6.2 [Hydrocarbons] Flashcards
What are the 3 main processes used to convert crude oil into fuels?
Fractional distillation
Cracking
Reforming
These processes are used in oil refineries that are located all over the world.
What is fractional distillation?
Crude oil is a complex mixture of compounds including hydrocarbons. Composition of mixtures vary depending on were the crude oil came from. It involves converting crude oil into a small number of fraction, the number of fractions vary between different refineries takes place in a distillation column.
Fractional distillation is the process used to separate a liquid mixture into fractions by boiling and condensing
What is the process for fractional distillation?
crude oil is heated in a furnace which turns most of the liquid into vapour, it is then passed into the column near the bottom. There is a temperature gradient in the column hotter at the bottom cooler at the top.
As vapour passes up the column different fractions condense at different highest in the column depending on the boiling temperature range of the molecules in the fraction.
Near the bottom the fractions contain larger molecules with longer chains and higher boiling temperature.
Near the top of the column the fractions contain smaller molecules with shorter chains and lower boiling temperatures.
Some hydrocarbons in crude oil are dissolved gases and they rise to top of column without condensing.
Why can’t some fractions undergo fractional distillation?
Some fractions contain many different compounds so may go further fractional distillation separately.
What is cracking?
Demand for shorter chain hydrocarbons is much higher as they are better fuels can be used to make other substances like polymers.
Cracking is the breakdown of molecules into shorter ones by heating with a catalyst.
What is the process involved for cracking?
Cracking is done by passing the hydrocarbons in the heavier fractions through a heated catalyst, usually of zeolite , this causes larger molecules to break up into smaller ones. One large molecule may form 2 small ones.
What is reforming?
During very rapid combustion that occurs in vehicle engines, not all hydrocarbons of the right size burn in the same way. Those with straight chains burn less efficiently than those with branched chain molecules and those with rings. Reforming is the process of the conversion of straight chain hydrocarbons into branched chain cyclic hydrocarbons.
What are Alkanes burned for?
Alkanes can burn, Alkanes can be burned in vast quantities to provide worlds energy.
What are the products of combustion?
One of the products of combustion is water, which adds to the total quantity of H20.
Other products is CO2. This is a greenhouse gas and its increasing production is considered to be responsible for global warming climate change and other problems.
Other problems are caused by using alkanes as fuels, water and carbon dioxide formed not considered to be pollutants but other compounds formed during combustion of alkanes are pollutants.
What is incomplete combustion?
The combustion of alkanes can be incomplete if there is insufficient oxygen or because the combustion is very rapid. Some of the atoms in the fuel are not fully oxidised. All of the hydrogen atoms in the alkane molecules are converted into water but some of the carbon atoms can form carbon monoxide or solid carbon (soot) smoke. Some of the products of incomplete combustion can be fatal.
What is carbon monoxide?
Carbon monoxide is a toxic gas that causes death to many people. It prevents the transport of oxygen around the body, it is colourless and odourless so breathe it in without knowing.
What occurs with unburned hydrocarbons?
An example of incomplete combustion is when the hydrocarbon does not burn at all, a small proportion of the hydrocarbons in a fuel are released into the atmosphere unchanged, they are known as unburned hydrocarbons.
How does combustion link to oxides of sulfur?
Some of the molecules in crude oil contain atoms of sulfur, and these many not be removed by the fractional distillation, cracking or reforming processes. During the combustion of alkanes these atoms of sulfur form sulfur dioxide and then react in atmosphere to form sulfur trioxide
Both of these acids contribute to formation of acid rain, responsible for environmental damages to aquatic life in lakes and rivers and damage to crops and forest
How does combustion link to oxides of nitrogen
Very few molecules used as alkane fuels contain atoms of nitrogen, combustion of these atoms occurs at a very high temperature, Under these conditions nitrogen molecules in the air react with oxygen molecules. these reactions leads to the formation of oxides of nitrogen represented by the formula NOx Nitrogen monoxide and Nitrogen dioxide contribute to environmental damage in the same way as sulfuric acid.
How do catalytic converts fight pollution?
Cars and other road vehicles are responsible for a lot of air pollution, widespread use of catalytic converters fitted to exhaust system made pollution less of a problem.
Different types but all use small quantities f precious metals like platinum these metals speady thinly over a honeycomb mesh to increase surface area for reaction. One common type is a three way catalyst it can remove 3 different types of pollutants CO, unburned hydrocarbons and oxides of nitrogen
As the exhaust gases from the engine passes through the catalytic converters different reactions occur. Examples are the oxidation of the carbon monoxide and the oxidation of unburned hydrocarbons.
Catalyst not very good at removing sulfur compounds best way to remove sulfur based pollution is to remove the sulfur compounds from the fuel before the fuel is burned.
Why are there needs for alternative fuels?
Other than pollution caused by the combustion of alkanes the other factors that need to be considered are the depletion of natural resources and global warming and climate change.
The alternatives to fossil fuels are biofuels (obtained from living matter) , furthermore renewable resources
What is carbon neutrality?
