Alkanes Flashcards
What are aliphatic
chemical compound that belonging to the organic class in which the atoms are connected by single, double, or triple bonds such as Alkanes, Alkenes and alkynes
What are aromatic
cyclic compounds containing double bonds such as arenes
What is crude oil
mixture of hydrocarbons containing alkanes, cycloalkanes and arenes
fractions from fractional distillation
refinery gas, gasoline, kerosene, diesel, fuel oil, bitumen
Cracking
excess heavier fractions are broken down into smaller, more useful compounds
These more useful compounds include alkanes and alkenes of lower relative formula
Product of cracking
When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed
Types of cracking
Thermal cracking requires high temperatures and high pressure and produces alkanes and a lot of alkenes
Catalytic cracking uses a lower temperature and slight pressure in the presence of a catalyst such as a zeolite or aluminium oxide to produce mainly aromatic hydrocarbons
Reforming alkanes
Many of the alkanes in these fuels are straight chain alkanes
These straight chain alkanes are more likely to explode, rather than combust, inside the engine
This is known as knocking and makes the combustion less efficient
To reduce this straight chain alkanes are reformed into
Branched alkanes
Catalyst for reforming
platinum
What is complete combustion
When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively
What is incomplete combustion
When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
Some carbon is only partially oxidised to form carbon monoxide.
carbon will be produced in the form of soot
what do Car exhaust fumes include
include toxic gases such as carbon monoxide (CO), oxides of nitrogen (NO/NO2) and volatile organic compounds (VOCs)
What is carbon monoxide
CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death
The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide
how are oxides of nitrogen caused
nitrogen is too unreactive to react with oxygen in air
However, in a car engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place
how is photochemical smog made
Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)
In air, the nitrogen oxides can react with these VOCs to form peroxyacetyl nitrate (PAN) which is the main pollutant found in photochemical smog
how is acid rain formed
Nitrogen oxides can also dissolve and react in water with oxygen to form nitric acid which can cause corrosion of buildings, endangers plant and aquatic life
how is soot made
Solid carbon particles (or particulates) released from incomplete combustion clump together to form soot which gradually falls back to the ground
what problems can particulates cause
If they are inhaled they can damage the lungs and cause respiratory problems
They can cover buildings and statues, making them look unclean and accelerating corrosion
They can reflect sunlight back into space reducing the amount of light reaching the earth, this is called global dimming
how can pollution be reduced
catalytic converters which have precious metals (such as platinum) are coated on a honeycomb to provide a large surface area
what reactions take place in catalytic converter
Oxidation of unburnt hydrocarbons
Oxidation of CO to CO2
Reduction of NO to N2
as more stable
Catalytic removal of NO and CO
The nitrogen oxides are reduced on the surface of the hot catalyst to form the unreactive and harmless nitrogen gas which is then released from the vehicle’s exhaust pipe into the atmosphere
2CO (g) + 2NO (g) → 2CO2 (g) + N2 (g)
Alternative fuels
Biofuels are renewable fuels
Renewable meaning that they can be replaced over a short period of time
The bio part of biofuel meaning that it comes from living matter
Types of biofuel
Biodiesel - made by refining renewable fats and oils
Bioethanol - made by fermentation
Biogas - made / released when organic waste breaks down
Benefit and disadvantages of biofuels
Biofuels are often considered as carbon neutral
This is because as the plants grow they absorb carbon dioxide which is equivalent to the amount of carbon dioxide that will be released when they are burnt
Biodiesel and biogas can reduce the amount of waste going to landfill as the waste can be used to produce them
Biofuel production could provide money for less developed countries as they have the space to grow the crops required
The cost of converting engines and machinery to run on biofuels instead of petrol / diesel
countries don’t have the space to be able to produce enough plants to make the biofuels because the land is needed for food production
What is free radical substitution
a hydrogen atom gets substituted by a halogen
why is ultraviolet needed for this substitution
alkanes are very unreactive
What are the three steps of free-radical substitution
In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals
These radicals create further radicals in a chain reaction called the propagation step
The reaction is terminated when two radicals collide with each other in a termination step
explain initiation step
Cl-Cl or Br-Br is broken by energy from the UV light
This produces two radicals in a homolytic fission reaction each atom takes one electron from covalent bond
what is heterolytic and homolytic
homolytic fission is splitting a bond to produce two particles which are free radicals.
Heterolytic fission produces two particles which are different because one is a positive ion and the other a negative ion.
explain propagation
Free radicals are very reactive and will attack the unreactive alkanes
A C-H bond breaks homolytically (each atom gets an electron from the covalent bond). An alkyl free radical is produced
This can attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radical
This free radical can then repeat the cycle
progression (growing) of the substitution reaction in a chain reaction
Will all hydrogen be eventually substituted
yes if there is enough halogen present.
explain termination step
when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule
different possible equations with the different alkyles
if we have propane how will substitution occur
put it next to the carbon with radical in brackets
characteristics homologous series
they have the same general formula
neighbouring compounds differ from each other by a -CH2 - group
they have the same functional group / display similar chemical properties
they show a gradual change / trend in physical properties
uses of cracking hydrocarbons
shorter chain alkanes and alkenes formed
Alkenes are useful starting materials in organic synthesis / used for making polymers / plastics
Shorter chain alkanes are more in demand / higher value / can be used as fuel
Why do isomers of alkanes have lower boiling temperatures
due to branching which decreases the surface area as the molecule will be less tightly packed therefore the intermolecular forces / van der vaals forces are decreased.
why are sigma bonds stronger
more effective overlap
one reason why cracking reactions are carried out in industry and suggest why high temperatures are used in this process other than to speed up the reaction.
(Greater) demand for smaller alkanes more useful and to break c-c bond or break down the hydrocarbon chain
what is a fuel
substance that produces heat or energy
reason why companies reform alkanes
has more efficient combustion
allows smoother burning
increases octane number
reduces knocking / less likely to produce pre-ignition
why can different alkanes in crude oil be seperated by fractional distillation
as they contain a different boiling temperature due to difference in chain length
why do isomers of the unbranched hexane have a lower boiling temperature
because isomers have branched chain that have lower surface areas so don’t pack closely hence london forces are weaker
what is the one product that is always produced during forming
Hydrogen
