Modification of alkanes by cracking Flashcards
What are 3 economic reasons for cracking?
1- the petroleum fractions with shorter C chains (e.g. petrol) are in more demand than larger fractions
2- to make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked
3- the products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene))
What is the general equation for cracking?
High Mr -> Smaller Mr alkanes + alkenes + (hydrogen)
This chemical process involves the splitting of which type of bond?
Strong covalent
Does cracking require high or low temperatures?
High temperatures
What are the two main types of cracking?
Thermal and catalytic
In thermal cracking what pressure is required?
A high pressure of 7000 kPa
In thermal cracking what temperature is required?
A high temperature of 400-900 degrees Celsius
What does thermal cracking produce mostly?
Alkenes - e.g. Ethene is used for making polymers and ethanol
What does thermal cracking sometimes produce?
Hydrogen - this is used in the Haber Process
In catalytic cracking what pressure is required?
Low pressure
In catalytic cracking what temperature is required?
A high temperature of 450 degrees Celsius
In catalytic cracking what type of catalyst is used?
Zeolite
What does catalytic cracking produce?
Branched and cyclic alkanes and aromatic hydrocarbons
Which is cheaper, catalytic or thermal cracking?
Catalytic is cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used
Do branched and cyclic hydrocarbons burn more or less cleanly?
They burn more cleanly and are used to give fuels a higher octane number