Fuels are considered in terms of their carbon neutrality. A fuel should be completely carbon neutral.
What does carbon neutral mean?
Carbon neutral is a term used to represent the idea of carbon dioxide neutrality.
Fossil fuels such as those from tress described as carbon neutral because they too absorb carbon dioxide during their growth to form the same amount of carbon dioxide when they are burned.
What are Biofuels?
Biofuels are fuels obtained from living matter that has died recently. Biofuels are not carbon neutral because the plants have been harvested and processed in factories. Both of these stages involve the use of energy from formation of carbon dioxide so overall use of biofuel involves forming more carbon dioxide than is absorbed. Much closer to being carbon neutral than other fossil fuels.
What is Biodiesel?
Biodiesel is very important. Starting materials for making biodiesel are present in vegetable oils like those obtained from sunflowers. Biodiesel is a very effective alternative to ordinary diesel. Biodiesel is a fuel made from vegetable oils obtained from plants.
What are Bioalcohols?
Bioethanol refers to ethanol produced by bacteria rather than enzymes. It is possible to use a much wider range of plants and plant waste. The upper limit to the amount of ethanol that can be obtained from a given amount of starting material is increasing more efficient form of fermentation?
What is a renewable resource?
Renewable energy sources use sources that can continuously be replaced
What is a non-renewable resource?
Non-renewable energy sources are not being replenished except over geological timescale.
What are the factors used to compare Biofuels?
Land use - How much land is used to grow the crops?
Yield - How much crop can be grown on a given piece of land and how quickly does it grow?
Manufacture and transport - How much energy is used in growing, processing and transporting the crops?
Carbon neutrality - How close to carbon neutral is the fuel?
Why are alkanes relatively unreactive?
Alkanes apart from readily undergoing combustion are fairly unreactive because they contain only carbon and hydrogen atoms and only single bonds. The bonds are not very polar and so do not undergo reactions with substances that are reactive like acid alkalis and metals.
What is a substitution reaction?
A substitution reaction is one in which an atom or group is replaced by another atom or group. Alkanes undergo these reactions.
CH4 + CL2 > CH3Cl + HCl
In the equation above one of the hydrogen atoms in methane has been replaced or substituted by an atom of chlorine. This reaction is described as chlorination.
What are mechanisms?
A mechanism is the sequence of steps in an overall reaction. Each step shows what happens to the electrons involved in a bond breaking bond formation. Mechanisms explain the actual changes that occur during a reaction. Each step is represented by an equation.
What mechanisms occur in the chlorination of methane?
When methane is mixed with chlorine no reaction occurs until the temperature is increased or until the mixture is exposed to UV radiation. Alkanes are not affected by UV radiation but UV can affect chlorine.
What is the first step for the chlorination of methane?
UV radiation breaks the chlorine molecule into chlorine atoms. As the bond in the chlorine molecule consists of a shared pair of electrons, which are shared between the 2 atoms, Each chlorine atom takes one electron from shared pair. This bond breaking is called Homolytic fission.
Represented by equation:
Cl2 > Cl. + Cl.
Dots represent an unpaired electron. The formula CL. represents a radical. A radical is a species with an unpaired pair of electrons.
This reaction involves one molecule forming 2 radicals, it is called an initiation.
What is the second step for the chlorination of methane?
Chlorine radicals are very reactive species when they collide with methane molecules. They remove a hydrogen atoms
Cl. + CH4 > HCl + CH3.
CH3. is formed this is a methyl radical it is very reactive it then reactive with chlorine molecules.
CH3. + Cl2 > CH3Cl + Cl.
Both equations involve one radical reactive with a molecule and the products are also a radical and a molecule this is known as propagation.
What is the 3rd step of the chlorination of methane?
When all radicals being formed likely that 2 radicals will collide with each other. When this happens react to form a molecule. 2 unpaired electrons share to form a covalent bond 2 radicals available 3 different possibilities.
Cl. + Cl. . Cl2
Cl. + CH3. > CH3Cl
CH3. + CH3. > C2H6
Involve 2 radicals reacting to form one molecule known as termination. sequence of reactions come to an end.
What is homolytic fission?
Homolytic fission is the breaking of a covalent bond where each of the bonding electrons leaves with one species forming a radical.
What is a radical?
A species with an unpaired electron.
What are initiation, propagation and termination reactions?
Initiation involves the formation of radicals, usually as a result of bond breaking by UV radiation.
Propagation are the 2 steps when repeated many times convert the starting materials into the products of the reaction
Termination steps involve the formation of a molecule from 2 radicals.
What are the differences between alkenes and alkanes?
Alkanes only contain single bonds, alkenes contain at least one C=C double bond, so they are unsaturated. Alkenes are much less common than alkanes can be made from alkanes. Formula CnH2n
What are the bond angles like for the alkenes?
For alkenes normally show the bond angles at 120
Some alkenes like alkanes are cyclic, cyclic alkenes like cyclohexene do not have the same molecular formula as hexene fewer hydrogen atoms.
