3.3.2.2. Modification of alkanes by cracking Flashcards
Looking at the data for the composition of north sea crude oil:
Shorter chain hydrocarbons are more in demand, but the top 3 fractions are the least abundant in crude oil, so cracking is needed.
What is cracking?
The process of breaking down long chain hydrocarbons into shorter chain hydrocarbons (which are more useful)
What is thermal cracking and what are the conditions?
Where long chains are heated.
High temperature of ~1000-1200K
High pressure ~ 70atm
What is the mechanism for thermal cracking?
Free-readical
What are the products of thermal cracking?
Shorter chain alkanes and alkenes
What happens in the extreme conditions of thermal cracking and how can this be prevented?
Could cause the decomposition of the molecules carbon and hydrogen.
Can be prevented by reducing the time spent under the extreme temperatures and pressures
What is catalytic cracking and what are the conditions?
Long chain hydrocarbons are heated.
Zeolite catalyst
High temperature ~700-1000K
Slight pressure ~1-2 atm
What are the products of catalytic cracking?
Cycloalkenes and aromatic hydrocarbons and branched alkanes
What is the mechanism of catalytic cracking?
A carbocation/ Heterolytic fission
Why is high temperature needed in cracking?
C-C covalent bond in alkane molecules is strong, so lots of energy needed to overcome
What is a zeolite catalyst?
An acidic material made of aluminium oxide and silicon dioxide.
It has a honeycomb structure, so greater surface area so faster rate of reaction.
Mainly used to make fuel for motor vehicles.
What are the economic reasons for cracking?
Short chain hydrocarbons are high in demand.
Very low abundance of these in crude oil so supply must meet demand.
What is reforming used for?
To form aromatic hydrocarbons
