alkanes Flashcards
what are alkanes
alkanes are saturated hydrocarbons - they contain only carbon -carbon and carbon - hydrogen singe binds
what are alkanes used for
alkanes are the least reactive organic compounds
they are used for:
- fuels
- lubricants
- starting materials for a range of other compounds - this means they are very important to industry
what is the source of alkanes
the main source for alkanes is crude oil
what is the general formula for alkanes
the general formula for chain alkanes is CnH2n+2
what are the different structure of alkanes
hydrocarbons may be unbranched chains , branched chains or rings
what are unbranched alkanes
unbranched chains are often called straight chains but the C-C-C angles is 109.5 degrees.
This means that the chains are not actually straight
In an unbranched alkane, each carbon has two hydrocarbons except the end carbons which have one extra
what is the general formula for ring alkanes
ring alknes have the general molecular formula of Cn H2n
what is the polarity of alkanes
alkanes are almost non - polar because the electronegativity of both carbon and hydrogen are so similar.
As a result, the only intermolecular forces between their molecules are weak Van der Waals forces, and the larger the molecule, the stronger the Van der Waals forces
what is the boiling point of alkanes
due to the increasing intermolecular forces, the boiling points of alkanes increase as the chain length increases
the shorter chains are gases at room temperature
Pentane, with five carbons, is a liquid with a low boiling point of 309K (36C)
At a chain length of about 18 carbons, the alkanes become solids at room temperature
what is the trend of melting points in branched chains
alkanes with branched chains have lower melting points than straight chain alkanes the the same number of carbon atoms
This is because they cannot pack together as closely as branched chains and so the Van der Waals forces are not effective
what s the solubility of alkanes
Alkanes are insoluble in water. This is because water molecules are held together by hydrogen bonds which are much stronger than he Van der Waal’s forces that act between alkane molecules.
However, alkanes do mix with other relatively non -polar liquids
how do alkanes react
alkanes are relatively unreactive, they have strong carbon - carbon and carbon - hydrogen bonds
Thy do not react with acids, bases, oxidising agents, and reducing agents
what happens when alkanes are burned in oxygen
when they burn in plentiful supply of oxygen, they form carbon dioxide and water
OR
in a restricted supply of oxygen, they form carbon monoxide and carbon )
why are crude oils important
crude oil is at the present the world’s main source of organic chemicals
how are fossil fuels formed
fossil fuels are formed by the breakdown of plant and animal remains at the high pressures and temperatures deep below the Earth’s surface
because it forms slowly, it effectively non - renewable
what is crude oil
crude oils are a mixture mostly of alkanes , both unbranded and branched
crude oils from different sources have different compositions
what is fractional distillation
to convert crude oil into useful products you have to separate the mixture
This is done by heating it and collecting the fractions that boil over different ranges of temperatures
what are the different fractions composed of
each fraction is composed of:
- hydrocarbons of similar chain lengths and therefore similar properties
how do you carry out fractional distillation/ how does it work
METHOD:
- the crude oil is first heated in a furnace ( at 350C)
- a mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom
- the vapours pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool ( at a lower temperature than their boiling point). Then they condense to liquid
- the mixture of liquids that condenses on each tray is piped off
- the shorter chain hydrocarbons condense in the trays nearer to the top of the tower, where it is cooler, because they have lower boiling points
what is the thick residue that collects at the base of the fractional tower called
the thick residue that collects at the base of the tower is called tar or bitumen. It can be used for road surfacing but, as suppy often exceeds demand, this fraction is often further processed to give more valuable products
what is cracking
longer chain fractions are not as useful and therefore of lower value economically
Most crude oil has more longer chain fractions than is wanted and not of the naphtha fraction ( which has a shorter chain and are not in high demand)
The shorter chain products are economically more valuable than the longer chain material
to meet the demand for the shorter chain hydrocarbons, many if the longer chains are broken into shorter lengths (cracked)
what are the products of thermal cracking
shorter more useful chains are produced especially petrol
some of the products of the long-chain alkanes are the more reactive alkenes
what are alkenes used for
alkenes are used as chemical feed stock (which means they supply industries with the starting materials to make different product) and are converted into a huge range of other compounds e.g. polyethene and a range of everyday materials
what are the different ways in which alkanes are cracked
since alkanes are very unreactive and harsh conditions are required to break them down. There are a number of different ways of carrying out cracking:
- Thermal cracking
- Catalytic cracking
what is thermal cracking
thermal cracking involves heating alkanes to a high temperature, 700K -1200K (1000C) under high pressure, up to 7000kPa (70atm)
the C-C bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom
These fragments are called free radicals
Free radicals are highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules
how do the C - C bonds in alkanes break
any number of C - C may break and the chain does not necessarily break in the middle
-Hydrogen may also be produced
why is there a high proportion of alkenes when hydrocarbons are cracked
as there is not enough hydrogen atoms to produce two alkanes, one of the new chains must have a C = C and therefore be an alkene
how do we reduce the decomposition of products formed by thermal cracking
thermal cracking tends to produce a high proportion of alkenes
to avoid too much decomposition (ultimately to carbon and hydrogen) the alkanes are kept in these conditions for a very short time (typically one second)
what is catalytic cracking
Catalytic cracking involves
catalysts to crack hydrocarbons
This form of cracking is used to produce motor fuels
at what temperature and pressure does catalytic cracking takes place
catalytic cracking takes place at a lower temperature (approximately 500C ) and lower pressure ( but more than atmosphere)
what type of catalyst is used in catalytic cracking
it uses zeolite catalyst, consisting of silicon dioxide and aluminium oxide (aluminium silicates)
what structure is the zeolite structure
zeolites have a honeycomb structure with an enormous surface area. They are also acidic.
