Cracking: Producing More Useful Materials

Describe the typical spatial arrangement of chemical bonds within simple covalent molecules.
Explain covalent bonding as the electrostatic attraction arising from the sharing of electron pairs between adjacent atoms.
Construct accurate dot and cross diagrams to model simple covalent substances.
Translate structural representations of simple molecules fluidly between three-dimensional space and two-dimensional geometric diagrams.
Evaluate the inherent theoretical limitations of models such as dot and cross diagrams, ball and stick models, and 2D/3D representations for simple molecules.

Why separate crude oil and then crack it?

Crude oil is a finite mixture of hydrocarbons: compounds containing only carbon and hydrogen. It supplies both fuels and the raw materials, or feedstocks, used to make many chemicals and consumer products.

Fractional distillation separates crude oil into fractions containing molecules of similar size. However, the amounts obtained do not necessarily match the amounts people need. There is high demand for shorter-chain hydrocarbons used in fuels such as petrol, while some fractions containing larger molecules are available in excess.

Cracking converts some of these larger hydrocarbon molecules into smaller, more useful molecules. Rather than simply separating what is already present, it changes the molecules themselves.

How industrial cracking works

In catalytic cracking, a fraction containing long-chain hydrocarbons is heated so that it vaporises. The vapour passes over a hot catalyst, such as a zeolite. The high temperature provides energy for the reaction, and the catalyst increases its rate without being used up.

Carbon–carbon covalent bonds in the larger molecules break, and the atoms rearrange to form smaller molecules. Cracking is a form of thermal decomposition: a substance is broken down using heat. Another method uses steam at very high temperatures without a catalyst.

A fraction containing long-chain hydrocarbons enters a heater and is vaporised. The hydrocarbon vapour passes into a hot zeolite catalyst region, where larger molecules break into smaller molecules. The catalyst remains in this region. Two product streams leave: shorter-chain alkanes used as fuels, including petrol, and alkenes such as ethene used as chemical feedstocks for polymers such as poly(ethene). — Catalytic cracking turns larger hydrocarbons into smaller molecules useful as fuels and chemical feedstocks.

Cracking is a chemical change because new substances form. This differs from fractional distillation, a physical separation in which the hydrocarbon molecules remain unchanged.

What molecules are produced?

Cracking long-chain alkanes produces a mixture containing shorter-chain alkanes and alkenes. An alkane has only single bonds between its carbon atoms; an alkene contains a carbon–carbon double bond.

One possible reaction is the breakdown of decane into octane and ethene:

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

The ten carbon atoms in the original molecule are shared between the two smaller molecules: eight in octane and two in ethene. The hydrogen atoms are also conserved: $22 = 18 + 4$ . Atoms are rearranged, not created or destroyed.

This equation shows one possible outcome, not a rule that every molecule splits into the same products. Different cracking conditions can produce different mixtures.

Why the products are more useful

The shorter-chain alkanes can be used as fuels. Cracking therefore helps increase the supply of fuel molecules for which demand is high.

The alkenes provide feedstocks for making new materials. For example, ethene can be used to make the polymer poly(ethene), a widely used plastic. Cracking is therefore important not just for supplying fuels, but also for manufacturing materials.

By converting surplus larger molecules into products with greater demand, cracking makes better use of crude oil. It does not, however, make crude oil renewable: the original resource is still finite.

Process

Cracking: larger hydrocarbons → smaller, more useful molecules.

Heat a long-chain hydrocarbon fraction to vaporise it.
Pass the vapour over a hot catalyst.

When explaining why cracking is useful, link both products to their uses: shorter-chain alkanes for fuels and alkenes for making polymers.

Distinguish cracking, which makes new substances, from fractional distillation, which separates an existing mixture.

Describe the conditions as well as the change: vaporise the hydrocarbons, then pass the vapour over a hot catalyst.

A compound containing only carbon and hydrogen atoms.

A group of hydrocarbons separated from crude oil that have similar molecular sizes and boiling points.

The breakdown of larger hydrocarbon molecules into smaller, more useful molecules using high temperatures, often with a catalyst.

A hydrocarbon with only single bonds between its carbon atoms.

A hydrocarbon containing a carbon–carbon double bond, $\mathrm{C=C}$ .

A substance that increases the rate of a chemical reaction without being used up.

A raw material used to manufacture other chemicals or materials.

A compound containing only carbon and hydrogen atoms.

A group of hydrocarbons separated from crude oil that have similar molecular sizes and boiling points.

The breakdown of larger hydrocarbon molecules into smaller, more useful molecules using high temperatures, often with a catalyst.

A hydrocarbon with only single bonds between its carbon atoms.

A hydrocarbon containing a carbon–carbon double bond, $\mathrm{C=C}$ .

A substance that increases the rate of a chemical reaction without being used up.

A raw material used to manufacture other chemicals or materials.

What you need to know

Why separate crude oil and then crack it?

How industrial cracking works

What molecules are produced?

Why the products are more useful

Process

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Key terms(7)

Key terms(7)

Products and importance

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