Crude oil is a hydrocarbon mixture and a feedstock

Definition

Hydrocarbon

A compound containing carbon and hydrogen atoms only.

Definition

Feedstock

A raw material that an industrial process turns into other useful substances.

Crude oil is a thick, dark mixture of many different hydrocarbons, pumped from rocks beneath the land and the seabed.
1. Most of these hydrocarbons are alkanes, with chains ranging from one carbon atom to more than fifty.
Crude oil is the main source of the hydrocarbons used as fuels, such as petrol, diesel and kerosene.
It is also a feedstock for the petrochemical industry, which turns its hydrocarbons into new substances.
1. Plastics, solvents, detergents, lubricants, dyes and many medicines are all made from crude oil hydrocarbons.
Crude oil is of little use as it comes out of the ground, because each use needs hydrocarbons with a particular chain length.

Key Idea

Crude oil has to be separated into simpler mixtures before its hydrocarbons can be used as fuels or feedstocks.

Each fraction contains alkanes of similar chain length

Definition

Fraction

A mixture of hydrocarbons with similar numbers of carbon atoms and similar boiling points, collected together from crude oil by fractional distillation.

Definition

Alkane

A hydrocarbon in which all the bonds between carbon atoms are single covalent bonds.

Fractional distillation separates crude oil into fractions with different boiling point ranges.
Each fraction is still a mixture, containing hydrocarbons with similar numbers of carbon atoms and similar boiling points.
The fractions are made up largely of alkanes, which form a homologous series.
1. Each alkane has one more $CH2\text{CH}_2$ unit than the one before, and all of them fit the general formula: $CnH2n+2\text{C}_n\text{H}_{2n+2}$
2. In the formula, $n$ is the number of carbon atoms, so octane, with $8$ carbon atoms, is $C8H18\text{C}_8\text{H}_{18}$ .
Alkanes contain only single bonds, so each carbon atom is bonded to as many hydrogen atoms as possible.

Example

Find the formula of the alkane with $12$ carbon atoms, then decide whether $C10H20\text{C}_{10}\text{H}_{20}$ is an alkane.
Substitute $n = 12$ : $\times 12) + 2 = 26$ , so the formula is $C12H26\text{C}_{12}\text{H}_{26}$ .
An alkane with $10$ carbon atoms needs $\times 10) + 2 = 22$ hydrogen atoms.
$C10H20\text{C}_{10}\text{H}_{20}$ has only $20$ hydrogen atoms, so it does not fit $CnH2n+2\text{C}_n\text{H}_{2n+2}$ and is not an alkane.

In fractional distillation, vapours condense at different heights

Definition

Fractionating column

A vertical tube in which vapour repeatedly condenses and evaporates, improving the separation of liquids with different boiling points.

Crude oil is heated to about $∘C350\ ^{\circ}\text{C}$ in a furnace, so most of it boils and turns to vapour.
The hot vapour is piped into the bottom of a tall fractionating column.
The column has a temperature gradient, being hottest at the bottom and coolest at the top.
The vapours rise up the column through holes in a series of trays, cooling as they go.
1. Each hydrocarbon condenses to a liquid when it reaches a tray that is cooler than its boiling point, and the liquid is piped off.
The smallest molecules never condense, so they leave the top of the column as refinery gases.
The largest molecules do not boil at all, so they run out of the bottom as a thick residue called bitumen.

Example

From the top of the column down, the main fractions are refinery gases, petrol, naphtha, kerosene, diesel oil, fuel oil and bitumen.
Refinery gases are bottled as liquefied petroleum gas, petrol and diesel fuel cars, and kerosene fuels aircraft.
Naphtha is the main feedstock for making chemicals, fuel oil powers ships and bitumen surfaces roads.

Larger molecules have higher boiling points

Definition

Intermolecular force

An attraction between separate molecules.

Definition

Boiling point

The temperature at which a liquid changes to a gas at a stated pressure.

Hydrocarbon molecules are held to each other by weak intermolecular forces.
Longer molecules have more surface in contact with their neighbours, so the intermolecular forces between them are stronger.
Stronger forces need more energy to overcome, so larger molecules have higher boiling points.
1. Molecules with high boiling points condense in the hot lower part of the column, while small molecules with low boiling points rise to the cooler top.
The covalent bonds inside the molecules do not break, so fractional distillation separates the hydrocarbons without changing them chemically.
Boiling point rises steadily with chain length, from $∘C-162\ ^{\circ}\text{C}$ for methane to $∘C174\ ^{\circ}\text{C}$ for decane.

Table showing the increase in boiling point of the first ten straight-chain alkanes, from methane to decane, as molar mass increases.

Exam technique

When a question asks you to explain fractional distillation, link the steps in order: chain length, strength of intermolecular forces, boiling point and height in the column.
Say that intermolecular forces are overcome, never that covalent bonds break.

Self review

What is a fraction?
What is the formula of the alkane with $7$ carbon atoms?
Where in the fractionating column do the fractions with the highest boiling points condense?
Why do larger hydrocarbon molecules have higher boiling points?
Name two types of product that the petrochemical industry makes from crude oil.

Crude oil is a hydrocarbon mixture and a feedstock

Definition

Hydrocarbon

A compound containing carbon and hydrogen atoms only.

Definition

Feedstock

A raw material that an industrial process turns into other useful substances.

Crude oil is a thick, dark mixture of many different hydrocarbons, pumped from rocks beneath the land and the seabed.
1. Most of these hydrocarbons are alkanes, with chains ranging from one carbon atom to more than fifty.
Crude oil is the main source of the hydrocarbons used as fuels, such as petrol, diesel and kerosene.
It is also a feedstock for the petrochemical industry, which turns its hydrocarbons into new substances.
1. Plastics, solvents, detergents, lubricants, dyes and many medicines are all made from crude oil hydrocarbons.
Crude oil is of little use as it comes out of the ground, because each use needs hydrocarbons with a particular chain length.

