Cracking and Alkenes

This lesson covers cracking — the process of breaking long-chain hydrocarbons into shorter, more useful molecules — as required by the Edexcel GCSE Chemistry specification (1CH0). You need to know why cracking is necessary, the two main methods (thermal and catalytic), and how to write balanced equations for cracking reactions.

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Why Is Cracking Needed?

As you learned in previous lessons, fractional distillation of crude oil produces a mismatch between supply and demand:

• Too much of the long-chain, heavy fractions (fuel oil, bitumen) — supply exceeds demand.
• Not enough of the short-chain, lighter fractions (petrol, kerosene, LPG) — demand exceeds supply.

Cracking solves this problem by converting long-chain hydrocarbons (for which there is low demand) into shorter-chain hydrocarbons (for which there is high demand), along with useful alkenes.

Products of Cracking

Cracking always produces a mixture of:

1. Shorter alkanes — used as fuels (e.g. petrol-range hydrocarbons).
2. Alkenes — unsaturated hydrocarbons used as raw materials to make polymers (plastics), alcohols and other chemicals.

Exam Tip: Cracking always produces at least one alkene as well as shorter alkanes. If the question asks why cracking is useful, mention both fuels and feedstock for the petrochemical industry (making polymers).

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What Happens During Cracking?

Cracking is a thermal decomposition reaction — a large molecule is broken down into smaller molecules by heating. Covalent bonds within the long-chain hydrocarbon are broken.

General Equation Pattern

long-chain alkane → shorter alkane + alkene(s)

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Methods of Cracking

There are two main methods of cracking:

1. Thermal Cracking

Feature	Detail
Temperature	Very high: 600–700 °C
Pressure	High pressure
Catalyst	None (heat alone provides the energy)
Products	Tends to produce a high proportion of alkenes

Thermal cracking is used when the primary goal is to produce alkenes for making polymers and other chemicals.

2. Catalytic Cracking

Feature	Detail
Temperature	High but slightly lower than thermal: approximately 500 °C
Pressure	Slight pressure (lower than thermal cracking)
Catalyst	Zeolite catalyst (an aluminosilicate with a honeycomb-like porous structure)
Products	Produces a mixture of alkanes and alkenes; more useful for making motor fuels

The zeolite catalyst provides an alternative reaction pathway with a lower activation energy, allowing the reaction to occur at a lower temperature than thermal cracking.

Exam Tip: Know the key difference: thermal cracking uses high temperature and high pressure with no catalyst; catalytic cracking uses a zeolite catalyst at a slightly lower temperature. Both break long chains into shorter ones.

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Balanced Equations for Cracking

You need to be able to write and balance equations for cracking reactions.

Example 1: Cracking Decane

Decane (C₁₀H₂₂) can be cracked into octane (C₈H₁₈) and ethene (C₂H₄):

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Word equation: decane → octane + ethene

Check: Left side: C = 10, H = 22. Right side: C = 8 + 2 = 10, H = 18 + 4 = 22. ✓ Balanced.

Example 2: Cracking Decane — Different Products

Decane can also be cracked in a different way:

C₁₀H₂₂ → C₃H₆ + C₂H₄ + C₅H₁₂

Word equation: decane → propene + ethene + pentane

Check: C = 3 + 2 + 5 = 10 ✓, H = 6 + 4 + 12 = 22 ✓

Example 3: Cracking Hexane

C₆H₁₄ → C₄H₁₀ + C₂H₄

Word equation: hexane → butane + ethene

Example 4: Cracking Dodecane

C₁₂H₂₆ → C₂H₄ + C₁₀H₂₂

Word equation: dodecane → ethene + decane

Exam Tip: When balancing a cracking equation, count all the carbon atoms and hydrogen atoms on both sides. The total must be the same. Remember: cracking is not combustion — there is no oxygen involved.

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Why Alkenes Are Valuable

The alkenes produced by cracking are extremely valuable to the chemical industry because they are reactive (due to their C=C double bond). They are used as the starting materials (monomers) for making:

• Polymers (plastics) — e.g. poly(ethene) from ethene, poly(propene) from propene.
• Alcohols — e.g. ethanol from ethene (by hydration).
• Many other chemicals used in manufacturing.

Without cracking, the chemical industry would not have a sufficient supply of alkenes.

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Cracking in Context — Economic Importance

Factor	Detail
Improves fuel supply	Converts surplus heavy fractions into petrol-range molecules
Provides feedstock	Produces alkenes for the polymer and chemical industries
Reduces waste	Less unwanted heavy oil is left over
Economic value	Short-chain fuels and alkenes are worth more than heavy oil

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Identifying Products — Worked Example

Question: A hydrocarbon with the formula C₈H₁₈ is cracked. One product is ethene (C₂H₄). Write a balanced equation for this reaction and name the other product. [3 marks]

Step 1: Write what you know:

C₈H₁₈ → C₂H₄ + ?

Step 2: Work out the unknown product by subtracting:

• Carbon: 8 − 2 = 6
• Hydrogen: 18 − 4 = 14
• Unknown product: C₆H₁₄

Step 3: Identify the product — C₆H₁₄ is hexane (an alkane, since it fits CₙH₂ₙ₊₂ with n = 6).

Balanced equation: C₈H₁₈ → C₂H₄ + C₆H₁₄

Exam Tip: When a cracking equation gives you one product and asks for the other, just subtract the atoms. Then check whether the unknown product is an alkane (CₙH₂ₙ₊₂) or an alkene (CₙH₂ₙ) by applying the general formulae.

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Key Definitions to Learn