Cracking and Alkenes — AQA GCSE Chemistry

SPEC 8462 • LESSON 7.03 • PAPER 2

Cracking & Alkenes

How surplus long-chain hydrocarbons are converted into petrol and the alkene starting materials for polymers. Spec 4.7.1.4 — ⭐ Priority — appears regularly on Paper 2.

What This Lesson Covers

Cracking is the supply-and-demand fix for petroleum. Distillation produces too much long-chain heavy fuel oil and not enough petrol or alkenes for polymers. Cracking solves both problems at once.

3

Marks · cracking conditions question

~50%

Scored 3/3 on P2 Jun22 Q04.6

2

Cracking methods to memorise

Why This Matters

Cracking has appeared on multiple recent Paper 2 sittings. The Jun22 examiner report on Q04.6 noted that "the name of the process was well known but a large proportion of students failed to mention either the high temperature or the use of steam or a catalyst." The marks come from conditions, not just the process name.

Key Definitions

Three terms that anchor every question in this lesson — every mark scheme uses at least one of them.

Cracking

The breaking down of long-chain hydrocarbons into shorter, more useful hydrocarbons (a shorter alkane and an alkene), using heat and either steam or a catalyst.

Alkene

An unsaturated hydrocarbon with a C=C double bond. General formula CₙH₂ₙ — two fewer hydrogens than the corresponding alkane.

Unsaturated

Contains at least one C=C double bond. By contrast, alkanes are saturated — only single bonds (Lesson 7.01).

Why Crack? · Supply & Demand

The fractions produced by distillation don't match what the market wants. Cracking is the economic fix.

From distillationMarket reality

Lots of heavy fuel oil (long chains, high b.p.)Too much supply — less commercially valuable

Limited petrol , diesel , alkenesHigh demand — petrol for cars, alkenes for polymers

Spec 4.7.1.4 — verbatim

"There is a high demand for fuels with small molecules and so some of the products of cracking are useful as fuels. Alkenes are used to produce polymers and as starting materials for the production of many other chemicals."

Big Picture

Cracking turns surplus long chains → useful short chains. Output: shorter alkane + alkene, every time. The alkene goes to polymerisation (Lesson 7.06); the shorter alkane goes to petrol/fuel.

Two Methods of Cracking

The spec requires you to "describe in general terms the conditions used for catalytic cracking and steam cracking." Each method needs two conditions.

1 · Catalytic Cracking

Condition 1: high temperature (~400 °C)

Condition 2: hot catalyst (e.g. aluminium oxide / zeolite)

Long-chain hydrocarbon vapour passed over the hot catalyst.

2 · Steam Cracking

Condition 1: very high temperature

Condition 2: mixed with steam

Vaporised hydrocarbons mixed with steam at high T.

EXAMINER ALERT (P2 Jun22 Q04.6, 3 marks)

Mark scheme awards marks independently: 1 for naming the process (cracking), 1 for "high temperature", 1 for "steam OR catalyst". A "stated temperature in the range 300–900 °C" is also accepted. Saying just "hot" caps you at 1/3.

Lab-Scale Catalytic Cracking

The classic GCSE catalytic cracking apparatus. Paraffin (a long-chain alkane) is vaporised, passed over a hot catalyst, and the gaseous products are collected over water.

Mineral wool (paraffin source) → catalyst (heated) → delivery → gas collection over water

Spec scope · Lab apparatus

The spec only requires you to "describe in general terms the conditions" — not to draw or describe specific apparatus. But understanding the lab setup makes the abstract conditions concrete: high temperature is supplied by the Bunsen, the catalyst is broken porcelain (an aluminium oxide), and the products are gaseous (short alkanes + alkenes).

The Cracking Reaction

Cracking always produces a shorter alkane plus an alkene. The equation must balance C and H — atoms aren't created or destroyed.

long alkane → shorter alkane + alkene

Worked Example · Decane

Step 1. Decane has formula C₁₀H₂₂ (CₙH₂ₙ₊₂ with n = 10).

Step 2. One possible cracking outcome: octane (C₈H₁₈) + ethene (C₂H₄).

Step 3. Check C: 10 = 8 + 2 ✓ Check H: 22 = 18 + 4 ✓.

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

A C–C bond breaks · long chain → shorter alkane + alkene (with new C=C)

Identifying the Alkene Product · CₙH₂ₙ

Alkenes follow CₙH₂ₙ — two fewer hydrogens than the corresponding alkane. So C₂H₄ (ethene), C₃H₆ (propene), C₄H₈ (butene), C₈H₁₆ (octene) are alkenes. C₈H₁₈, C₈H₁₄ are not. P2 Jun22 Q04.7 tested exactly this recognition (correct answer: C₇H₁₄ and C₈H₁₆).

Cracking Products · Multiple Outcomes

One starting alkane can crack in different ways — AQA accepts any equation that balances C and H and produces a shorter alkane + alkene.

