Alkanes from Crude Oil

Alkanes

What is crude oil?

A mixture of compounds, mostly hydrocarbons.

What are the main sources of energy currently used worldwide?

Fossil fuels such as coal, crude oil and natural gas.

What are three main processes used to convert crude oil into useful fuels?

Fractional distillation, cracking and reforming.

What is fractional distillation?

The process used to separate a liquid mixture into fractions by boiling and condensing.

Why can crude oil be separated by fractional distillation?

The hydrocarbons in crude oil have different boiling temperatures.

What is a fraction?

A mixture of compounds with a similar range of boiling temperatures.

What happens to crude oil at the start of fractional distillation?

It is heated in a furnace so that most of it vaporises.

Where does the vaporised crude oil go during fractional distillation?

It passes into a fractionating column near the bottom.

What is a temperature gradient?

The way in which temperature changes up and down a column.

How does temperature change up a fractionating column?

It is hotter near the bottom and cooler near the top.

What happens to hydrocarbon vapours as they rise through a fractionating column?

They condense at different heights depending on their boiling temperatures.

Where do hydrocarbons with higher boiling temperatures condense in a fractionating column?

Nearer the bottom of the column.

Where do hydrocarbons with lower boiling temperatures condense in a fractionating column?

Nearer the top of the column.

What types of hydrocarbons are found near the bottom of a fractionating column?

Larger molecules with longer carbon chains and higher boiling temperatures.

What types of hydrocarbons are found near the top of a fractionating column?

Smaller molecules with shorter carbon chains and lower boiling temperatures.

What happens to some hydrocarbons with very low boiling temperatures during fractional distillation?

They remain as gases and leave the top of the column without condensing.

Why may fractions from crude oil need further fractional distillation?

Some fractions still contain many different compounds and need further separation.

Why is cracking needed?

There is less demand for long-chain hydrocarbons and greater demand for useful short-chain hydrocarbons.

Why are short-chain hydrocarbons particularly useful?

They are useful as fuels and can be used to make other substances such as polymers.

What is cracking?

The breakdown of molecules into shorter ones by heating with a catalyst.

How is catalytic cracking carried out?

Hydrocarbons from heavier fractions are passed through a heated catalyst.

What catalysts can be used for cracking?

Compounds containing aluminium, silicon and oxygen.

What happens to long-chain hydrocarbon molecules during cracking?

They break into smaller hydrocarbon molecules.

How many smaller molecules are formed from one molecule during cracking?

At least two smaller molecules are formed.

Give an example of a cracking reaction.

Decane can crack to form octane and ethene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.

Why is cracking decane into octane and ethene useful?

Octane is used in petrol and ethene is used to make polymers.

Does cracking involve a chemical reaction?

Yes, new compounds are formed.

Does fractional distillation involve a chemical reaction?

No, it only separates compounds that already exist in the mixture.

What is the key difference between fractional distillation and cracking?

Fractional distillation separates existing compounds, whereas cracking chemically changes molecules to form new compounds.

What is reforming?

The conversion of straight-chain hydrocarbons into branched-chain and cyclic hydrocarbons.

Why is reforming used on hydrocarbons used as fuels?

Branched-chain and cyclic hydrocarbons burn more smoothly in vehicle engines than straight-chain hydrocarbons.

How are hydrocarbons reformed?

They are heated with a catalyst.

What catalyst is commonly used in reforming?

Platinum.

What types of hydrocarbons are produced by reforming?

Branched-chain alkanes and cyclic hydrocarbons.

Give an example of reforming pentane.

Pentane can be converted into cyclopentane and hydrogen: C₅H₁₂ → C₅H₁₀ + H₂.

What happens when heptane is reformed into methylbenzene?

Heptane is converted into methylbenzene and hydrogen: C₇H₁₆ → C₇H₈ + 4H₂.

What useful by-product is commonly formed during reforming?

Hydrogen.

What is the purpose of fractional distillation of crude oil?

To separate crude oil into useful fractions based on differences in boiling temperatures.

What is the purpose of cracking?

To convert less useful long-chain hydrocarbons into more useful shorter-chain hydrocarbons.

What is the purpose of reforming?

To convert straight-chain hydrocarbons into branched-chain or cyclic hydrocarbons that burn more smoothly.