Organic chemistry: cracking and alkenes

What you'll learn

This revision guide covers cracking and alkenes as specified in the AQA GCSE Chemistry specification. You'll understand why and how large hydrocarbon molecules are broken down into smaller, more useful molecules, and learn the properties and reactions of alkenes. This topic links directly to crude oil chemistry and is essential for understanding how the petrochemical industry produces fuels and plastics.

Key terms and definitions

Cracking — the thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules including alkanes and alkenes.

Alkenes — unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C) with the general formula CₙH₂ₙ.

Saturated hydrocarbon — a hydrocarbon containing only single carbon-carbon bonds (C-C), such as alkanes.

Unsaturated hydrocarbon — a hydrocarbon containing at least one carbon-carbon double bond (C=C) or triple bond, such as alkenes.

Catalytic cracking — the breaking down of large hydrocarbon molecules by passing vapour over a hot catalyst (typically aluminium oxide or silicon dioxide at 600-700°C).

Steam cracking — the breaking down of large hydrocarbon molecules by mixing vapour with steam and heating to very high temperatures (800-900°C).

Polymerisation — the chemical reaction in which many small alkene molecules (monomers) join together to form very large molecules (polymers).

Functional group — an atom or group of atoms that determines the characteristic chemical properties of a compound; in alkenes, this is the C=C double bond.

Core concepts

Why cracking is necessary

The fractional distillation of crude oil produces fractions in fixed proportions determined by the composition of the crude oil. However, these proportions don't match consumer demand:

• High demand exists for: short-chain hydrocarbons (petrol, diesel, chemicals for plastics)
• Low demand exists for: long-chain hydrocarbons (heavy fuel oil, bitumen)

Cracking solves this supply-and-demand problem by converting less useful long-chain alkanes into more valuable shorter alkanes and alkenes. Without cracking, the petrochemical industry would have surplus heavy fractions and insufficient lighter fractions.

The products of cracking include:

• Shorter-chain alkanes used as fuels (petrol, diesel)
• Alkenes used as chemical feedstock for making polymers and other chemicals
• Hydrogen (sometimes produced as a by-product)

The cracking process

Cracking involves breaking strong C-C bonds, which requires significant energy input. There are two main industrial methods:

Catalytic cracking:

• Long-chain hydrocarbon vapour is passed over a hot catalyst
• Temperature: 600-700°C
• Catalyst: aluminium oxide (Al₂O₃) or silicon dioxide (SiO₂)
• Lower temperature than steam cracking due to catalyst reducing activation energy
• Produces high percentage of alkenes suitable for polymer production

Steam cracking:

• Long-chain hydrocarbon vapour is mixed with steam
• Temperature: 800-900°C
• No catalyst required
• Higher temperature compensates for absence of catalyst
• Produces different product distribution, often more ethene

Both methods break C-C bonds in the middle of long chains, producing a mixture of products. The exact products depend on the length of the original hydrocarbon and the conditions used.

Laboratory demonstration of cracking

In the laboratory, you can demonstrate catalytic cracking using simple apparatus:

Method:

1. Heat liquid paraffin (a mixture of long-chain alkanes) to produce vapour
2. Pass the vapour over hot broken porcelain or aluminium oxide catalyst
3. Collect the gaseous products over water
4. Test the products

Safety considerations:

• Ensure the delivery tube is removed from water before heating stops (prevents suck-back)
• Use a safety screen
• Wear eye protection
• Ensure good ventilation

The products can be tested to show they include alkenes by adding to bromine water, which decolourises.

Structure and bonding in alkenes

Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ. The presence of the C=C double bond makes them different from alkanes.

