# Crude Oil

> Chemistry · Edexcel IGCSE 4CH1 (2017)
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-crude-oil/

This guide covers all core Edexcel IGCSE Chemistry content for crude oil, including fractional distillation, fraction properties and uses, hydrocarbon combustion, pollutants, and catalytic cracking, aligned to spec points 4.7–4.18.

**Prerequisites:** [Mixtures and separation techniques](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-separation-techniques/); [Introduction to organic chemistry](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-introduction-to-organic-chemistry/)

## Learning objectives

- State that crude oil is a mixture of hydrocarbons
- Describe how fractional distillation separates crude oil into useful fractions
- Recall the names, uses, and property trends of main crude oil fractions
- Identify products of complete and incomplete combustion of hydrocarbons, and explain risks of carbon monoxide
- Explain origins of sulfur dioxide and oxides of nitrogen, and their role in acid rain
- Describe catalytic cracking conditions, products, and purpose for matching supply and demand

## Composition of Crude Oil

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.

**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.

## Fractional Distillation of Crude Oil

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).

**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 |

> **mnemonic**
>
> LPL: *L*ower (top of column) = *P*aler, *L*ess viscous, Lower boiling point

**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.

## Combustion of Hydrocarbon Fuels

**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: $C_3H_8(g) + 2O_2(g) \rightarrow 3C(s) + 4H_2O(g)$

> **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.

## Combustion Pollutants and Acid Rain

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.

## Catalytic Cracking of Long-Chain Alkanes

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.

**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.

## Common pitfalls

- **Wrong:** Stating that crude oil is a pure substance
  - Why it fails: Crude oil is a mixture of hundreds of different hydrocarbons, not a single compound
  - Correct: Always explicitly state that crude oil is a mixture of hydrocarbons
- **Wrong:** Mixing up the order of fractions in the distillation column, e.g. putting bitumen at the top
  - Why it fails: Longer-chain hydrocarbons have higher boiling points, so they condense at the hotter bottom of the column
  - Correct: Recall the top-to-bottom order: refinery gases → gasoline → kerosene → diesel → fuel oil → bitumen
- **Wrong:** Writing carbon dioxide as a product of incomplete combustion
  - Why it fails: Incomplete combustion has limited oxygen, so carbon is only partially oxidised to CO or C, not fully to CO₂
  - Correct: Only include CO₂ as a product if the question specifies complete combustion
- **Wrong:** Stating oxides of nitrogen come from sulfur impurities in fuel
  - Why it fails: NOₓ form when nitrogen and oxygen from air react at high engine temperatures, not from fuel impurities
  - Correct: Separate sources: SO₂ from sulfur impurities, NOₓ from air in hot engines
- **Wrong:** Forgetting cracking conditions or only writing alkenes as products
  - Why it fails: Exam questions award marks for recalling exact conditions and both alkane and alkene products
  - Correct: Always state silica/alumina catalyst, 600–700°C, and include one alkane + one alkene in cracking equations

## 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 |

## 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.

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