Cracking Alkanes: An Introduction Revision Notes | A Level AQA Chemistry

1. Why Cracking Exists: The Economic Problem

Crude oil is not a single substance — it is a complex mixture of hydrocarbons. Fractional distillation separates it into fractions based on boiling point, and each fraction contains alkanes of a particular chain-length range. Here is the problem: the fractions that come out of distillation in the largest quantities are not the fractions the world needs most.

Heavy fractions — those with long carbon chains, such as fuel oil and bitumen — are produced in excess. Meanwhile, lighter fractions such as petrol (gasoline) and naphtha, which contain shorter-chain alkanes, are in far greater demand than distillation alone can supply. The market is structurally mismatched, and that mismatch is the economic driver behind cracking.

The chart above illustrates the core issue: heavy fractions are over-supplied relative to demand, while petrol and naphtha fractions fall short. Cracking converts the surplus heavy fractions into the shorter-chain products the market actually wants, making it an economically essential process rather than an optional one.

There is a second economic incentive beyond fuel supply. Cracking produces alkenes — particularly ethene and propene — as by-products. Alkenes are the feedstocks for the polymer industry: they are used to manufacture poly(ethene), poly(propene), and a vast range of plastics. Without cracking, the petrochemical industry would have no affordable source of these reactive molecules.

2. What Actually Happens: Breaking C–C Bonds

Now that the economic motivation is clear, the chemistry itself becomes the focus. At the molecular level, cracking is the breaking of C–C bonds within long-chain alkane molecules. Carbon–carbon single bonds are strong (average bond enthalpy approximately 347 kJ mol⁻¹), so significant energy input is required to break them — which is why high temperatures are always involved.

An alkene can form when a radical fragment undergoes further bond breaking (beta-scission), giving a smaller radical and a molecule containing a C=C double bond.

A straightforward example helps to anchor this. Consider decane ($C_{10}H_{22}$), a long-chain alkane found in the kerosene fraction. Under cracking conditions, one possible outcome is its conversion into octane ($C_8H_{18}$) and ethene ($C_2H_4$):

Notice that the equation is balanced: 10 carbons and 22 hydrogens on the left, 10 carbons and 22 hydrogens on the right. Cracking typically produces a shorter-chain alkane (saturated, general formula $C_nH_{2n+2}$) and at least one alkene (unsaturated, general formula $C_nH_{2n}$); hydrogen gas may also be produced, particularly in thermal cracking. The formation of at least one alkene is a defining feature of cracking reactions.

3. Two Routes: Thermal Cracking vs Catalytic Cracking

Cracking is not a single industrial process — there are two main variants, each optimised for different products. Understanding the conditions and outputs of each is essential for AQA examination questions that ask you to compare or evaluate them.

Thermal cracking relies entirely on heat to supply the activation energy needed to break C–C bonds homolytically. Because no catalyst is present, very high temperatures are required, and the radical chain mechanism favours the production of small, reactive alkenes. Ethene is the most commercially valuable product of thermal cracking — it is the monomer for poly(ethene) and a starting material for ethanol manufacture.

Catalytic cracking uses a zeolite catalyst, which provides an alternative lower-energy pathway for bond breaking. The zeolite has a large surface area and an acidic character that promotes the formation of carbocation intermediates rather than free radicals. This mechanism favours the production of branched-chain alkanes and aromatic compounds, both of which have higher octane ratings and are therefore more valuable as petrol additives.

A key point for exam answers: the zeolite catalyst lowers the activation energy of the cracking reaction, allowing it to proceed at a lower temperature. This reduces energy costs compared with thermal cracking, which is an additional economic advantage of the catalytic route when the target products are motor fuels rather than alkenes.

4. Worked Example: Writing and Balancing a Cracking Equation

Exam questions frequently ask you to write or complete a cracking equation. The method is systematic: use atom conservation to identify the missing product, then verify the equation is balanced. Work through the example below step by step before checking each stage.

Worked example: Writing a Balanced Cracking Equation

Dodecane ($C_{12}H_{26}$) is cracked to produce propene ($C_3H_6$) and one other product. Identify the other product and write the balanced equation.

1. $C_{12}H_{26}$ contains 12 carbon atoms — The reactant is dodecane. All 12 carbons must appear in the products — carbon atoms are conserved.
2. Carbons in unknown = 12 − 3 = 9 — Propene ($C_3H_6$) accounts for 3 carbons. The remaining 9 carbons must be in the second product.
3. $C_{12}H_{26}$ contains 26 hydrogen atoms — All 26 hydrogens must be conserved across the two products.
4. Hydrogens in unknown = 26 − 6 = 20 — Propene ($C_3H_6$) accounts for 6 hydrogens. The remaining 20 hydrogens go to the second product.
5. Second product: $C_9H_{20}$ — this is nonane — Check: nonane fits the alkane general formula $C_nH_{2n+2}$. For $n = 9$: $2(9) + 2 = 20$. Confirmed as a saturated alkane.
6. $C_{12}H_{26} \rightarrow C_9H_{20} + C_3H_6$ Left: 12 C, 26 H. Right: 9 + 3 = 12 C, 20 + 6 = 26 H. — Both sides balance. The equation shows one alkane reactant producing one shorter alkane and one alkene — the hallmark of a cracking reaction.

Answer: $C_{12}H_{26} \rightarrow C_9H_{20} + C_3H_6$. The second product is nonane ($C_9H_{20}$).

The atom-conservation method works for any cracking equation. Check that products include at least one alkene ($C_nH_{2n}$) and typically one alkane ($C_nH_{2n+2}$); hydrogen gas ($H_2$) may also appear as a product in some cracking reactions, particularly thermal cracking. If no alkene is present among the products, recheck your arithmetic before assuming an unusual outcome.

5. Exam Technique and Common Mistakes

The two specification requirements for this topic — economic reasons for cracking and the breaking of C–C bonds — appear in different question styles. Economic questions tend to use 'explain' or 'evaluate' command words and reward multi-point answers that link supply, demand, and product value. Mechanism questions tend to use 'describe' or 'state' and reward precise molecular-level language.

Bringing both specification requirements together: the economic case for cracking (supply–demand mismatch and alkene production) and the molecular mechanism (C–C bond breaking to give alkanes and alkenes) are two sides of the same story. The industry carries out cracking because the chemistry allows it to convert low-value surplus into high-value products — and that is precisely the argument examiners want to see made explicitly in extended-response answers.