Crude Oil, Hydrocarbons and Alkanes — AQA GCSE Chemistry

SPEC 8462 • LESSON 7.01 • PAPER 2

Crude Oil, Hydrocarbons & Alkanes

Where fuels and plastics begin. Crude oil formation, the alkane homologous series (CₙH₂ₙ₊₂), and balancing complete combustion — the three pillars of organic chemistry on Paper 2.

What This Lesson Covers

Three concepts that dominate Paper 2 organic chemistry. Master these and the rest of Module 7 has a foundation to build on.

3

Marking points (formation)

CₙH₂ₙ₊₂

Alkane formula

<5%

Score full marks on fossil-fuel formation

Why This Matters

Module 7 is examined on Paper 2 and lessons 7.01–7.03 are flagged as ⭐ priority. The AQA examiner report on the most recent fossil-fuel-formation question (P2 Jun 2022 Q06.5, on natural gas) noted fewer than 1 in 20 students gained all 3 marks — pure technique, no difficult science. The same marking principles apply to crude oil formation. We fix that today.

Definition · Hydrocarbon

The simplest term in organic chemistry — and the foundation for everything in Module 7.

Hydrocarbon

A compound containing only carbon and hydrogen atoms — nothing else.

Quick Test

If a molecule contains any oxygen, nitrogen, sulfur, halogens, or other atoms, it is not a hydrocarbon.

✓ Hydrocarbons

CH₄ (methane)

C₂H₆ (ethane)

C₈H₁₈ (octane)

C₂H₄ (ethene)

✗ Not Hydrocarbons

CH₃OH (has O)

CH₃COOH (has O)

NH₃ (has N, no C)

CO₂ (no H)

Definition · Crude Oil

Crude oil is the source of nearly all hydrocarbons in this module. Get the definition word-perfect — every phrase corresponds to a separate marking point.

Crude Oil

A finite resource found in rocks. It is a mixture of hydrocarbons formed from the remains of ancient biomass — mainly plankton — that was buried in sediment over millions of years under high temperature and pressure.

PropertyValue / Description

Type of substanceMixture (not a compound, not pure)

Main componentsHydrocarbons (mostly alkanes)

OriginAncient marine plankton

FormationBuried in sediment (spec says "mud") under high T & P, millions of years

Renewable?No — finite / non-renewable on human timescales

Spec Source Note

Spec 4.7.1.1 explicitly says "buried in mud" and lists plankton + finite resource. "High temperature and pressure" is from the related mark scheme (P2 Jun22 Q06.5, natural gas) — accept both terms as equivalent for fossil-fuel formation.

How Crude Oil Forms

A four-stage process. Each stage corresponds to a marking point. Mentally rehearse this sequence — you'll need it for any "explain how crude oil was formed" question.

Plankton → buried in sediment → high T & P over millions of years → crude oil

Examiner Alert · Formation

The closest past-paper question on fossil-fuel formation is P2 Jun 2022 Q06.5 — about natural gas formation. The principles are identical to crude oil (same biogenic process), and AQA could ask the same of crude oil at any sitting. Students lost marks on three specific phrases.

"Some students were insufficiently precise about the size of the organisms involved in natural gas formation, and there was some confusion with the formation of coal. Similarly, the idea of coverage by sediments was sometimes inaccurately described as 'trapped under rocks', which is not part of the process, only of the outcome. The third mark was often not awarded as heat was stated instead of high temperature." — AQA Examiner Report, P2 Jun 2022 Q06.5

WrongRight (Mark Scheme)Why It Matters

"Dead dinosaurs"Marine plankton / tiny marine organismsCoal forms from land plants. Crude oil and natural gas from plankton.

"Trapped under rocks"Covered by sediment / buried"Trapped" describes the outcome, not the formation process.

"Heat" aloneHigh temperature AND high pressureBoth conditions required. "Heat" loses the mark.

Memorise This Sentence

"Marine plankton died, were buried in sediment, and over millions of years were exposed to high temperature and high pressure." Three marking points in one rehearsed sentence — works for crude oil OR natural gas.

Definition · Alkanes

Most hydrocarbons in crude oil are alkanes — the simplest homologous series of organic compounds. Three definitions you must know word-for-word.

