Crude oil, hydrocarbons and cracking – GCSE Combined Science Chemistry

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Crude oil is a thick, dark liquid found underground. On its own it is not much use. Once it is separated, it gives us petrol, diesel, jet fuel and the starting chemicals for plastics.

It matters because crude oil powers most of the transport you use. It is also a finite resource, so we need to understand how we use it. Burning it affects the air too.

By the end you will be able to explain how crude oil is separated. You will describe how properties change with chain length and write combustion equations. You will also explain why cracking is done.

Crude oil and alkanes

Crude oil is a mixture of hydrocarbons. A hydrocarbon is a compound made of hydrogen and carbon only. It formed from the remains of ancient sea creatures, mainly plankton. They were buried in mud and squashed and heated for millions of years.

Crude oil is finite, which means it will run out, because we use it far faster than it forms. Most of its hydrocarbons are alkanes. Alkanes are saturated, so they have only single bonds between carbon atoms.

The general formula is CₙH₂ₙ₊₂. Double the carbons and add 2. The first four are CH₄, C₂H₆, C₃H₈ and C₄H₁₀. Six carbons gives C₆H₁₄.

Petrol, kerosene and diesel oil are fuels obtained from crude oil, and methane is the main part of natural gas. All of these are fossil fuels. They are non-renewable, because they take millions of years to form and we use them much faster than they are made.

Fractional distillation

Crude oil is heated until most of it is a vapour. The vapour goes into a fractionating column. The column is hot at the bottom and cooler at the top.

Each hydrocarbon condenses where the temperature is just below its boiling point. Long chains have high boiling points, so they condense near the bottom. Short chains have low boiling points, so they rise to the top. The bars show boiling points rising with chain length.

A fraction is a group of hydrocarbons with similar boiling points. Think of climbing a ladder of temperatures. Each hydrocarbon climbs until it is cool enough to turn back into a liquid.

Fractions and their uses

There are six main fractions. From top to bottom they are refinery gases, petrol, kerosene, diesel oil, fuel oil and bitumen.

Refinery gases are used for heating and cooking. Petrol fuels cars. Kerosene fuels aircraft. Diesel oil fuels lorries and trains. Fuel oil is used in ships and for heating. Bitumen is used to surface roads.

Fractions are also feedstock, which means starting materials, for making chemicals such as solvents and polymers.

Trends in properties

As the chain gets longer, three things change. The boiling point rises. The viscosity increases, which means the liquid is thicker and flows less easily. The flammability decreases, which means it ignites less easily.

The reason is intermolecular forces, the weak forces between molecules. They get stronger between longer molecules, so more energy is needed to separate them.

So petrol is runny and catches fire easily. Think of water compared with syrup. Bitumen is thick and sticky, like cold syrup. Small molecules are volatile, which means they evaporate easily.

Combustion

Hydrocarbons burn in plenty of oxygen to make carbon dioxide and water. This is complete combustion, and it releases energy. CH₄ + 2O₂ → CO₂ + 2H₂O. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

In these reactions the carbon and the hydrogen are both oxidised. Oxidised means they gain oxygen.

If the oxygen supply is limited, combustion is incomplete. It makes carbon monoxide or carbon (soot) as well as water. For example, 2CH₄ + 3O₂ → 2CO + 4H₂O. Carbon monoxide is toxic, so gas appliances must be serviced and rooms ventilated.

Cracking and why we do it

Cracking breaks long hydrocarbon molecules into smaller, more useful ones. It is a thermal decomposition reaction. In catalytic cracking, the vapour passes over a hot catalyst at about 550 °C. In steam cracking, the hydrocarbon is mixed with steam and heated to a very high temperature.

Cracking makes an alkane and an alkene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄. An alkene has a double bond, so it is unsaturated. The atoms balance on both sides.

We crack because the demand for small hydrocarbons, such as petrol, is greater than the supply. Alkenes are also used to make polymers.

Recap

Crude oil is a finite mixture of hydrocarbons. Most are alkanes, with the formula CₙH₂ₙ₊₂.

Fractional distillation separates it by boiling point. Long chains condense at the bottom.

Longer chains have higher boiling points, are more viscous and are less flammable.

Complete combustion makes carbon dioxide and water. Incomplete combustion can make carbon monoxide.

Cracking turns long chains into smaller alkanes and alkenes, because small molecules are in higher demand.