what are the products produced by catalytic cracking
the products are mostly branched alkanes, cycloalkanes (rings) and aromatic compounds
what are the advantages of catalytic cracking
it is cheaper because it does not require high pressure or temperatures
Catalysts speed up reactions so less time and money is spent
how can we carry out catalytic cracking in a laboratory
in the laboratory, catalytic cracking can be carried out using an aluminium oxide catalyst
the products are mostly gases, showing that they have chain lengths of less than C5 and the mixture decolourises bromine solution. This test is a test for carbon-carbon double bond showing that the product contains alkenes
what happens when alkanes are burned in a limited supply of oxygen
in a limited supply of oxygen, the poisonous gas carbon monoxide, CO is formed
This is called incomplete combustion
with even less oxygen, carbon (soot) is produced e.g. when a bunsen burner is used with a closed air hole, the flame is yellow and a black sooty deposit appears on the apparatus
water is also produced in each of these reactions
when does incomplete combustion usually happen
incomplete combustion often happens with longer chain hydrocarbons, which need more oxygen to burn compared with shorter chains
what are the pollutants that are released when burning fuels
all hydrocarbon-based fuels derived form crude oil may produce polluting when they burn:
They include the following:
- nitrogen oxides
- sulfur dioxide
- carbon particulates
- unburnt hydrocarbons
- CO2
what are nitrogen oxides
nitrogen oxides (NOx) is produced when there is enough energy for nitrogen and oxygen in the air to combine
e.g.
N2(g) + O2(g) - 2NO(g)
The nitrogen monoxide can reach further to produce nitrogen dioxide:
2NO(g) + O2 - 2NO2
This happens in a petrol engine at the high temps present, when the sparks ignite the fuel. These oxides react with water vapour and oxygen in the air to form acid. They are therefore contributors to acid rain and photochemical smog
Nitrogen monoxide is produced when the high pressure and temperature in a car engine cause the nitrogen and oxygen atoms in the air to react
how are sulfur dioxides formed
Some fossil fuels contain sulfur
When they are burnt, the sulfur reactor formsukfur dioxide gas (SO2)
When sulfur dioxide gets in the atmosphere, it dissolves the moisture and is converted to sulfuric acid. This is what causes acid rain- the same process occurs with nitrogen oxides
These oxides combine with water vapour and oxygen in the air to form sulfuric acid
why is acid rain damaging
acid rain destroys trees and vegetation, as well as corroding buildings and statues and killing fish in lakes
what are carbon particles
carbon particles, are called particulates which can exacerbate asthma and cause cancer
What are unburned hydrocarbons
Engined don’t burn all the fuel molecules
Some these come out as unburned
what is CO2
CO2, is a greenhouse gas. It is always produced hydrocarbons burn
Greenhouse gases in the atmosphere are very good at absorbing infrared energy. Try emit some of the energy back towards the earth keeping it warm- this js called the greenhouse effect
Although carbon is necessary in the atmosphere, its level is rising and this is a cause of the earth’s temperature and consequent climate change
what is flue gas desulfurisation
large numbers of power stations generate electricity by burning fossil fuels such as coal or natural gas
These compounds contain sulfur compounds and one of the products of their combustion is sulfur dioxide, SO2, a gas that causes acid rain by combining with oxygen and water in the atmosphere to form sulfuric acids
SO2(g)+1/2O2(g) +H2(l) - H2SO4
these gases that are called flue gases so the process of removing the sulfur dioxide is called flue gas desulfuration
how can we carry out flu gas desulfurisation
one method:
a slurry of calcium oxide (lime =) and water is sprayed onto the flue gas which reacts with the calcium oxide ad water to form calcium sulfite, which can be further oxidised to calcium sulfate
CaO(s) +2H2O(l) +SO2 +1/2O - CaSO4 .2H2O
Alternatively:
you can use calcium carbonate rater than calcium oxide:
CaCO3(s+ 1/2O3(g) +SO(g) - CaSO4+CO2
what do catalytic converters do
internal combustion engines produce most of the pollutants listed previously
(except sulfur dioxide because sulfur is removed from petrol)
therefore all new cars with petrol engines are now equipped with catalytic converters in their exhaust systems to reduce the output of carbon monoxide, nitrogen oxides, and unburnt hydrocarbons in the exhaust gas mixture
what is the shape of catalytic converters/ what are they made out of
honeycomb made of ceramic material coated with platinum and rhodium metals are the catalysts
why are catalytic converters the shape of a honeycomb
honeycombs provide an enormous surface are, so a little of these expensive metals goes a long way
how do catalytic converters work
as the polluting gases pass over the catalyst, they react with each other to form less harmful products by the following reactions:
2CO + 2NO(g) - N2(g) +2CO2
carbon monoxide+ nitrogen oxide - nitrogen + carbon dioxide
hydrocarbon + nitrogen oxide - nitrogen + carbon dioxide + water
2NO(g) - N2 + O2
what is the greenhouse effect
greenhouses (CO2) traps infrared radiation so the earth’s atmosphere heats up
This is the greenhouse effect
why is the greenhouse effect important
the greenhouse effect it is impostant for life because, without other greenhouse gases, the earth would be too cold to sustain human life
why has there been an increase in the levels of greenhouse gases in the atmosphere
due to the industrial revolution, fossil fuels have been used to fuel industrial plants and the levels of CO2 has been rising.
