ALKANES 3.3.2 Flashcards
(43 cards)
Formula of alkanes
C n H 2n+2
Physical properties of alkanes
Non-polar therefore, the only intermolecular forces between molecules are weak van Der Waals
Saturated hydrocarbon
No carbon carbon double bonds
Crude oil
Mixture of different hydrocarbons formed from the decay of sea creatures over millions of years
How to separate crude oil
Fractional distillation
Fractional distillation
- Crude oil is heated and vapourised
- Vapours enter the fractionating column which is cooler at the top
- Vapours rise up the tower and cool at condense at their boiling points- Shorter chains condense near the top
- Liquid is piped off for different uses
4b. Bitumen is collected at the very bottom of the tower and is used for road surfacing
Boiling point of long chain alkanes
Higher because there are more van Der Waals forces to break so this requires more energy
Boiling point of short chain alkanes
Lower as there aren’t as many van Der Waals forces to break
How to break down products of fractional distillation even more
Cracking
Why do we use cracking and why is it important in industry
Long carbon chains are not useful so they must be broken down to form smaller, ore useful molecules
There is a bigger supply of long arbon chains but a higher demand for shorter chains
Cracking
Converts useless long chains into more useful short chains, makes more fuel for more useful products
Thermal cracking
Uses free radical mechanism
Temperatures must be around 1200k and 7000kPa (high temp and high pressure)
Produces high proportion of alkanes and alkenes
Catalytic cracking
Uses carbonation intermediates mechanism
Requires lower temps (than thermal cracking) (720K) and a normal pressure
Zeolite catalyst is needed
Produces aromatic compounds with carbon rings
Do cycloalkanes brn more efficiently
Yes
Which cracking method is cheaper
Catalytic as it requires lower temp and pressure
Why are alkanes good fuels
Release a lot of energy when burnt
Complete combustion
Plentiful supply of oxygen
Fuel + oxygen = carbon dioxide + water
CH₄ + 2O₂ -> CO₂ + 2H₂O
Incomplete combustion
Limited supply of oxygen
Produces carbon monoxide (toxic gas) or carbon (solid particulates)
CH₄ + 1.5 H₂O -> CO + H₂O
CH₄ + O₂ -> C + 2 H₂O
What is reforming
Where straight chain hydrocarbons are converted into branched chain alkanes and cyclic alkanes. These burn more efficiently and are used in petrol for cars
Pollutants form burning fuels
Sulfur dioxide
Particulates
Unburnt hydrocarbons
Carbon monoxide
Carbon dioxide
Sulfur dioxide -How is it formed and harmful effects and ways to reduce the problem
Impurities in the crude oil
S + O₂ -> SO₂
Combines with water vapour and oxygen in the atmosphere to form sulfuric acid (acid rain) which damages lakes, fish and plant life
Remove S from fuel before burning, flue gas desulfurization
Particulates -How is it formed and harmful effects and ways to reduce the problem
Incomplete combustion of a fossil fuel
CH₄ + O₂ -> C + 2 H₂O
Can cause asthma, it is a carcinogenic (cause cancer) and global dimming
Ensure a good supply of oxygen
Unburnt hydrocarbons- How is it formed, harmful effects and ways to reduce the problem
Some of the fuel isn’t burnt
Wastes fuel and is a green house gas. Causes photochemical smog
Ensure engines are well-tuned and there is a good supply of oxygen
Carbon monoxide -How is it formed, harmful effects and ways to reduce the problem
Incomplete combustion
CH₄ + 1.5 H₂O -> CO + H₂O
Toxic to humans as it prevents oxygen from binding to haemoglobin
Ensure good supply of oxygen