Alkane

A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂. Contains only single C–C and C–H bonds.

Saturated

Contains only single bonds — no C=C double bonds. (Alkenes, by contrast, are unsaturated because they contain a C=C — covered in Lesson 7.03.)

Homologous Series

A family of compounds sharing the same general formula, similar chemical properties, and a gradual change in physical properties as chain length increases.

The First Four Alkanes

Memorise these four. The spec is explicit: "Students do not need to know the names of specific alkanes other than methane, ethane, propane and butane." For larger alkanes (e.g. nonane in past papers), you'll be given the name and derive the formula from CₙH₂ₙ₊₂.

Methane

CH₄

n = 1

Ethane

C₂H₆

n = 2

Propane

C₃H₈

n = 3

Butane

C₄H₁₀

n = 4

Displayed formulae · methane, ethane, propane

Complete Combustion

The most-tested reaction in Module 7. With plenty of oxygen, hydrocarbons burn completely to give only carbon dioxide and water. The reactions are exothermic — which is why hydrocarbons are used as fuels.

hydrocarbon + O₂ → CO₂ + H₂O

Worked Example · Methane

Step 1. Write reactants and products: CH₄ + O₂ → CO₂ + H₂O

Step 2. Balance C: 1 C on each side ✓

Step 3. Balance H: 4 H on left → need 2 H₂O on right

Step 4. Balance O: right has 2 + 2 = 4 O → need 2 O₂ on left

CH₄ + 2O₂ → CO₂ + 2H₂O

Balancing Order — Always C, H, O

Balance carbon first (only appears in CO₂ on the right), hydrogen second (only in H₂O), oxygen last (because O₂ is flexible — adjust the coefficient last and check both sides).

Worked Example · Propane

Apply the C → H → O method to propane (C₃H₈). This is a classic 2-mark exam question — 1 mark for correct formulae, 1 mark for correct balancing.

Question · 2 Marks

Write the balanced equation for the complete combustion of propane (C₃H₈).

Step 1 (C). Complete combustion gives only CO₂ + H₂O. 3 carbons → produce 3 CO₂.

Step 2 (H). 8 hydrogens → produce 4 H₂O (2 H per H₂O).

Step 3 (O). Right side has 3(2) + 4(1) = 10 oxygen atoms → need 5 O₂ on the left.

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Verify Before You Stop

C: 3 = 3 ✓ | H: 8 = 8 ✓ | O: 10 = 6 + 4 = 10 ✓ — always check all three counts before moving on. The mark is in the verification.

Examiner Alert · Combustion

The examiner reports a recurring pattern of avoidable mistakes on combustion-balancing questions. Two are about handwriting, not chemistry.

"Students should be advised to take care when writing numbers in equations. A quickly written 14 sometimes looked like 19 and 19 H₂O was occasionally written by students who had double-counted the oxygen atoms in water. Similarly, excessively large subscript numbers and excessively small capital letters (especially O) are likely not to gain credit." — AQA Examiner Report, P2 Jun 2022 Q04.3

The correct answer for the combustion of nonane (C₉H₂₀): C₉H₂₀ + 14O₂ → 9CO₂ + 10H₂O. Nonane isn't on the spec memorise list — you derive C₉H₂₀ from CₙH₂ₙ₊₂ with n = 9. Nearly half the cohort scored both marks; the rest lost them on three specific errors.

ErrorFix

Forgetting to balance ("CH₄ + O₂ → CO₂ + H₂O")Always check C, H, O counts on both sides before stopping.

Including hydrocarbons in the products (e.g. CO₂ + H₂O + CH₂)Complete combustion gives only CO₂ and H₂O — nothing else.

Writing "19 H₂O" by double-counting O atoms in waterOne H₂O = one O. For nonane: 10 H₂O contributes 10 O, not 20. The 14 O₂ supplies all 28 O atoms.

Writing "14" so it looks like "19"Write large coefficients clearly. The examiner cannot guess.

Exam Technique

Subscripts noticeably smaller than the letters; write capital O big and round so it can't be confused with a 0. Per the examiner: oversized subscripts or undersized capital letters lose marks.

Practice · Crude Oil Formation

3 MARKS

Anticipatory question. Past papers have tested natural gas formation (P2 Jun 2022 Q06.5) but not crude oil directly — the spec covers both, and the 3 marking points are identical.