This causes the eath’s temp has been rising too and scientists believe this is due to the increased level of CO2
what happens when you expose an alkane and a halogen to UV light
When you put a mixture of an alkane and a halogen into bright sunlight (UV light) to form a halogenalkane
This reaction is called a photochemical reaction as it is started by uv light
how is the halogenalkane bromohexane formed
if you put a mixture of hexane and a little liquid of bromine into a test tube and leave it in the dark, it will stay a red-brown colour ( the colour of bromine)
However, if we shine ultraviolet light onto it, the mixture becomes colourless and mist fumes of hydrogen bromide appear
This is a substitution reaction :
one or more of the hydrogen atoms in the alkane has been replaced by a bromine atom and hydrogen bromide is given off as a gas
the haloalkane is also produced
what is the stages in a chain reaction
the reaction from above: an alkane and bromine
is a chain reaction
the three stages are:
initiation, propagation and termination
the reaction between any alkane and halogen goes by the same mechanism
what is initiation
imagine a reaction between methane and chlorine
first, (initiation) step of the reaction is breaking the CL -CL bond to form two chlorine atoms
the chlorine molecule absorbs the energy of a single quantum of ultraviolet (UV) light
The energy of one quantum of UV light is more than the CL -CL bond energy, so the bond will break
Since both atoms are the same, the CL- CL bond breaks homolytically, that is one electron going to each chlorine atom
This results in two separate chlorine atoms. They are called free radicals. The dot next to the CL is used to show the unpaired electron
free radicals are highly reactive
The C-H bond in the alkane needs more energy to break than is available in a quantum of UV radiation and therefore does not break
what happens during propagation
we are still considering the chlorine and alkane reaction
propagation takes place in two stages:
- the chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride, a stable compound. This leaves a methyl free radical CH3
- The methyl free radical is also very reactive and reacts with a chlorine molecule. This produces another chlorine free radical and a molecule of chloromethane - a stable compound
the effect of these two steps is to produce hydrogen chloride and chloromethane, and a new CL free radical.
This is ready to react with more methane and the two steps are repeated. This is the chain part of the chain reaction
These steps may take place thousand of times before the radicals are destroyed in the termination
what is termination
Termination is when free radicals are removed. This can happen in any of the following three ways ( in regards to the reaction with alkanes and chlorine) :
- Two chlorine free radicals react together to give chlorine CL2
- A chlorine free radical and a methyl free radical react to give chloromethane
- Two methyl free radical react together to give ethane
what are the other products that are produced in the chain reaction of chlorine with alkanes
the main ones are chloromethane and hydrogen chloride
some ethane
dichloromethane - in the propagation stage if a chlorine radical reacts with some chloromethane that has already formed
with longer chain alkanes there w9ill be many isomers formed because the CL can replace any of the hydrogen atoms
chain reactions are not very useful because they produce such a mixture of products. They will also occur without light at high temperatures
why are chain reactions not very useful
chain reactions are not very useful because they produce such a mixture of products. They will also occur without light at high temperatures
what is ozone
ozone is a molecule made from three oxygen atoms O3
The ozone layer is important because it protects the earth from the harmful exposure to too many ultraviolet (UV) rays. Ultraviolet can cause skin cancer in people by damaging DNA
how is the ozone layer damaged by the free radical
chlorine free radicals are formed from CFCs because the C - CL bonds breaks homolytically in the presence of UV radiation to produce free radicals, CL
Cl•+ O3• - O2(g) + ClO•
ClO• + O3 - 2O2 + Cl•
Because the chlorine free radical is regenerated, it only takes a little of it to destroy loads of ozone molecules
Why are these unburned hydrocarbons bad dor the environment
These hydrocarbons react with nitrogen oxides in the presence of sunlight to form ground level ozone (O3)
This is a major component of smog
Ground level ozone irritates people’s eyes, aggravates respiratory problems and even causes lung damage
What are the three main pollutants from vehicle exhausts
Nitrogen oxides
Unburnt hydrocarbons
Carbon monoxide
Catalytic convertes of Cars remove these pollutants from the exhaust
What is the equation for the removal for unburnt hydrocarbons
It is just the equation for the complete combustion of a hydrocarbons
