12.3 Industrial cracking Flashcards
Which are more useful,
long carbon chains or short ones
Short carbon chains are more useful as they have lower boiling points.
Short chains are more economically valuable
What is cracking
Why are harsh conditions needed to crack them
What are the products
The process of long hydrocarbon chains
(alkanes) broken into shorter lengths
So it results in…
-Shorter chains
- Some alkene products which are more reactive than alkanes
Alkanes are un-reactive due to being saturated so harsh conditions are required to break them
What are alkenes used for in industry
chemical feedstock
So supply industries with starting materials to make different products
They can be converted into a huge range of other compounds eg polymers and products eg paints and drugs
ethene is an example, and is used as a starting material for poly(ethene)
What are the two types of cracking
. Thermal cracking
. Catalytic cracking
Describe the temperature and pressure of thermal cracking
Very high temperatures (1000K) and high pressure (7000kPa)
Describe process of thermal cracking
What are free radicals
What are the products
Very high temperature and pressure;
. Carbon-Carbon bonds break in the alkanes
. So an electron from the pair in the covalent bond goes to each carbon atom
. So initially, two shorter chains are produced, each ending in a carbon atom with an unpaired electron
These are called free radicals
They are highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules
However…there aren’t enough hydrogen atoms to produce two alkanes, so one of the new chains must have a c=c so there is one alkene produced.
So usually there are more alkenes produced by this method
How are the products of thermal cracking made into short chain alkanes
Why are alkanes kept in these conditions for a very short amount of time
. Any number of C=C bonds in the alkene can break , and chain doesn’t necessarily break in the middle
. Once there are lots of alkenes, and a couple alkanes produced
. In this state, there can be lots of decomposition (ultimately to carbon and hydrogen)
. So alkanes are kept in these conditions for a very short time
What temperature and pressure does catalytic cracking occur
Quite high temperatures but lower than in thermal eg 720k and lower pressure (but still more than atmospheric)
What catalyst is used for catalytic cracking
What is it made of
Describe the structure of it
What is the PH
. Zeolite catalyst
Consisting of silicon dioxide and aluminium oxide
. They have a honeycomb structure with a very large surface area
. They are acidic so have a low PH
What is catalytic cracking mainly used to produce
How long are the chain lengths usually
How are the products separated
Motor fuels
Their products are mainly branched alkanes, cycloalkanes (rings), and aromatic compounds
. So the products are mostly gases so have very short chain lengths, less than C5.
The products from cracking are separated by fractional distillation
What is the test for a double carbon bond to separate alkanes and alkenes
. When added to bromine solution, it turns the solution from orange to colourless