Key alkenes in the homologous series:

• Ethene: C₂H₄ (CH₂=CH₂)
• Propene: C₃H₆ (CH₂=CH-CH₃)
• Butene: C₄H₈ (CH₂=CH-CH₂-CH₃)
• Pentene: C₅H₁₀

The C=C double bond consists of:

• One sigma (σ) bond formed by direct orbital overlap
• One pi (π) bond formed by sideways overlap (this detail is beyond GCSE but explains reactivity)

At GCSE level, you need to know that the double bond:

• Makes alkenes more reactive than alkanes
• Prevents rotation around that bond
• Is the site of chemical reactions

Physical properties:

• First few members are gases at room temperature
• Boiling points increase with chain length (like alkanes)
• Insoluble in water
• Less dense than water

Chemical reactions of alkenes

The C=C double bond is the reactive functional group in alkenes. The main reaction type is addition reactions, where the double bond breaks and atoms add to the carbon atoms.

Testing for unsaturation — bromine water test:

This is the standard test to distinguish alkenes from alkanes.

• Add orange/brown bromine water to the unknown hydrocarbon
• Alkene present: bromine water decolourises (turns colourless)
• Alkane present: bromine water remains orange/brown

The reaction with ethene:

C₂H₄ + Br₂ → C₂H₄Br₂

Ethene + bromine → 1,2-dibromoethane

This is an addition reaction where bromine atoms add across the double bond.

Hydrogenation:

Alkenes react with hydrogen gas to form alkanes when passed over a nickel catalyst at 150°C.

Ethene + hydrogen → ethane

C₂H₄ + H₂ → C₂H₆

This reaction is used in the food industry to convert liquid vegetable oils (containing C=C bonds) into solid fats (margarine).

Reaction with steam (hydration):

Alkenes react with steam in the presence of a phosphoric acid catalyst to form alcohols.

Ethene + steam → ethanol

C₂H₄ + H₂O → C₂H₅OH

This is the industrial method for producing ethanol from ethene (derived from crude oil). The reaction occurs at:

• Temperature: 300°C
• Pressure: 60-70 atmospheres
• Catalyst: phosphoric acid (H₃PO₄)

This method contrasts with fermentation, which produces ethanol from renewable resources but gives more dilute solutions.

Combustion:

Like all hydrocarbons, alkenes burn in oxygen:

• Complete combustion (excess oxygen): produces carbon dioxide and water
• Incomplete combustion (limited oxygen): produces carbon monoxide, carbon (soot), and water

Complete combustion of ethene:

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Alkenes are not typically used as fuels because they're more valuable as chemical feedstock for making polymers and other products.

Polymerisation of alkenes

Alkenes undergo addition polymerisation to form polymers (plastics). Many small alkene molecules (monomers) join together by breaking their double bonds and forming long chains.

Conditions required:

• High pressure
• Catalyst (typically a transition metal compound)
• Often elevated temperature

Formation of poly(ethene):

The monomer ethene (C₂H₄) polymerises to form poly(ethene), commonly called polythene:

n C₂H₄ → (C₂H₄)ₙ

Where n represents a very large number (often thousands)

Formation of poly(propene):

Propene (C₃H₆) polymerises to form poly(propene):

n C₃H₆ → (C₃H₆)ₙ

Drawing polymer structures:

When drawing the repeating unit:

1. Remove the double bond from the monomer
2. Draw single bonds to carbon atoms
3. Add extension bonds (continuation lines) on each side
4. Place the structure in brackets with subscript n

Example: For poly(ethene), the repeating unit is:

    H  H
    |  |
- [ C--C ] -
    |  |  n
    H  H

Other important polymers formed from substituted alkenes include:

• Poly(chloroethene) or PVC from chloroethene
• Poly(styrene) from styrene
• Poly(tetrafluoroethene) or PTFE from tetrafluoroethene

Worked examples

Example 1: Cracking equation

Question: Decane (C₁₀H₂₂) can be cracked to produce octane and ethene. Write a balanced symbol equation for this reaction. (2 marks)

Answer:

C₁₀H₂₂ → C₈H₁₈ + C₂H₄ ✓

Mark scheme:

• Correct formulae for all species (1 mark)
• Balanced equation (1 mark)

Examiner tip: Always check your equation is balanced by counting atoms on each side. For cracking, you can check: carbons (10 = 8 + 2) and hydrogens (22 = 18 + 4).

