ALKANES Flashcards
What is petroleum
A mixture consisting mainly of alkane hydrocarbons
Define petroleum fraction
A mixture of hydrocarbons with a similar chain length and boiling point range
Outline fractional distillation
Oil is heated and passed into a column
The fractions condense at different height as the temperature of the column decreases upwards
Therefor separation depends on boiling point which depends on molecule size
Larger molecules have larger van der waals forces and condense at the bottom of the column at higher temperatures
Smaller molecules have smaller van der waals forces and condense at the top of the column at lower temperatures
Similar (size/Bp/mass) molecules condense together and are tapped off in a petroleum fraction
What is fractional distillation
The physical process involving the splitting of weak van der waals forces between molecules
Outline vacuum distillation
Heavy residues from the fractionating column are distilled under a vacuum
Lowering the pressure over a liquid lowers its boiling point
Outline fractional distillation in a lab
Heat the flask with a Bunsen burner or electric mantle so vapours of all the components are produced
Vapours pass up the fractionating column
Lower bp molecules reach the top of the fractionating column first
The vapours with higher bp condense into the flask
Only the most volatile vapour passes into the condenser which cools the vapour into a liquid
Define cracking
Conversion of large hydrocarbons to smaller hydrocarbon molecules by breaking C-C bonds
Outline the general formula for cracking
High Mr alkane -> smaller Mr alkane + alkenes + (hydrogen)
Give an economic reason for cracking
Petroleum fractions with shorter C chains are in more demand than larger fractions
Make use of excess larger hydrocarbons
The products of cracking are more valuable than the starting materials
Why does cracking require high temperatures
It is a chemical process involving the splitting of strong covalent bonds
What are the two types of cracking
Thermal
Catalytic
Outline the conditions for thermal cracking
High pressure (7000 kPa)
High temperature (400-900C)
Outline the conditions for catalytic cracking
Low pressure
High temperature (450C)
Zeolite catalyst
What are the products of thermal cracking? What can they be used for?
Mostly alkenes
- making polymers/ethanol
Sometimes hydrogen
- haber process and margarine manufacturing
What are the products of catalytic cracking?
Branched and cyclic alkanes
Aromatic Hydrocarbons
Why is catalytic cracking useful
Branched and cyclic hydrocarbons burn more cleanly and give fuels a higher octane number
Cheaper than thermal cracking because of lower temperature and pressures used
Define a fuel
Releases heat energy when burnt
Why are alkanes useful as fuels
Alkanes readily burn in the presence of oxygen
The reaction is highly exit hermit
What are the products of complete combustion?
CO2
H2O
What are products of incomplete combustion?
CO
C (soot)
H2O
What defines complete combustion
Excess O2
What defines incomplete combustion
A limited amount of O2
Why is incomplete combustion unbenefitial?
Produces less energy per mole
Carbon (s) can cause global dimming
What are the two reactions producing SO2
S+O2->SO2
CH3SH+3O2 -> SO2 + CO2 + 2H2O