MP1

Origin. Crude oil formed from the remains of ancient marine plankton. (NOT "dead dinosaurs" or "dead plants" — that's coal.)

MP2

Burial. The dead organisms were buried under layers of sediment on the sea floor. (NOT "trapped under rocks" — that's the outcome, not the process.)

MP3

Conditions. Over millions of years, exposed to high temperature AND high pressure, converting the biomass into a mixture of hydrocarbons. (NOT "heat" alone — needs both.)

Practice · Combustion Equation

Nonane is an alkane with 9 carbon atoms. Write a balanced equation for the complete combustion of nonane.2 MARKS

F

Formula. CₙH₂ₙ₊₂ with n = 9 → C₉H₂₀. Complete combustion gives only CO₂ + H₂O.

C

Carbon. 9 C → 9 CO₂.

H

Hydrogen. 20 H → 10 H₂O (2 H per H₂O).

O

Oxygen. Right side: 9(2) + 10(1) = 28 O atoms → 14 O₂ on left.

✓

Final answer:C₉H₂₀ + 14O₂ → 9CO₂ + 10H₂O. 1 mark for correct formulae, 1 mark for correct balancing.

Mark-Scheme Language

Score the marks by writing what the AQA mark scheme accepts. Below: the precise vocabulary you need vs. the loose wording that loses marks.

ConceptMark-scoring phraseDo not accept

Hydrocarbon"contains only carbon and hydrogen""made of carbon and hydrogen" (could imply other atoms)

Crude oil origin"marine plankton" / tiny marine organisms"dead plants" (coal); "dead dinosaurs"

Burial"covered by sediment" / buried"trapped under rocks" (outcome, not process)

Conditions"high temperature AND high pressure""heat" alone

Alkane formulaCₙH₂ₙ₊₂ → e.g. C₉H₂₀C₉H₁₈ (alkene formula)

Saturated"only single bonds""full of hydrogen" (vague)

CombustionHC + O₂ → CO₂ + H₂O (balanced)Unbalanced; products containing other hydrocarbons

Common Errors

Recurring mistakes from real exam scripts (paraphrased from AQA examiner reports). Recognise them — write the right version.

✗ Common error

"Crude oil from dead dinosaurs"

✓ Mark-scheme

Marine plankton.

✗ Common error

"Heat formed crude oil"

✓ Mark-scheme

High temperature AND pressure.

✗ Common error

"Trapped under rocks"

✓ Mark-scheme

Covered by sediment / buried.

✗ Common error

Unbalanced combustion equation

✓ Mark-scheme

Always verify C, H, O counts on both sides.

Exam Strategy

Three habits to lock in for every Module 7 question. Each one corresponds to a marking point students consistently lose.

Strategy 1 · Memorise the Formation Sentence

"Marine plankton died, were buried in sediment, and over millions of years exposed to high temperature and high pressure." Three marking points in one rehearsed sentence.

Strategy 2 · Always Balance C → H → O

Carbon first (single product), hydrogen second (single product), oxygen last (flexible coefficient). Then double-count both sides before writing the final equation.

Strategy 3 · Write Numbers Clearly

Subscripts noticeably smaller than letters. Capital O round and large. Coefficients written so 14 cannot be misread as 19. The examiner cannot give the benefit of the doubt.

Lesson Summary

Key Definitions

Hydrocarbon — compound containing only carbon and hydrogen.

Crude oil — finite mixture of hydrocarbons formed from ancient marine plankton, buried in sediment under high T and P.

Alkane — saturated hydrocarbon with general formula CₙH₂ₙ₊₂.

Saturated — only single bonds.

Homologous series — same general formula, gradual change in physical properties.

First 4 Alkanes

CH₄ methane

C₂H₆ ethane

C₃H₈ propane

C₄H₁₀ butane

Combustion

HC + O₂ → CO₂ + H₂O. Balance C, H, O in order. Exothermic — fuels.

Top Exam Tip

For crude oil formation: rehearse the three marking points as one memorised sentence — marine plankton, buried in sediment, high temperature AND pressure over millions of years. Fewer than 1 in 20 students score full marks here. Your sentence is your edge.