Key Idea

Crude oil has to be separated into simpler mixtures before its hydrocarbons can be used as fuels or feedstocks.

Each fraction contains alkanes of similar chain length

Definition

Fraction

A mixture of hydrocarbons with similar numbers of carbon atoms and similar boiling points, collected together from crude oil by fractional distillation.

Definition

Alkane

A hydrocarbon in which all the bonds between carbon atoms are single covalent bonds.

Fractional distillation separates crude oil into fractions with different boiling point ranges.
Each fraction is still a mixture, containing hydrocarbons with similar numbers of carbon atoms and similar boiling points.
The fractions are made up largely of alkanes, which form a homologous series.
1. Each alkane has one more $CH2\text{CH}_2$ unit than the one before, and all of them fit the general formula: $CnH2n+2\text{C}_n\text{H}_{2n+2}$
2. In the formula, $n$ is the number of carbon atoms, so octane, with $8$ carbon atoms, is $C8H18\text{C}_8\text{H}_{18}$ .
Alkanes contain only single bonds, so each carbon atom is bonded to as many hydrogen atoms as possible.

Example

Find the formula of the alkane with $12$ carbon atoms, then decide whether $C10H20\text{C}_{10}\text{H}_{20}$ is an alkane.
Substitute $n = 12$ : $\times 12) + 2 = 26$ , so the formula is $C12H26\text{C}_{12}\text{H}_{26}$ .
An alkane with $10$ carbon atoms needs $\times 10) + 2 = 22$ hydrogen atoms.
$C10H20\text{C}_{10}\text{H}_{20}$ has only $20$ hydrogen atoms, so it does not fit $CnH2n+2\text{C}_n\text{H}_{2n+2}$ and is not an alkane.

In fractional distillation, vapours condense at different heights

Definition

Fractionating column

A vertical tube in which vapour repeatedly condenses and evaporates, improving the separation of liquids with different boiling points.

Crude oil is heated to about $∘C350\ ^{\circ}\text{C}$ in a furnace, so most of it boils and turns to vapour.
The hot vapour is piped into the bottom of a tall fractionating column.
The column has a temperature gradient, being hottest at the bottom and coolest at the top.
The vapours rise up the column through holes in a series of trays, cooling as they go.
1. Each hydrocarbon condenses to a liquid when it reaches a tray that is cooler than its boiling point, and the liquid is piped off.
The smallest molecules never condense, so they leave the top of the column as refinery gases.
The largest molecules do not boil at all, so they run out of the bottom as a thick residue called bitumen.

Example

From the top of the column down, the main fractions are refinery gases, petrol, naphtha, kerosene, diesel oil, fuel oil and bitumen.
Refinery gases are bottled as liquefied petroleum gas, petrol and diesel fuel cars, and kerosene fuels aircraft.
Naphtha is the main feedstock for making chemicals, fuel oil powers ships and bitumen surfaces roads.

Larger molecules have higher boiling points

Definition

Intermolecular force

An attraction between separate molecules.

Definition

Boiling point

The temperature at which a liquid changes to a gas at a stated pressure.

Hydrocarbon molecules are held to each other by weak intermolecular forces.
Longer molecules have more surface in contact with their neighbours, so the intermolecular forces between them are stronger.
Stronger forces need more energy to overcome, so larger molecules have higher boiling points.
1. Molecules with high boiling points condense in the hot lower part of the column, while small molecules with low boiling points rise to the cooler top.
The covalent bonds inside the molecules do not break, so fractional distillation separates the hydrocarbons without changing them chemically.
Boiling point rises steadily with chain length, from $∘C-162\ ^{\circ}\text{C}$ for methane to $∘C174\ ^{\circ}\text{C}$ for decane.

Table showing the increase in boiling point of the first ten straight-chain alkanes, from methane to decane, as molar mass increases.

Exam technique

When a question asks you to explain fractional distillation, link the steps in order: chain length, strength of intermolecular forces, boiling point and height in the column.
Say that intermolecular forces are overcome, never that covalent bonds break.

Self review

What is a fraction?
What is the formula of the alkane with $7$ carbon atoms?
Where in the fractionating column do the fractions with the highest boiling points condense?
Why do larger hydrocarbon molecules have higher boiling points?
Name two types of product that the petrochemical industry makes from crude oil.

6.2.1 Crude oil and fractional distillation

Crude oil is a hydrocarbon mixture and a feedstock

Each fraction contains alkanes of similar chain length

In fractional distillation, vapours condense at different heights

Larger molecules have higher boiling points

Review 6.2.1 Crude oil and fractional distillation by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

6.2.1 Crude oil and fractional distillation

Crude oil is a hydrocarbon mixture and a feedstock

Each fraction contains alkanes of similar chain length

In fractional distillation, vapours condense at different heights

Larger molecules have higher boiling points

Review 6.2.1 Crude oil and fractional distillation by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

6.2.1 Crude oil and fractional distillation

Crude oil is a hydrocarbon mixture and a feedstock

Each fraction contains alkanes of similar chain length

In fractional distillation, vapours condense at different heights

Larger molecules have higher boiling points

Review 6.2.1 Crude oil and fractional distillation by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

6.2.1 Crude oil and fractional distillation Revision Guide

6.2.1 Crude oil and fractional distillation

Crude oil is a hydrocarbon mixture and a feedstock

Each fraction contains alkanes of similar chain length

In fractional distillation, vapours condense at different heights

Larger molecules have higher boiling points

Review 6.2.1 Crude oil and fractional distillation by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

6.2.1 Crude oil and fractional distillation Revision Guide