ReactantPossible cracking equations

Decane (C₁₀H₂₂)C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (octane + ethene) C₁₀H₂₂ → C₆H₁₄ + C₄H₈ (hexane + butene) C₁₀H₂₂ → C₅H₁₂ + C₃H₆ + C₂H₄ (multiple alkenes possible)

Nonane (C₉H₂₀)C₉H₂₀ → C₇H₁₆ + C₂H₄ (heptane + ethene) C₉H₂₀ → C₆H₁₄ + C₃H₆ (hexane + propene)

Octane (C₈H₁₈)C₈H₁₈ → C₆H₁₄ + C₂H₄ (hexane + ethene) C₈H₁₈ → C₅H₁₂ + C₃H₆ (pentane + propene)

Mark-scoring strategy

When asked "give a possible product" or "complete the equation," you only need one valid combination. Always check: (a) at least one alkane, (b) at least one alkene (CₙH₂ₙ), (c) C and H balance.

Alkenes · More Reactive

Spec 4.7.1.4: "Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes."

Why More Reactive?

The C=C double bond is the reactive site (the "functional group"). It's a region of high electron density that opens to allow other atoms to add across — making alkenes far more reactive than alkanes, which only have stable single bonds.

PropertyAlkane (saturated)Alkene (unsaturated)

General formulaCₙH₂ₙ₊₂CₙH₂ₙ

BondsOnly C–C single bondsOne C=C double bond

ReactivityLess reactiveMore reactive

Bromine waterNo reaction — stays orangeReacts — turns colourless

Spec Scope · 7.03 vs 7.04

In scope for 7.03 (4.7.1.4): alkenes are products of cracking; CₙH₂ₙ formula recognition; "more reactive than alkanes"; the bromine water test and its colour change. Reserved for 7.04 (4.7.2, chemistry only): ethene/propene structural formulae (CH₂=CH₂, etc.), the C=C functional group concept, and full addition reactions with H₂, Br₂ (mechanism), and H₂O.

The Bromine Water Test

The single most-tested practical observation in this topic. The mark scheme is brutally precise about colour wording.

The Test (Verbatim Mark Scheme · Nov21 Q06.1)

Test: add bromine (water).

Result: changes from brown / orange to colourless. "Ignore clear." (i.e. "clear" alone scores zero — must use "colourless").

ORANGE / BROWN→COLOURLESS

Substance addedObservationReason

Alkene (e.g. C₂H₄)Bromine water turns from orange/brown to colourlessBromine adds across the C=C double bond

Alkane (e.g. C₂H₆)Bromine water stays orange/brown — no changeNo C=C bond — no addition reaction occurs

Examiner Alert · Bromine Water

Two bromine-water past-paper questions show exactly how marks are lost on this topic. The errors are linguistic, not scientific.

"The bromine water test was known by many students, but some of these were then unable to gain the second mark, usually because an incorrect colour change was stated." — AQA Examiner Report, P2 Nov 2021 Q06.1

WrongRightWhy it matters

"Bromine water goes clear ""Bromine water turns colourless ""Clear" describes transparency, not colour. Mark scheme literally says "ignore clear".

"Decolourised" (alone)"Changes from orange to colourless"State the colour change — both start and end.

"Red" or "purple" bromine water"Orange" / "brown" / "yellow-brown"Wrong start colour fails MP1.

"Disappears""Turns colourless"The bromine doesn't disappear — its colour does.

Memorise the Phrase

"Bromine water turns from orange to colourless." Both ends of the colour change. Both words exact. This is one of the highest-frequency 1-mark observations in Module 7.

Uses of Cracking Products

The spec is clear about why cracking matters economically. Two distinct product streams, two distinct destinations.

Shorter Alkane Products

Used as fuels — especially petrol (high demand for short-chain hydrocarbon fuels with small molecules).

Alkene Products

Used to make polymers (Lesson 7.06) and as starting materials for the production of many other chemicals.

Spec 4.7.1.4 (verbatim)

"Students should be able to give examples to illustrate the usefulness of cracking. They should also be able to explain how modern life depends on the uses of hydrocarbons."

Exam Application

If asked "why is cracking useful?" — give two reasons: (1) produces fuels with smaller molecules to meet demand for petrol; (2) produces alkenes used as feedstock for polymers and other chemicals. Two reasons, two marks.

Practice · Cracking Conditions

(a)

two

(b)

3 MARKS

Inspired by AQA Q04.6 (Jun22) examiner-alert pattern. Style: 3-mark methods + conditions.

(a)

Two methods:catalytic cracking and steam cracking.

(b1)

Catalytic cracking conditions: high temperature (~400 °C) AND a hot catalyst (aluminium oxide / zeolite).

(b2)

Steam cracking conditions: very high temperature AND mixed with steam.

⚠

Examiner trap (Q04.6 pattern): the name of cracking is well-known but many students forget either "high temperature" or the second condition (steam or catalyst). Marks are awarded independently — state every condition, every time.

