Lesson Explainer: Cracking of Hydrocarbons Chemistry

In this explainer, we will learn how to explain the catalytic cracking of alkanes and its importance and describe the process on an industrial scale.

Cracking is the process of taking bigger organic molecules, like the hydrocarbons from crude oil, and turning them into smaller ones. So, broadly speaking, cracking reactions are decomposition reactions.

In this explainer, cracking will be used to refer exclusively to the cracking of hydrocarbons.

Definition: Decomposition Reaction

A type of reaction where a substance breaks down to form two or more other substances.

Definition: Cracking Reaction

A type of decomposition reaction where larger organic molecules are broken down in smaller molecules.

While cracking can be performed on unsaturated hydrocarbons, like alkenes, we will focus on cracking as it applies to the abundant saturated hydrocarbons in crude oil. Cracking crude oil will generally produce a mixture rich in alkanes and alkenes.

Cracking can produce pure carbon, pure hydrogen, and other types of hydrocarbon, as well as alkanes and alkenes, but the process is controlled to give the most desirable products.

Typically, the overall effect of cracking a large hydrocarbon molecule is to

  1. break carbon–carbon single bonds,
    CHHCHHCHHCHH
  2. rearrange hydrogen atoms by breaking and forming carbon–hydrogen bonds,
    CHCHHHCHCHHH
  3. form other carbon–carbon bonds like carbon–carbon double bonds.
    CHCHHHHCHCHH

The process of cracking is endothermic, as the bonds formed are weaker than the bonds that are broken. Cracking relies on high temperatures to drive the process forward, and high pressures are used to increase the reaction rate.

Commonly, catalysts are zeolites—compounds of aluminum, silicon, and oxygen. There are many types of zeolite, some natural and some synthetic. In the lab, you can perform the cracking of a hydrocarbon like paraffin using pieces of pottery, or use silicon dioxide (SiO2, also known as silica) or aluminum oxide (AlO23, also known as alumina).

Steam cracking is done using extremely hot steam, with or without a catalyst.

Steam cracking without a catalyst is typically done at >800C and is generally used to produce very short alkenes in large quantities. This form of cracking is often called thermal cracking, because we rely entirely on the high temperature to drive the reaction.

Catalytic cracking is typically done at 500C700C, reducing the amount of energy needed to heat the reaction mixture. Catalytic cracking is generally used for converting crude oil fractions into lighter, more profitable ones.

The two applications of cracking we are going to look at here are the production of alkenes and the conversion of heavier fractions of crude oil into lighter ones.

Example 1: Recalling the Conditions for Thermal Cracking

Under which of the following conditions is thermal cracking usually performed?

  1. Low temperature and high pressure
  2. High temperature and low pressure
  3. High temperature and high pressure
  4. Low temperature and low pressure

Answer

Thermal cracking has a name with a hint in it. Thermal indicates that high temperatures are involved. However, we need to know a little more about cracking to figure out the rest.

Cracking—and here, we are talking about cracking hydrocarbons—is an endothermic process. This means that the hotter it is, the greater the equilibrium yield is.

When cracking hydrocarbons, we typically use temperatures in excess of 500C with a catalyst. Without a catalyst, we typically use temperatures in excess of 800C. Thermal cracking is the name for cracking done without a catalyst.

Without a catalyst, the only other way to increase the reaction rate is to use high pressures. Cracking produces more molecules of gas than we start with, so the pressure cannot be too high, or the equilibrium yield will decrease too much.

Generally speaking, cracking is performed at a high temperature and pressure, providing the highest reaction rate and a good equilibrium yield, without being too expensive.

The answer is C.

Alkenes like ethene and propene are extremely valuable. They can be used to make polymers like poly(ethene), also known as polythene or PE, and poly(propene), also known as polypropylene or PP.

Crude oil does not naturally contain a lot of these chemicals, so they can be made by cracking larger hydrocarbons.

Let’s have a look at octane:

CCCCCCCCHHHHHHHHHHHHHHHHHH

Octane is one of the many hydrocarbons that might be in the mix that is used to make ethene and propene.

The cracking of octane might occur as follows:

In the process, a carbon–carbon bond is broken, carbon–hydrogen bonds break and reform, and a carbon–carbon single bond is converted to a carbon–carbon double bond.

When calculating the enthalpy change of the reaction, we can ignore the carbon–hydrogen bonds (since as many break in the reactants as are formed in the products). Instead, we can focus on the carbon–carbon single and double bonds.

These are the bonds that are formed and broken during a single cracking reaction:

BondCHCCCC
Bonds Broken11
Bonds Formed11

However, there is one critical thing to observe: the carbon–carbon double bond is not forming from two unbonded carbon atoms; it is forming between carbon atoms that already have a single bond between them, so the energy released is the difference between the bond energies of the single and double bonds.

