Industrial cracking

What is the economic purpose behind cracking?

To ease the higher demand for shorter-chained alkanes, such as naphtha or petrol

What is cracking?

Large alkanes are broken up into smaller alkanes

What is another product of cracking besides smaller-chained alkanes?

Alkenes

Why is the production of alkenes good economically?

They are very valuable

What is decane, C₁₀H₂₂ broken up into when cracked?

Decane → octane + ethene

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

What are the two types of cracking?

Thermal cracking and catalytic cracking

What does thermal cracking involve?

Heating alkanes to a high temperature, 700-1200K, under high pressure, up to 7000kPa

What happens when the C—C bonds break?

One electron from the pair in the covalent bond goes to each carbon atom

What is initially formed when the C—C bond of a long-chained alkane breaks?

Two shorter chains that each end with a carbon atom with an unpaired electron - free radicals

The free radicals produced from the thermal cracking of butane:

C₄H₁₀ → C₂H₅

What do the free radicals do due to their high reactivity?

They reactive in a number of different ways to form a variety of shorter chain molecules

Why is one of the new molecules produced from the free radicals always an alkene?

Because there are not enough hydrogen atoms to produce two alkanes so one of the new chains must have a C=C bond

Products from the thermal cracking of butane

Ethane and ethene

Is the example of the cracking of butane above reflective of industrial cracking?

No - the chain does not always break in the middle and any number of of C—C bonds could break

What is another possible product of thermal cracking?

Hydrogen - H₂

Which type of cracking produces a higher proportion of alkenes?

Thermal cracking

What are the conditions of catalytic cracking compared to thermal cracking?

• Takes place at a lower temperature - around 750K

• Lower pressure

• A catalyst is used

What is the catalyst that is typically used in catalytic cracking?

Zeolites

What are zeolites make up of?

A mixture of silicon dioxide and aluminium oxide

What type of structure do zeolite catalysts have and how is this useful?

Honeycomb structure which has an enormous surface area - small amount can be used

What products does catalytic cracking produce? (3)

Not so many alkenes but branched alkanes, cycloalkanes and aromatic compounds

What are aromatic compounds based on?

The benzene ring C₆H₆

Catalytic cracking in a lab experiment

What catalyst is used in the experiment?

Aluminium oxide catalyst

What does the product of gases show about their chain lengths?

They are short hydrocarbons - less than C₅

What is the product of gases a mixture of?

Alkanes and some alkenes

How do you test for alkenes?

Adding the sample to bromine water will decolourise it if alkenes are present

Does cracking always provide you with similar-chain hydrocarbon products?

No, it gives you a mixture of products

How would you obtain refined products from the products from cracking?

By fractional distillation