hydrocarbons in multiple exam questions, so memorise this core fact explicitly.

2. Fractional Distillation of Crude Oil★★☆☆☆⏱ 4 min

Fractional distillation separates crude oil into groups of hydrocarbons with similar boiling points, called fractions, in an industrial fractionating column. The column is hottest at the bottom (~350°C) and coolest at the top (~20°C).

📘 Definition

Fraction

A group of hydrocarbons with similar chain lengths and boiling points, separated from crude oil for a specific commercial use.

Crude oil is heated to ~400°C to vaporise almost all of the mixture, then pumped into the bottom of the column. Shorter-chain hydrocarbons have lower boiling points, so they rise up the column and condense at cooler, higher levels. Longer-chain hydrocarbons have higher boiling points, so they condense at hotter, lower levels of the column, or remain liquid and are removed from the bottom.

Fraction (top to bottom)	Boiling point	Viscosity	Colour	Main use
Refinery gases	Lowest (<40°C)	Runniest	Colourless	Bottled heating/cooking fuel
Gasoline	40–100°C	Very runny	Pale yellow	Petrol for cars
Kerosene	100–250°C	Runny	Yellow	Aircraft fuel, paraffin lamps
Diesel	250–350°C	Thick	Dark yellow	Lorry, car and generator fuel
Fuel oil	350–600°C	Very thick	Dark brown	Ship and power station fuel
Bitumen	Highest (>600°C)	Most viscous	Black	Road surfacing, roof sealant

📐 Worked Example

State two properties of kerosene that differ from fuel oil, and explain the reason for the difference.

1
Kerosene has a much lower boiling point than fuel oil. This is because kerosene is made of shorter hydrocarbon chains that require less energy to turn to gas.
2
Kerosene is also far less viscous than fuel oil, as shorter hydrocarbon chains flow more easily than long, tangled chains.

Exam tip:

You will often be asked to order fractions by boiling point, viscosity or colour, so memorise the top-to-bottom sequence of fractions carefully.

3. Combustion of Hydrocarbon Fuels★★★☆☆⏱ 4 min

📘 Definition

Fuel

A substance that releases heat energy when burned (combusted) with oxygen.

Hydrocarbons are used widely as fuels because they release large amounts of heat energy when burned. There are two types of combustion, depending on the amount of oxygen available.

Complete combustion occurs when there is a plentiful supply of oxygen. The only products are carbon dioxide and water vapour, and all energy stored in the hydrocarbon is released.

CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)

Incomplete combustion occurs when there is a limited supply of oxygen. Products are water vapour plus carbon monoxide (a toxic gas) and/or solid carbon (soot). Less energy is released than in complete combustion.

2CH_4(g) + 3O_2(g) \rightarrow 2CO(g) + 4H_2O(g)

Carbon monoxide is a colourless, odourless toxic gas. It reduces the ability of blood to transport oxygen around the body, leading to unconsciousness or death if exposure is prolonged. You do not need to mention haemoglobin in your answers for this specification.

📐 Worked Example

Write a balanced equation for the incomplete combustion of propane (C₃H₈) to produce solid carbon and water vapour, including state symbols.

1
First write unbalanced reactants and products: C₃H₈ + O₂ → C + H₂O
2
$latex: C_3H_8 + O_2 \rightarrow 3C + H_2O (balance carbon)$
3
$latex: C_3H_8 + O_2 \rightarrow 3C + 4H_2O (balance hydrogen)$
4
$latex: C_3H_8 + 2O_2 \rightarrow 3C + 4H_2O (balance oxygen)$
5
Add state symbols for final answer: $C_{3} H_{8} (g) + 2 O_{2} (g) \to 3 C (s) + 4 H_{2} O (g)$

Exam tip:

Always check if a question specifies complete or incomplete combustion before writing an equation. If it says 'plenty of air', use complete combustion; if it says 'limited air', use incomplete.

4. Combustion Pollutants and Acid Rain★★★☆☆⏱ 3 min

Burning hydrocarbon fuels releases several harmful atmospheric pollutants, in addition to carbon dioxide (a greenhouse gas).

Oxides of nitrogen (NOₓ): In car engines, the very high temperature of combustion allows nitrogen and oxygen from the air to react together, forming nitrogen monoxide and nitrogen dioxide (collectively called NOₓ).

Sulfur dioxide: Many hydrocarbon fuels contain small amounts of sulfur impurities. When the fuel is burned, these sulfur impurities react with oxygen to form sulfur dioxide gas.

Both NOₓ and sulfur dioxide dissolve in rainwater in the atmosphere, reacting to form acidic solutions. This falls as acid rain, which damages stone buildings, kills aquatic life, and harms trees and plants.

📐 Worked Example

Explain how burning petrol in a car engine leads to acid rain formation, even if the petrol has no sulfur impurities.

1
The high temperature inside the car engine causes nitrogen and oxygen from the air to react, forming oxides of nitrogen (NOₓ).
2
NOₓ gases rise into the atmosphere and dissolve in rainwater, forming acidic solutions that fall as acid rain.

