12.3- INDUSTRIAL CRACKING Flashcards
Is the naphtha fraction from the fractional distillation of crude oil in huge demand?
yes
Why is the naphtha fraction in huge demand?
for petrol and by the chemical industry
What are longer chain fractions described as, as they’re not as useful?
they’re of lower value economically
What happens to meet the demand for shorter chain hydrocarbons?
many of the longer chain fractions are broken into shorter lengths (cracked)
What are the useful results of cracking? (2)
shorter, more useful chains produced, especially petrol
some of the products are alkenes, which are more reactive than alkanes
What is petrol a mixture of?
mixture of mainly alkanes containing between 4-12 carbon atoms
What are alkenes used as?
as chemical feedstock and are converted into a huge range of other compounds including polymers + variety of products from paint to drugs
What is probably the most important alkene?
ethene
Why is ethene perhaps the most important alkene?
starting material for poly(ethene) and wide range of other everyday materials
Why are harsh conditions needed to break down alkanes?
because they’re very unreactive
Two different types of cracking?
thermal cracking
catalytic cracking
What does thermal cracking involve?
heating alkanes to a high temperature, under high pressure
What high temperature is used for thermal cracking?
700-1200 K
What high pressure is used for thermal cracking?
up to 7000 kPa
What happens to carbon-carbon bond in thermal cracking?
carbon-carbon bond breaks in such way that one electron from the pair in the covalent bond goes to each carbon atom
So initially what is produced when the carbon-carbon bond is broken? (cracking)
initially two shorter chains are produced, each ending in a carbon atoms with an unpaired electron
What are the fragments ending in a carbon atom with unpaired electrons called?
free radicals
How reactive are free radicals and in what way do they react?
highly reactive intermediates + react in number of ways to form a variety of shorter chain molecules
What is produced in thermal cracking?
alkane and alkene
Why is an alkene produced in thermal cracking?
there’s not enough hydrogen atoms to produce 2 alkanes
How many carbon-carbon bonds may break and does it have to be in the middle in thermal cracking?
any number of carbon-carbon atoms may break
chain does not necessarily have to break in the middle
What may be also produced in thermal cracking?
hydrogen
What does thermal cracking tend to produce a high proportion of?
high proportion of alkenes
What is done to avoid too much decomposition (ultimately to carbon and hydrogen) after thermal decomposition?
alkanes are kept in thermal cracking conditions for a very short time, typically one second
At what conditions is catalytic cracking carried out? (3)
lower temperature- about 720K
lower pressure- but more than atmospheric
zeolite catalyst- consisting of silicon dioxide and aluminium oxide
What is the structure of zeolite like?
honeycomb structure with enormous surface area
Is zeolite acidic or alkaline?
acidic
What is catalytic cracking mainly used for?
mainly to produce motor fuels
What are the products of catalytic cracking mostly like? (3)
mostly branched alkanes, cycloalkanes (rings), and aromatic compounds
How are the products obtained from catalytic cracking separated?
by fractional distillation
In a lab what can be the catalyst for catalytic cracking?
lumps of aluminium oxide
What are the products mostly in catalytic cracking in a lab?
mostly gases- shows they have chain lengths of less than C5
What does the mixture from catalytic cracking in a lab do to bromine solution?
decolourises bromine solution
What is the bromine solution a test for?
test for carbon-carbon double bond showing the products contain alkenes