CC Bond Energy345 kJ/mol
CC Bond Energy611 kJ/mol
Energy In (Breaking a CC Bond)345 kJ/mol
Energy Out (Converting a CC Bond to a CC Bond)(611345)/=266/kJmolkJmol
Enthalpy Change(345266)/=79/kJmolkJmol

So, we can see that cracking octane is an endothermic reaction. The reaction generates two molecules from one molecule, so the reaction is driven by the change in entropy and the high temperature.

The cracking of a straight-chain alkane like octane produces a shorter alkane (hexane) and an alkene (ethene). This pattern can be used to deduce the identity of one product from the identity of the other.

Also, the products from one cracking reaction may undergo cracking themselves.

Here are a number of ways that octane might be cracked:

Products from these reactions, like hexane, could be cracked themselves, producing even smaller alkanes and alkenes.

Example 2: Predicting the Second Product of the Cracking of Heptane given the Displayed Formula of the Other Product

Shown in the equation is one possible reaction in the cracking of heptane:

+XCHHHCCHHCHHHCHHHCHHCHHHCHHHCHCHH

Compound X is an unbranched hydrocarbon. What is the displayed formula of compound X?

Answer

Heptane is an alkane with the chemical formula CH716. The products of a single cracking reaction of an alkane will be an alkane and an alkene, both of which will be shorter than the starting alkane.

The first product in this cracking reaction is propene, an alkene with the chemical formula CH36. From this, we can deduce that the chemical formula of the second product is CH410. This matches the general formula for an alkane, CH2+2, where 𝑛 is 4. The question tells us that compound X is unbranched, so compound X must be butane.

CHCHHHHCCHHHHH

Crude oil is a complex mixture of many different hydrocarbons, not just simple alkanes and alkenes. It is distilled into fractions. Each fraction contains a smaller range of these hydrocarbons, all with similar boiling points.

Definition: Fraction

The pure substance or mixture that can be collected over a temperature range from a fractional distillation.

Fractions like gasoline and liquified petroleum gas are in high demand, but some of the heavier fractions (fractions with larger hydrocarbons), like lubricating oil, fuel oil, and tar, are not as desirable.

Cracking of heavier crude oil fractions is usually done at lower temperatures than for the production of alkenes, around 535C, at a little under 2 bar of pressure, with a zeolite catalyst.

When the heavier fractions are cracked, the longer hydrocarbons are broken down into smaller ones. For example, fuel oil goes in and gasoline comes out.

So, theoretically, a heavier fraction could be converted into any lighter fraction by cracking.

These are the typical fractions of crude oil:

If we crack a fraction, we will be able to produce a mixture that is more like a lighter fraction. As hydrocarbons get bigger, their properties change in a predictable way:

As hydrocarbons get bigger, their boiling points decrease and they become less volatile and less flammable. At the same time, they become more viscous.

So, cracking tends to produce mixtures with lower boiling points, which are more volatile, more flammable, and less viscous.

Example 3: Identifying the Property That Increases for Hydrocarbons That Undergo Cracking in a Set of Properties

In which of the following properties is an increase observed when hydrocarbon fuels undergo cracking?

  1. Melting point
  2. Volatility
  3. Viscosity
  4. Molecular size
  5. Ignition temperature

Answer

Hydrocarbons, like alkanes and alkenes, tend to have predictable changes in their properties as their chain length decreases. During a cracking reaction, hydrocarbons are generally decomposed into shorter alkanes and alkenes.

Hydrocarbon fuels (like those produced by fractional distillation of crude oil) will have properties that are tailored to suit their final applications. Since some fractions of crude oil are more valuable than others, cracking is performed to change the properties of a heavier fraction (a fraction containing larger hydrocarbons) to resemble those of a lighter fraction (a fraction containing smaller hydrocarbons).

As hydrocarbons decrease in size, we tend to see a decrease in their melting points, viscosity (thickness as a liquid), and the temperature required to set them alight (their ignition temperature). These decrease because of the decrease in intermolecular force strength (largely London dispersion forces) as the molecules get smaller.

However, one feature that increases as hydrocarbons get smaller is their boiling point. Since the intermolecular forces get weaker as the molecules get smaller, less energy is required to overcome them and turn the molecules in gases. A volatile substance is a substance with a low boiling point, so we expect cracking of a hydrocarbon fuel to produce products with lower boiling points (that are therefore more volatile) than the starting material.

The answer is B.

Key Points

  • Cracking is the process of converting large organic molecules (like hydrocarbons) into smaller ones.
  • Cracking hydrocarbons is an endothermic process and requires high temperatures to work.
  • Cracking of hydrocarbons is done thermally (without a catalyst) or catalytically (with a catalyst); thermal cracking is generally done at higher temperatures.
  • Alkanes can be cracked into a mixture of alkanes and alkenes.
  • Heavier fractions of crude oil can be converted into lighter fractions by cracking.

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