Exam tip:

Do not confuse the source of NOₓ: they come from air in the engine, not from the fuel itself. SO₂ comes from sulfur impurities in the fuel.

5. Catalytic Cracking of Long-Chain Alkanes★★★☆☆⏱ 4 min

Fractional distillation of crude oil produces more long-chain fractions (like fuel oil) than the global market demands, and not enough short-chain fractions (like gasoline for petrol). Cracking solves this supply-demand imbalance.

📘 Definition

Cracking

A thermal decomposition reaction that breaks long-chain alkanes into shorter, more useful alkanes and alkenes.

Catalytic cracking uses a catalyst of silica or alumina, at a temperature of 600–700°C, to break the carbon-carbon bonds in long alkane chains. The products are always a mixture of at least one shorter alkane and at least one alkene.

C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4

The shorter alkanes are used to make higher-demand fuels like petrol, while the alkenes are used as feedstock to make polymers (plastics) and other organic chemicals.

📐 Worked Example

Long-chain alkane C₁₆H₃₄ is cracked to produce propene (C₃H₆) and one other alkane product. Write the balanced equation for this reaction.

1
The number of carbon and hydrogen atoms must be equal on both sides of the equation.
2
$latex: \text{Carbon in alkane product} = 16 - 3 = 13$
3
$latex: \text{Hydrogen in alkane product} = 34 - 6 = 28$
4
$latex: C_{16}H_{34} \rightarrow C_{13}H_{28} + C_3H_6$

Exam tip:

You must memorise the exact conditions for catalytic cracking: silica/alumina catalyst, 600–700°C. Marks are often lost for missing these details.

6. Common Pitfalls

Wrong move:

Stating that crude oil is a pure substance

Why:

Crude oil is a mixture of hundreds of different hydrocarbons, not a single compound

Correct move:

Always explicitly state that crude oil is a mixture of hydrocarbons

Wrong move:

Mixing up the order of fractions in the distillation column, e.g. putting bitumen at the top

Why:

Longer-chain hydrocarbons have higher boiling points, so they condense at the hotter bottom of the column

Correct move:

Recall the top-to-bottom order: refinery gases → gasoline → kerosene → diesel → fuel oil → bitumen

Wrong move:

Writing carbon dioxide as a product of incomplete combustion

Why:

Incomplete combustion has limited oxygen, so carbon is only partially oxidised to CO or C, not fully to CO₂

Correct move:

Only include CO₂ as a product if the question specifies complete combustion

Wrong move:

Stating oxides of nitrogen come from sulfur impurities in fuel

Why:

NOₓ form when nitrogen and oxygen from air react at high engine temperatures, not from fuel impurities

Correct move:

Separate sources: SO₂ from sulfur impurities, NOₓ from air in hot engines

Wrong move:

Forgetting cracking conditions or only writing alkenes as products

Why:

Exam questions award marks for recalling exact conditions and both alkane and alkene products

Correct move:

Always state silica/alumina catalyst, 600–700°C, and include one alkane + one alkene in cracking equations

7. Quick Reference Cheatsheet

Key Concept	Facts to Recall
Crude oil	Mixture of hydrocarbons
Fraction order (top to bottom)	Refinery gases → Gasoline → Kerosene → Diesel → Fuel oil → Bitumen
Top column fraction trend	Lower boiling point, paler, less viscous
Complete combustion products	CO₂ + H₂O
Incomplete combustion products	CO/C + H₂O
CO toxicity	Reduces blood oxygen transport capacity
NOₓ source	High temp reaction of N₂ and O₂ in car engines
SO₂ source	Combustion of sulfur impurities in fuels
Acid rain cause	NOₓ and SO₂ dissolve in rainwater to form acids
Cracking conditions	Silica/alumina catalyst, 600–700°C
Cracking products	Shorter alkane + alkene
Cracking purpose	Matches supply of long fractions to demand for short fractions

8. Frequently Asked

What is the order of fractions from top to bottom of the fractional distillation column?

The order from top (lowest boiling point) to bottom (highest boiling point) is: refinery gases, gasoline, kerosene, diesel, fuel oil, bitumen.

Why is cracking of long-chain alkanes necessary?

Cracking solves the supply-demand imbalance: fractional distillation produces excess low-demand long-chain fractions (e.g. fuel oil) and insufficient high-demand short-chain fractions (e.g. gasoline for petrol).

What is the difference between complete and incomplete combustion products?

Complete combustion (plenty of oxygen) produces only carbon dioxide and water. Incomplete combustion (limited oxygen) produces water plus carbon monoxide and/or solid carbon (soot).

What's Next

Now that you have mastered crude oil content, you are ready to move on to the properties and reactions of alkanes, the main component of crude oil fractions. You will then learn about alkenes, the valuable product of cracking, including their characteristic reactions and use in polymer production. Make sure to practice past paper questions on fractional distillation and cracking, as these are high-frequency exam topics worth 3–6 marks per question. You should also practice balancing combustion and cracking equations to avoid losing easy marks in your exam.