Cracking (Chemistry)

Cracking (Chemistry)

Cracking in Chemistry is like cracking open an egg. You break down large molecules into smaller, more useful ones. This process is commonly used to break long-chain hydrocarbons into shorter-chain alkanes and alkenes. It's like revealing the yummy inner white and yolk of an egg when you crack it open. In this article, we'll explore the fascinating world of cracking in Chemistry.

Why do we crack hydrocarbons?

When crude oil is refined through Fractional Distillation, it produces various hydrocarbon fractions that have different purposes and demands. Naphtha, for instance, which has carbon chain lengths between 6 and 12, is in high demand as it is used for petrol and in the chemical industry. On the other hand, there is less demand for longer-chain hydrocarbons with over 16 carbon atoms. Unfortunately, crude oil from the North Sea commonly contains only 10 percent naphtha, but 66 percent gas oil and fuel oil fractions, which means we have a surplus of longer-chain hydrocarbons that have little economic value. This poses a challenge as we don't have much use for them.

Composition of the North Sea crude oil. | Download Table
The composition of typical North Sea crude oil

To make longer-chain fractions more economically valuable, we can crack them to produce shorter-chain molecules. This has the benefit of producing not only shorter alkanes, but also alkenes, which we’ll introduce below. These hydrocarbons are in much higher demand and are a lot more useful to us than longer-chain hydrocarbons, making cracking such an important reaction. But what exactly are these products used for?

Short-chain alkanes

You’ll probably find examples of short-chain alkanes such as butane, , everywhere in your local environment. Butane is commonly used in cigarette lighters, in aerosols, as a food additive, in fridges, and even as fuel for your car.



Alkenes are unsaturated hydrocarbons. An unsaturated molecule contains at least one C=C double bond. Alkenes are more reactive than alkanes and are used as a chemical feedstock, which means that they supply industries with the starting materials used to make further products. Reacting multiple alkene molecules together forms plastic polymers, such as those used in plastic bags, bottles, and nylon clothing. We also use them as a base for making alcohols, paints, and medicines.

The simplest alkene, known as ethene
The simplest alkene, known as ethene

To find out more about these hydrocarbons, check out Alkenes.

What are the different types of cracking?

There are two different types of cracking we commonly use to split hydrocarbons. These are known as thermal cracking and catalytic cracking. Because hydrocarbons like alkanes are relatively unreactive due to their strong, non-polar C-C and C-H bonds, both types of cracking require certain harsh conditions to break them down.

Thermal cracking

Thermal cracking is a process that involves applying extreme heat and pressure to alkanes for a brief period, typically only one second. This splits the alkane into two free radicals, which are highly reactive molecules with an unpaired outer shell electron. Free radicals then react to produce various hydrocarbons, especially alkenes. Although this process is effective, it requires a lot of fuel and has a significant economic and environmental impact.

Catalytic cracking, on the other hand, uses a zeolite catalyst to increase the rate of reaction by lowering the activation energy needed for a reaction to occur. This process occurs at a lower temperature and pressure than thermal cracking, with a pressure slightly above atmospheric pressure and a temperature of 700K. However, larger alkanes cannot be cracked in this way. Catalytic cracking produces a high proportion of cyclic and branched alkanes, as well as aromatic compounds like benzene. It's a more efficient process than thermal cracking and requires less fuel. Here's a comparison table of the two types of cracking:

A table comparing thermal and catalytic cracking
A table comparing thermal and catalytic cracking

Cracking is a process that randomly produces multiple different equations and potential products. The total number of carbons and hydrogens on each side of the equation must be equal. Let's look at an example: Decane, , can be cracked to produce octane, and one other molecule. The balanced equation for the reaction is:

C10H22 → C8H18 + C2H4

In this equation, x = 2 and y = 4. The other product is ethene, .

Here's another example: One molecule of alkane X can be cracked to produce one molecule of heptane and two molecules of propene. The equation for this reaction is:

C(x)H(y) → C7H16 + 2C3H6

There are 13 carbons and 28 hydrogens on the right-hand side of the equation. As there is only one molecule reacting, the formula for X must be C13H28.

Overall, cracking is a process used to turn longer-chain hydrocarbons into more economically valuable shorter-chain alkanes and alkenes. It can be done thermally or catalytically, but it requires harsh conditions to break the strong bonds within the alkanes. Cracking is random and produces a mixture of products.

Cracking (Chemistry)

What is cracking?

Cracking is the process of breaking down-longer chain fractions from the fractional distillation of crude oil into shorter lengths.

Why do we crack hydrocarbons?

Longer-chain hydrocarbons have a low demand, so are cracked to produce more economically valuable shorter-chain hydrocarbons.

What are the two common types of cracking?

Catalytic cracking and thermal cracking.

What conditions are needed for cracking?

Thermal cracking requires a temperature of 700-1200K and a pressure of 7000 kPa. Catalytic cracking requires a zeolite catalyst, a temperature of 700K, and a slightly raised pressure.

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