Alkanes Flashcards
What is the general formula for alkanes ?
CnH2n + 2
What does it mean if the alkanes are hydrocarbons ?
they contain hydrogen and carbon atoms only
Two types of hydrocarbons
aliphatic and aromatic hydrocarbons
aliphatic hydrocarbons
chains of carbon atoms which may be branched or unbranched and with rings that are not aromatic
aromatic hydrocarbons
rings in which there are delocalised electrons - named because of their smells - sometimes called arenes
Saturated compunds
only single bonds between the atoms in their molecules
Where are alkanes found ? - fractional distillation
alkanes are found in crude oil which is a mixture of different length hydrocarbons
Fractional Distillation
- vaporised oil enters the column and rises through the trays
- the longest hydrocarbons don’t vaporise and run down to the bottom
What does the column in fractional distillation have ?
It has a temperature gradient
- cooler at the top
- as the vapour rises parts of the mixture condense at different temperatures - because there are different chain lengths and hence different boiling points
- the fractions are drawn off at different levels
What happens to the shortest hydrocarbons ?
they won’t even condense at 20 degrees Celsius and so comes off as a gas at the top of the column
How can the fractions be used ?
gas - used in LPG and stove gas
petrol - used in petrol cars
kerosine - used as jet fuel and heating
diesel oil - used in diesel oil
fuel oil - used in ships and power stations
bitumen - used in roofing and tarmac
Cracking
heavier fractions can be cracked to higher demand lighter fractions
What does fractional distillation produce ?
it produces a variety of fractions however demand for each fraction varies
Is heavier fractions like fuel oil higher or lower demand ?
Lower demand than lighter fractions such petrol which are more valuable
What are the two types of cracking ?
thermal and catalytic cracking
Thermal cracking
- high temperatures and pressure used in 10000 degrees celcius
- 70 atm of pressure
- products = alkenes
- alkenes are used to make polymers such as plastics
Catalytic cracking
- high temperature and slight pressure used 450 degrees celcius
- a zeolite is used which helps to lower the temperature needed
- products = aromatic hydrocarbons useful in fuel in vehicles
- using a zeolite cataylst lowers the temperature and pressure needed for cracking to occur - lowers costs and speeds up process
Reforming alkanes
alkanes can be reformed into cycloalkanes and aromatic molecules
What are petrol and diesel made up of ?
a mixture of hydrocarbons some of which are straight chain hydrocarbons
What is the problem with straight chain alkanes ?
create knocking in car engine - knocking is where straight chain alkanes explode upon compression
- adding branched and cyclic alkanes reduces knocking and increases engine efficiency
How can cyclic and branched alkanes be made ?
by using straight chain alkanes and a platinum in a process called reforming
Complete combustion of alkanes
- alkanes burn in oxygen completely to form carbon dioxide and water
Why are alkanes good fuels ?
they burn readily to produce large amount of energy - the longer the alkane the more energy they produce
What is the combustion of alkanes used for ?
power vehicles and most of Britain’s electricity
Incomplete combustion of alkanes
when alkanes burn with a limited oxygen supply we produce carbon monoxide and carbon (soot)
How is carbon monoxide poisonous ?
bonds to haemoglobin in the blood and prevents oxygen bonding - can be removed using catalytic converter
What can soot cause ?
causes breathing problems and make up building dirty and clog up engines
Acid rain
burning fossil fuels can release sulfur dioxide and oxides of nitrogen which contribute to acid rain
What does acid rain cause ?
damage to plants, kills fish and causes erosion of buildings
Some fossil fuels contain sulfur based impurities …
when burned the sulfur reacts with oxygen producing sulfur dioxide
What is sulfur dioxide ?
- an acidic gas
- reacts with water in the atmosphere to form sulfuric acid which falls as acid rain
Oxides of nitrogen
- these are produced when nitrogen and oxygen are heated in the engine and react
- dissolve in water in the atmosphere to form nitric acid when contributes to acid rain
Where are catalytic converter found ?
they are found mainly in vehicles and help to reduce harmful pollutants entering the atmosphere
What does catalytic converter help to reduce ?
the amount of unburnt hydrocarbons and oxides of nitrogen going into the atmosphere
What does catalytic converter contain ?
platinum, rhodium and indium metals
What do catalytic converter convert ?
they convert harmful gases like :
- carbon monoxide
- oxides of nitrogen
- unburnt hydrocarbons
into less harmful products like
- water vapour
- nitrogen
- carbon dioxide
What are fossil fuels ?
created from dead animals and plants that lived millions of years ago
Is fossil fuels renewable or non - renewable ?
non - renewable ( will run out ) and manily include :
- coal
- gas
- oil
Can fossil fuels run out ?
Fossil fuels will run out. Oil in particular is in a short supply and is becoming increasingly more difficult to extract. This means the cost of extraction increases too.
Coal, gas and oil can be …
extracted easily and are a good source of energy as a fuel
Biofuels
adv -biofules are renewable so they are a more sustainable than crude oil
- produce CO2 when they are burnt however they are classes as carbon neutral as the CO2 is absorbed by the plants when its growing
dis - expensive to convert existing petrol engines to take fuels with higher concentration of ethanol
- land that could have been used to grow food is being used to make food shortages in countries that grow sugar cane
what are the two types of chemical reactions of alkanes ?
combustion ( complete and incomplete )
free radical subsistution
free radical
a chemical species with an unpaired electron
substitution
a reaction in which one atom is replaced by another atom or group
what do mechanisms show ?
the movement of electrons and bonds that allow reactant to change into products
arrows will always travel ..
from negative to postitive when forming a bond
what does a double headed arrow represent ?
2 electrons moving
what does a single headed ( fish hook ) represent ?
1 electron moving
what are the two types of bond fission ?
homolytic and heterolytic
bond fission
the breaking of a covalent bond. the electron pair in the bond can be distributed into 2 ways
Heterolytic fission
the bond breaks but the electrons are distributed unequally to form 2 different ions ( an anion and anion )
what does hetero mean ?
different
Homolytic fission
the bond breaks with the pair of electrons in the bond being shared equally to form 2 uncharged radicals
what does the dot mean ?
there is an unpaired electron on the atom
what does homo mean ?
same
state the 3 main stages in chain reactions
initiation
propagation
termination
initiation
radicals are produced normally using visible light or ultra violet - called photochemical reactions.
the bond breaks producing 2 radicals
Propagation
when a radical reacts with a non radical molecule. New radicals are created which then go to react with other non-radicals - chain reaction
Termination
when 2 radicals react they form a non-radical molecule - ends the chain reaction
If there is loads of chlorine in the reaction we will get ..
di, tri, tetra haloalkane
By adding excess methane it will ..
reduce the amount of multiple substitutions - greater chance of chlorine reacting with methane
Ina addition to multiple substitution what else would form ?
isomers - because free radical substitution will occur anywhere on a hydrocarbon chain
