which fractions of hydrocarbons are most in demand?
fractions with shorter C chains
how is demand for shorter C chain fractions met?
longer hydrocarbons are cracked - breaking long-chain alkanes into smaller hydrocarbons, involves breaking C-C bonds
what are the two useful results of cracking?
- shorter, more useful chains produced, especially petrol
- some of the products are alkenes, which are more reactive than alkanes
what is petrol made up of?
mixture of mainly alkanes containing between 4 and 12 carbon atoms
what are alkenes commonly used for?
chemical feedstock - converted into other compounds such as polymers and products such as paint
why are harsh conditions needed to break down alkanes with cracking?
because they are very unreactive (strong C-C and C-H bonds)
Give 2 conditions of thermal cracking
- high temps (700-1200K)
- high pressure (up tp 7000kPa)
What does thermal cracking produce?
high proportion of alkanes and alkenes
what happens in thermal cracking?
- C-C bonds break and one electron from covalent pair goes to each C
- produces 2 free radical intermediates
- intermediated react in variety of ways to produce variety of shorter chain molecules
- not enough H atoms to produce 2 alkanes so one alkane and one alkene produced
how is decomposition
(to carbon and hydrogen) prevented in thermal cracking?
alkanes are kept under cracking conditions for very short time, typically 1 second
Name 3 conditions of catalytic cracking
- Uses zeolite catalyst
- lower pressure
- 450°C / 700K - lower temperatures
why is a zeolite catalyst used in catalytic cracking?
needed to compensate for less harsh conditions (lower pressure and temperature)
also speeds up reactions, saving time and money
What does catalytic cracking normally produce?
- branched alkanes
- cycloalkanes
- aromatic compounds
Why doesn’t thermal cracking produce 2 alkanes?
Since there isn’t enough hydrogen atoms to produce 2 alkanes = one of new chains has C=C
How are the products obtained from cracking separated?
fractional distillation
what is reforming?
straight chain hydrocarbons converted to branched chain alkanes and cyclic alkanes
why are hydrocarbons reformed?
to burn more efficiently and used in petrol for cars
