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
Why is SO2 hazardous?
It dissolves in atmospheric water which can produce acid rain
How can the effects of SO2 be reduced?
Remove SO2 from waste gases from fur crave
By flue has desulphurisation
Outline flue gas desulphurisation
Gases pass through a scrubber
containing basic calcium oxide
Which reacts with acidic sulphur dioxide
In a neutralisation reaction
SO2 + CaO -> CaSO3
What can the product of flue gas desulphurisation be used for? What is the product?
Calcium sulphite
Can be used to make calcium sulphate
Used in plaster board
What reactions form Nitrogen oxides
N2 + O2 -> 2NO
N2 + 2O2 -> 2NO2
Why do engines form nitrogen oxides
The high temperature and spark in the engine
Provides sufficient energy to break strong N2 bonds
What is the environmental consequence of nitrogen oxides?
NO is toxic and can form NO2
NO2 is toxic and acid and forms acid rain
What is the environmental consequence of carbon monoxide?
Toxic
What is the environmental consequence of carbon dioxide?
Contributed to global warming
What is the environmental consequence of I burnt hydrocarbons?
Contributed to smog formation
What is the environmental consequence of soot?
Global dimming
Respiratory problems
What is a catalytic converter?
A ceramic honeycomb coated with thin layers of catalyst metals
Platinum palladium rhodium
Remove CO, NOx and unburnt hydrocarbons from exhaust gases
What do catalytic converters convert pollutants into
CO2, N2 and H2O
What is a greenhouse gas?
A gas that traps Earths radiated infra red energy in the atmosphere
What are the green house gases?
CO2
CH4
H2O
Why are alkanes generally unreactive with reagents?
The C-C bind and the C-H bond are relatively strong
What conditions must synthesis of chloro alkanes occur under?
Why?
UV light
Supplies the energy to break the Cl-Cl bond
Broken in preference to others as it’s the weakest bond
What are the three steps of free radical substitution
Initiation
Propagation
Termination
Outline initiation
Cl-Cl bond is broken in homolytic fission
Under UV Libby
Forming free radicals
Outline propagation
Chlorine free radicals react with a hydrogen on the alkane
Forming a free radical and HCl
The new free radical reacts with Cl2
to form the final product and a Cl free radical
Outline termination
Two free radicals react
To form whole molecules
Left in structural formula
What are the potential products of chloroalkane synthesis in excess Cl2
CH2Cl2
CHCl3
CCl4
