Cracking Hydrocarbons: GCSE Chemistry Guide

Cracking hydrocarbons GCSE chemistry means breaking long-chain alkane molecules from crude oil into shorter, more useful hydrocarbons using heat, and sometimes a catalyst. It converts an oversupply of long chains, such as those used for fuel oil, into smaller alkanes and alkenes that are in higher demand, including petrol and the alkenes used to make polymers.

Why do we crack hydrocarbons?

Crude oil is separated into fractions by fractional distillation, but the fractions produced do not match what society actually needs. Fractional distillation naturally yields far more long-chain hydrocarbons, such as fuel oil and bitumen, than there is demand for, while shorter-chain fractions like petrol and liquefied petroleum gas (LPG) are in much higher demand than crude oil alone can supply.

Cracking solves this imbalance by breaking the surplus long-chain molecules into two useful groups:

• Smaller alkanes, which are more useful as fuels because they are more volatile and ignite more easily.
• Alkenes, unsaturated hydrocarbons containing a carbon–carbon double bond, which are not found naturally in crude oil in large quantities but are essential as the starting material (feedstock) for manufacturing polymers such as poly(ethene) and poly(propene).

What is catalytic cracking and how is it carried out?

Catalytic cracking vaporises long-chain hydrocarbons and passes them over a hot catalyst, commonly a zeolite (an aluminosilicate) or a mixture of silica and alumina, at a moderate temperature of around 450 °C and only slight pressure. The catalyst lowers the energy needed for the carbon–carbon bonds to break, speeding up the reaction without needing extremely high temperatures. Catalytic cracking tends to produce a high proportion of branched, cyclic and aromatic hydrocarbons, which burn more efficiently and are especially valuable as motor fuels.

What is thermal cracking and how does it differ?

Thermal cracking uses heat alone, without a catalyst, at much higher temperatures of around 600–700 °C and high pressure, sometimes up to about 70 atmospheres. The intense heat and pressure break carbon–carbon bonds directly, and thermal cracking characteristically produces a high proportion of alkenes, such as ethene and propene, alongside shorter alkanes.

How do you write a balanced cracking equation?

Cracking always follows the same pattern: one longer alkane breaks into one shorter alkane plus one shorter alkene, with the total number of carbon and hydrogen atoms conserved.

General form: $$C_nH_{2n+2} \rightarrow C_mH_{2m+2} + C_xH_{2x} \quad \text{where } n = m + x$$

Worked example: Crack decane (C₁₀H₂₂) to produce octane (C₈H₁₈) and an alkene. Find the formula of the alkene and check the equation balances.

Since decane has 10 carbon atoms and octane has 8, the alkene must contain the remaining 2 carbon atoms, giving the formula C₂H₄ (ethene), following the alkene pattern CₓH₂ₓ.

$$C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4$$

Checking the atoms balance:

Both sides balance exactly, confirming decane cracks into octane and ethene.

What products does cracking produce and why are alkenes useful?

Cracking always produces a mixture of shorter alkanes and alkenes, and the exact ratio depends on which type of cracking is used and the exact conditions applied. Alkenes are unsaturated, meaning they contain at least one carbon–carbon double bond, which makes them far more chemically reactive than the saturated alkanes they come from. This reactivity is exactly why alkenes are so valuable: they can undergo addition reactions to form polymers, such as ethene molecules joining together to form poly(ethene), or react with other small molecules to make solvents, alcohols and other everyday chemicals that alkanes cannot easily produce.

Frequently asked questions

What's the difference between catalytic and thermal cracking?

Catalytic cracking uses a hot catalyst, such as a zeolite, at a moderate temperature and low pressure, and mainly produces branched and cyclic hydrocarbons useful as motor fuels. Thermal cracking uses no catalyst at all, relying instead on much higher temperatures and pressures, and produces a higher proportion of alkenes such as ethene. Both processes achieve the same basic goal of breaking long hydrocarbon chains into shorter, more useful ones, but they favour different products.

Why can't we just extract more petrol directly from crude oil?

Crude oil contains a fixed natural mixture of hydrocarbon chain lengths, and fractional distillation only separates that existing mixture into fractions — it cannot create more of any particular fraction. Because crude oil naturally contains more long-chain hydrocarbons than short-chain ones, but demand for products like petrol is much higher than demand for long-chain fractions, cracking is needed to convert the surplus long chains into the shorter molecules that are actually needed.

How do you test whether a product of cracking is an alkene?

Bromine water is used to test for alkenes. When an alkene is shaken with orange bromine water, the double bond reacts with the bromine and the solution rapidly turns colourless. Alkanes, which contain no double bond, do not react with bromine water in the same way, so the solution stays orange, making bromine water a reliable way to distinguish a cracking product that is an alkene from one that is an alkane.

Is cracking a physical or chemical change?

Cracking is a chemical change. Strong covalent carbon–carbon bonds within the long-chain hydrocarbon molecules are broken, and entirely new substances, with different chemical formulae and different properties, are formed as a result. This distinguishes cracking clearly from fractional distillation, which is a physical process that only separates existing substances by boiling point without breaking any chemical bonds.

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