Practice · Identify the Alkenes

two

2 MARKS

☐ A C₃H₈

☐ B C₄H₈

☐ C C₅H₁₂

☐ D C₅H₁₀

☐ E C₆H₁₄

Inspired by AQA Q04.7 (Jun22) — formula recognition pattern with different carbon counts and a non-MCQ framing.

Logic

Bromine water decolourises only with alkenes (C=C double bond). Alkenes follow CₙH₂ₙ; alkanes follow CₙH₂ₙ₊₂.

A

C₃H₈ — alkane (2(3)+2 = 8). No reaction.

B

C₄H₈ — alkene (2(4) = 8) ✓. Decolourises bromine water.

C

C₅H₁₂ — alkane (2(5)+2 = 12). No reaction.

D

C₅H₁₀ — alkene (2(5) = 10) ✓. Decolourises bromine water.

E

C₆H₁₄ — alkane (2(6)+2 = 14). No reaction.

✓

Answer: B (C₄H₈) and D (C₅H₁₀) — both fit CₙH₂ₙ, both have C=C, both decolourise bromine water.

Practice · Bromine Water Test

chemical test

observation

3 MARKS

Inspired by AQA Q06.1 (Nov21) — bromine water observation pattern with a different scenario (alkane/alkene comparison instead of cycloalkene).

MP1

Test: add bromine water to each liquid (and shake).

MP2

Hexene observation: bromine water changes from orange (or brown) to colourless — hexene contains a C=C double bond, so bromine adds across it.

MP3

Hexane observation: bromine water stays orange (or brown) — hexane is saturated (no C=C), so no reaction occurs.

⚠

Mark scheme rule (Q06.1): "ignore clear" — using "clear" alone for the final colour scores zero on the colour-change mark. Always say "colourless". Both ends of the colour change must be stated.

Mark-Scheme Language

The exact phrasing AQA accepts vs the loose wording that loses marks. Memorise the column on the left.

ConceptMark-scoring phraseDo not accept

Cracking process" cracking ""distillation"; "fractionating"

Cracking conditions" high temperature (300–900 °C) AND ( steam OR catalyst )""hot" alone; just "high temperature"

Cracking products"shorter alkane + alkene " — both named"two alkanes"; "small molecules" alone

Why crack?"high demand for fuels with small molecules"; "alkenes for polymers / other chemicals""to make oil useful" (vague)

Bromine water start colour" orange " / " brown " / "yellow-brown""red"; "purple"

Bromine water end colour" colourless ""clear"; "decolourised" alone; "disappears"

Alkene formulaCₙH₂ₙ — e.g. C₂H₄, C₇H₁₄CₙH₂ₙ₊₂ formulas (those are alkanes)

Common Errors

The four mistakes that account for most lost marks across Module 7 cracking and bromine water questions. Pattern-match these in your own answers.

✗ Common error

"Cracking happens at high temperature" (alone)

✓ Mark-scheme

"high temperature AND steam (or catalyst)" — both conditions

✗ Common error

"Bromine water goes clear"

✓ Mark-scheme

"Turns from orange to colourless" — MS literally says "ignore clear"

✗ Common error

Cracking product: "two alkanes"

✓ Mark-scheme

"shorter alkane + alkene" — every cracking gives an alkene

✗ Common error

Unbalanced cracking equation: C₁₀H₂₂ → C₈H₁₈ + C₂H₂

✓ Mark-scheme

Always check C and H balance. Correct: C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Exam Strategy

Three habits that secure marks in every cracking and alkenes question.

Strategy 1 · Always Two Conditions

For cracking: high temperature + (catalyst OR steam). Never one without the other. The marks are awarded independently — both must be stated to score full marks on a 3-mark conditions question.

Strategy 2 · Memorise the Colour Phrase

"Bromine water turns from orange to colourless." Both ends. Both words exact. Never write "clear", "decolourised" alone, or "disappears". This is the highest-frequency observation phrase in Module 7.

Strategy 3 · Verify the Alkene

Every cracking equation produces at least one alkene. To identify alkenes from formula: count atoms, apply CₙH₂ₙ. If H = 2C, it's an alkene; if H = 2C + 2, it's an alkane. Always balance the C and H on both sides.

Lesson Summary

Key Definitions

Cracking — breaking long-chain hydrocarbons into shorter alkane + alkene, using heat with steam or a catalyst.

Alkene — unsaturated hydrocarbon with a C=C double bond.

Unsaturated — contains at least one C=C double bond.

Two Methods · Two Conditions Each

Catalytic: high T + catalyst (Al₂O₃ / zeolite). Steam: high T + steam.

Bromine Water Test

Alkene: orange → colourless. Alkane: stays orange. Never say "clear".

Top Exam Tip

For any cracking question, give both conditions (high T + steam OR catalyst). For any bromine water question, give both colour states (orange → colourless). For any cracking equation, ensure C and H balance AND at least one alkene (CₙH₂ₙ) is produced.