Cracking

This lesson covers cracking of hydrocarbons as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand why cracking is carried out, the conditions for thermal and catalytic cracking, and the types of products formed.

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Why Is Cracking Needed?

Fractional distillation of crude oil produces fractions in fixed proportions. However, the demand for different fractions does not match the supply:

• There is high demand for short-chain hydrocarbons (petrol, gases) and alkenes (for making plastics).
• There is a surplus of long-chain hydrocarbons (heavy fuel oil, bitumen) that are less useful as fuels.

Cracking solves this problem by breaking long-chain hydrocarbons into shorter, more useful molecules.

What Cracking Produces

Cracking always produces:

• Shorter alkanes — used as fuels
• Alkenes — used to make polymers (plastics) and other chemicals

The general idea:

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

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Types of Cracking

There are two main types of cracking required by the specification:

1. Catalytic Cracking

Feature	Detail
Temperature	~500 °C
Catalyst	Zeolite (aluminium silicate) or broken pottery (alumina)
Pressure	Slight pressure
Products	Shorter alkanes + alkenes (often branched)

Catalytic cracking uses a hot catalyst to break down the long-chain hydrocarbons. The catalyst provides an alternative route with a lower activation energy.

2. Thermal Cracking (Steam Cracking)

Feature	Detail
Temperature	800–1000 °C (higher than catalytic)
Catalyst	None — heat alone is used
Pressure	High pressure, with steam
Products	Shorter alkanes + alkenes (including ethene for polymers)

Thermal cracking uses very high temperatures and high pressure with steam to break the bonds.

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Example Equations

Example 1: Decane → Octane + Ethene

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

• Decane (10 carbons) is a long-chain alkane found in surplus.
• It is cracked into octane (used in petrol) and ethene (used to make polyethene).

Example 2: Decane → Pentane + Propene + Ethene

C₁₀H₂₂ → C₅H₁₂ + C₃H₆ + C₂H₄

This shows that one long molecule can be broken into multiple shorter molecules.

Checking the Equation

Always check that the number of carbon atoms and hydrogen atoms is the same on both sides of the equation:

	C atoms	H atoms
Left: C₁₀H₂₂	10	22
Right: C₈H₁₈ + C₂H₄	8 + 2 = 10	18 + 4 = 22

graph LR
    A["Long-chain alkane<br/>(e.g. C₁₀H₂₂)"] --> B["CRACKING<br/>High temperature<br/>± Catalyst"]
    B --> C["Shorter alkane<br/>(e.g. C₈H₁₈)<br/>→ Fuel"]
    B --> D["Alkene<br/>(e.g. C₂H₄)<br/>→ Plastics"]

    style A fill:#8e44ad,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#2980b9,color:#fff

Exam Tip: In cracking equations, the total number of carbon and hydrogen atoms must be balanced. You can check by counting atoms on each side. At least one product must be an alkene (contains a C=C double bond).

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Identifying Products of Cracking

You can tell an alkane from an alkene by their formulae:

Property	Alkane	Alkene
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Bonding	All single bonds (C–C)	Contains at least one C=C double bond
Saturated/unsaturated	Saturated	Unsaturated
Test with bromine water	No change (stays orange)	Decolourises (turns colourless)

Using the General Formulae to Identify Products

If a product has the formula C₃H₆:

• Check alkane: CₙH₂ₙ₊₂ where n = 3 → C₃H₈. Not a match.
• Check alkene: CₙH₂ₙ where n = 3 → C₃H₆. Match → it is propene (an alkene).

Exam Tip: If you are given the formula of a product and asked whether it is an alkane or alkene, use the general formulae. If it fits CₙH₂ₙ₊₂ it is an alkane; if it fits CₙH₂ₙ it is an alkene.

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Economic Importance of Cracking

Cracking is economically important because it:

• Converts low-value long-chain hydrocarbons into high-value shorter ones.
• Produces alkenes needed as feedstock for making polymers (plastics such as polyethene and polypropene).
• Helps match the supply of products from fractional distillation to the demand.
• Increases the yield of petrol and other fuels from a barrel of crude oil.

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Summary

• Cracking breaks long-chain hydrocarbons into shorter, more useful molecules.
• Products are shorter alkanes (fuels) and alkenes (for polymers).
• Catalytic cracking: ~500 °C with a zeolite catalyst.
• Thermal cracking (steam cracking): 800–1000 °C with high pressure and steam; no catalyst.
• Cracking is needed because demand for short-chain hydrocarbons exceeds supply from fractional distillation.
• Alkenes produced by cracking are essential for the plastics industry.

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Worked Examples — Cracking Products

Worked Example 1 — Simple split

Question: Decane (C₁₀H₂₂) is cracked into octane and one other molecule. Write a balanced equation.

Step 1 — Carbon balance: 10 C on the left. Octane = C₈H₁₈ uses 8 C, leaving 2 C for the other molecule.

Step 2 — Hydrogen balance: 22 H on the left. Octane uses 18 H, leaving 4 H. A molecule with 2 C and 4 H is ethene (C₂H₄) — an alkene.

Equation: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.

Check atoms: left 10 C and 22 H; right (8 + 2) = 10 C and (18 + 4) = 22 H. Balanced.

Worked Example 2 — Cracking to give propene

Question: Cracking hexadecane (C₁₆H₃₄) gives one alkane and propene (C₃H₆). What is the alkane?

Step 1 — Carbons: 16 − 3 = 13 C in the alkane. Step 2 — Hydrogens: 34 − 6 = 28 H in the alkane. Step 3 — Check against alkane general formula (CₙH₂ₙ₊₂): for n = 13, H = 28. Matches.

Answer: The alkane is C₁₃H₂₈ (tridecane).

Worked Example 3 — Identifying the alkene

Question: A cracking reaction produces C₆H₁₄ and another hydrocarbon with formula C₂H₄. Is the second product an alkane or alkene, and how do you know?

Answer: Alkene. The formula C₂H₄ fits the alkene general formula CₙH₂ₙ (for n = 2, H = 4), not the alkane formula CₙH₂ₙ₊₂. C₂H₄ is ethene and contains a C=C double bond (functional group of the alkene homologous series).

Common Mistake: Writing two alkanes as products. Cracking always produces at least one alkene, because breaking a C–C bond in a saturated chain leaves a deficit of hydrogen in one fragment, forcing a C=C double bond.

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Comparison Tables

Table 1 — Catalytic vs thermal (steam) cracking