Thermal vs. Catalytic Cracking Revision Notes | A Level AQA Chemistry

1. Why Cracking Exists: The Industrial Problem

Fractional distillation of crude oil gives us a range of hydrocarbon fractions — but the amounts we get do not match the amounts industry needs. There is far too much of the heavy, long-chain fractions and nowhere near enough of the lighter, more useful ones.

Cracking is the industrial solution: long-chain alkane molecules are broken down into shorter, more commercially valuable hydrocarbons. The word 'cracking' is precise — you are literally cracking C–C bonds apart.

There are two distinct methods used in industry: thermal cracking and catalytic cracking. They share the same broad goal but differ significantly in their conditions, mechanisms, and the products they favour. Understanding those differences is exactly what AQA will test you on.

2. Thermal Cracking: Conditions and Products

Thermal cracking relies entirely on energy — no catalyst is involved. The process operates at high temperature (typically in the range of 400–900 °C) and high pressure. These extreme conditions supply enough energy to break the strong C–C covalent bonds in long-chain alkanes.

The key characteristic of thermal cracking is the high proportion of alkenes produced. At these extreme temperatures, C–C bonds break homolytically (each atom takes one electron from the bond), generating highly reactive radical intermediates that lead to alkene formation. AQA does not require you to know the mechanism in detail — but you must know that alkenes are the dominant product.

Alkenes are enormously valuable as chemical feedstocks. Ethene, for example, is polymerised to make poly(ethene), and propene is used to make poly(propene). The polymer and plastics industries depend on the alkene output from thermal cracking.

3. Catalytic Cracking: Conditions and Products

Now compare that with catalytic cracking, which takes a very different approach. Instead of brute-force energy, it uses a zeolite catalyst to lower the energy required for bond breaking. This means the process can run at high temperature but only slight pressure — a significant difference from thermal cracking.

The zeolite catalyst has a large surface area and contains acidic sites that interact with the hydrocarbon molecules, promoting C–C bond cleavage at much lower pressures than thermal cracking demands. The products are predominantly branched-chain alkanes (which burn more efficiently in petrol engines), cycloalkanes, and aromatic hydrocarbons such as benzene and methylbenzene.

This makes catalytic cracking the process of choice for producing motor fuels. Branched-chain alkanes have higher octane ratings than their straight-chain equivalents, meaning they resist 'knocking' in petrol engines and perform better as fuels.

4. Comparing the Two Processes Side by Side

At this point you have the core facts for both processes. The most reliable way to lock them in — and to prepare for the 'compare' command word in exam questions — is to place them directly alongside each other.

Notice that both processes use high temperature — this is a similarity that examiners sometimes exploit. The critical difference is pressure: high for thermal cracking, slight for catalytic cracking. Get this distinction wrong and you will lose marks on a 'compare' question.

5. The Industrial Significance of Each Process

Both cracking processes address the same supply-demand mismatch in crude oil refining, but they serve different sectors of the economy. Thermal cracking feeds the polymer industry: the alkenes it produces — particularly ethene and propene — are the monomers from which plastics are made. Without thermal cracking, the supply of these monomers would be severely limited.

Catalytic cracking, on the other hand, feeds the fuel industry. The branched-chain alkanes it produces are blended into petrol, and the aromatic hydrocarbons it generates are used both as fuel components and as starting materials for the synthesis of dyes, pharmaceuticals, and other chemicals.

From an economic standpoint, catalytic cracking is the more widely used process in modern refineries, because the demand for motor fuels is enormous. Thermal cracking remains essential wherever a high alkene yield is specifically required — for example, when a refinery is supplying a nearby petrochemical plant making polymers.

6. Exam Technique: Answering Cracking Questions

Cracking questions on AQA papers tend to use the command words 'describe', 'compare', 'explain', and 'evaluate'. Each demands a slightly different response structure, and knowing the difference is what separates a 4-mark answer from a 2-mark one.

For a 'describe' question about cracking conditions, list the conditions precisely: temperature, pressure, and whether a catalyst is present. Do not add 'because' — that is explanation, not description. For a 'compare' question, you must address both processes and explicitly state similarities and differences. Saying 'thermal cracking uses high pressure' is worth a mark; saying 'thermal cracking uses high pressure whereas catalytic cracking uses only slight pressure' is worth more because it makes the contrast explicit.

For an 'evaluate' question about which process is more suitable for a given purpose, structure your answer as: state which process you recommend, give the relevant condition or product as evidence, then address the counter-argument briefly before reaching a conclusion. Always tie your conclusion back to the specific context given in the question.

7. Consolidation: Pulling It All Together

You now have the full picture. Cracking exists because crude oil distillation produces too many long-chain hydrocarbons and not enough of the shorter, more useful ones. Thermal cracking — high temperature, high pressure, no catalyst — solves this by generating a high proportion of alkenes for the polymer industry. Catalytic cracking — high temperature, slight pressure, zeolite catalyst — solves it differently, producing motor fuels and aromatic hydrocarbons for the fuel and chemical industries.

The two processes are not in competition; they are complementary. A modern refinery uses both, directing different feedstocks through each process depending on what the market currently needs most. That industrial context is worth keeping in mind when you encounter an 'evaluate' or 'suggest' question in the exam.

Both cracking processes address the supply-demand imbalance in crude oil refining: thermal cracking supplies alkene feedstocks for polymers, while catalytic cracking supplies motor fuels and aromatic hydrocarbons — together covering the two largest sectors of the petrochemical industry.