Example 2: Identifying products

Question: A student cracks a long-chain hydrocarbon and collects the gaseous products. Describe a test the student could use to show that the products contain an alkene, and state the result. (3 marks)

Answer:

Test: Add bromine water to the gas ✓

Result if alkene present: The orange/brown bromine water turns colourless ✓ / is decolourised ✓

Mark scheme:

• Bromine water test identified (1 mark)
• Colour change from orange/brown (1 mark)
• To colourless/decolourised (1 mark)

Examiner tip: Be specific about colours — "goes clear" is less precise than "turns colourless" and might not score the mark.

Example 3: Explaining why cracking is used

Question: Explain why the petroleum industry uses cracking. (4 marks)

Answer:

Fractional distillation produces large amounts of long-chain hydrocarbons ✓ but there is low demand for these products ✓. There is high demand for shorter-chain hydrocarbons like petrol ✓. Cracking breaks down long-chain molecules into shorter, more useful ones including alkenes ✓.

Alternative acceptable points:

• Cracking helps match supply with demand
• Alkenes are needed for making polymers/plastics
• Short-chain hydrocarbons are more valuable

Mark scheme:

• Recognition of surplus of long-chain hydrocarbons (1 mark)
• Recognition of shortage/high demand for short-chain hydrocarbons (1 mark)
• Economic reason/matching supply and demand (1 mark)
• Production of useful alkenes/specific products (1 mark)

Common mistakes and how to avoid them

• Forgetting that cracking produces both alkanes and alkenes — Always show at least one alkene product in cracking equations. If you produce only alkanes, the equation doesn't represent cracking.

• Incorrect general formula for alkenes — The general formula is CₙH₂ₙ, not CₙH₂ₙ₊₂ (which is for alkanes). If a molecule has 4 carbons, an alkene has 8 hydrogens (C₄H₈), not 10.

• Confusing the bromine water test — The colour change is orange/brown to colourless, not the reverse. Also, this tests for unsaturation (C=C bonds), not specifically for alkenes alone.

• Drawing addition reactions incorrectly — When drawing products of addition reactions, remember the double bond breaks completely. Both carbons that were double-bonded each gain one new atom/group.

• Unbalanced cracking equations — Always check that the number of carbon and hydrogen atoms is the same on both sides. C₁₀H₂₂ cannot produce C₈H₁₈ + C₂H₆ because the hydrogens don't balance (22 ≠ 24).

• Thinking all alkenes are gases — While smaller alkenes (ethene, propene) are gases at room temperature, larger alkenes can be liquids or solids, just like alkanes.

Exam technique for "Organic chemistry: cracking and alkenes"

• "Describe a test" questions require three elements: the reagent added, the observation with the original substance, and the observation with the test substance. For alkenes: "Add bromine water; it remains orange/brown with alkanes; it turns colourless with alkenes."

• "Explain why cracking is used" questions require economic reasoning, not just a description of the process. Link supply (what fractional distillation produces) to demand (what consumers want) to earn full marks.

• Balancing cracking equations — You may be given one product and asked to work out the other. Use atom counting: if C₁₂H₂₆ → C₈H₁₈ + ?, then the unknown must be C₄H₈ (12-8=4 carbons, 26-18=8 hydrogens). This must be an alkene.

• Drawing displayed formulae — When asked to draw structures, show all atoms and all bonds clearly. For the repeating unit of a polymer, include the extension bonds and brackets with subscript n, or you'll lose marks even if the structure is otherwise correct.

Quick revision summary

Cracking thermally decomposes long-chain alkanes into shorter alkanes and alkenes using high temperatures and catalysts (catalytic cracking) or steam (steam cracking). This process meets demand for shorter hydrocarbons and produces alkenes for polymer manufacture. Alkenes (CₙH₂ₙ) are unsaturated hydrocarbons containing C=C double bonds that make them reactive. They undergo addition reactions with bromine (decolourising bromine water), hydrogen (forming alkanes), and steam (forming alcohols). Addition polymerisation joins many alkene monomers to form plastics like poly(ethene). Understanding cracking and alkenes is essential for explaining how crude oil is converted into valuable products.