How is a C=C bond relevant to alkenes?
Carbon double bond is an area of high electron density.
What importance to sigma bonds have to alkenes?
Covalent bond between 2 hydrogen atoms in a hydrogen molecule are shown as H-H referred to as sigma bonds. When sigma bonds are formed between carbon atoms in an alkene their formation is called an overlap
What importance do Pi bonds have to alkenes?
When 3 sigma bonds around each carbon atom are formed in ethene. After they are formed each carbon atom has one electron in a P orbital that ha not been used in bond formation p orbitals are parallel to each other not able to overlap in the 2 regions of negative charge above and below the C-C sigma bonds. type of bond that is formed by the sideways overlap of orbitals and bond types are known as pi bonds. Each of these regions of negative charge contains one electron so they make a second shared pair of electrons between the carbon atoms.
Why are sigma bonds stronger than pi bonds?
Because the sigma bond electrons are more tightly held between 2 carbon atoms, a sigma bond is stronger than a pi bond. So a double bond is a stronger than a single bond.
How do addition reactions occur in Alkenes?
Most reactions of alkenes involve the double bond becoming a single bond, in these reactions sigma bond remains unchanged but the pi-bond electrons are used to form new bonds with an attacking molecule. Reaction forms a saturated molecule as the product contains sigma bonds not pi, products bonds are stronger to more stable.
Reaction between ethene and bromine C2H4 + Br2 >C2H4Br
How is the reaction between ethene and bromine a addition reaction?
C2H4 + Br2 > C2H4Br2
This reaction used as a chemical test for the presence of C=C in a compound, because the products of these reactions are colourless. So when this reaction occurs colour of bromine is decolorised
2 Molecule become 1
What is hydrogenation?
Hydrogenation is an addition reaction in which hydrogen is added to an alkene in hydrogenation of ethene + Hydrogen > ethane is done using a nickel catalyst
How is hydrogenation used in the manufacturing of margarine?
Type of reaction is extensively used in industry to manufacture margarine. Naturally occurring vegetable oils are unsaturated and contain C=C double bonds. When these react with hydrogen some of the C=C double bonds become C-C single bonds this process changes the properties of the vegetable oils converts it into a solid.
What is Halogenation?
Reactions between alkenes and bromine examples of halogenation products of these reactions are dihalogenalkanes. Reactions with chlorine are chlorination.
What are hydrations?
Hydrations is adding water adding H and OH to 2 C=C double bonds this reaction is usually done by heating the alkene with steam passing mixture over a catalyst of phosphoric acid.
What is addition of hydrogen halides?
Another addition reaction is addition of a hydrogen halide often hydrogen bromide or chlo9ride to form halogenoalkane.
What happens in oxidation to diols?
Both addition and oxidation A diol is a compound containing to OH groups. Oxidising agent is potassium manganate (VII) in acidic conditions. The potassium manganate provides an oxygen at, and the water in the solution provides another oxygen atom and 2 hydrogen
atoms addition of 2 OH groups across double bond.
During this reaction Potassium manganate (VII) solution changes from purple to colourless this col9our used to distinguish between alkanes and alkenes
How do alkenes attract other molecules?
Alkenes such as ethene have a pi bond, region of high electron density (rich C=C) double bond. This makes it attractive to other species that are electron deficient, including molecules with polar bonds. Hydrogen bromide is a polar molecules because bromine is more electronegative than hydrogen H-Br
What are electrophiles?
When a hydrogen bromide molecule approaches an ethene molecule the slightly positive end of the HBr molecule is attracted to the electron in the pi bonds in C=C. The HBr molecule is described as electrophile when it does this. Phile meaning liking electro meaning electro parts to negative charge species attracted to negative charge.
The curly arrow used in the reaction must start from a bond and move to an atom. Or start from a lone pair of electrons and move to an atom.
How does electrophilic addition occur for hydrogen halides?
Involves the addition and an attack by electrophile. This mechanism of reaction between ethene and hydrogen bromide. Hydrogen bromide molecule breaks so that both electrons in H-Br bond go to one atom. This bond breaking is called heterolytic fission.
What is step 1 of the addition reaction of hydrogen halides?
In this step 2 ions are formed. Positive ion has its charge on a carbon atom so it is known as carbonation.
Bromide ion is shown with 4 lone pairs which is correct often only the lone pair moves.
What is step 2 of the electrophilic addition of hydrogen halides?
The 2 opposite charged ions attract each other and react to form a new covalent bond.
How does electrophilic addition occur in halogens?
Reaction is very similar to hydrogen halides. Attacking bromine molecule does not have a polar bond. As it approaches the C=C bond, the electrons in the pi bond repel the electrons in the Br-Br bond and induce the molecule to become polar. After this same as hydrogen halide.
How are alkenes and used in addition polymerisation?
Many compounds containing C=C double bonds can be polymerised. They are addition reactions because the alkene molecules add together in vast numbers to form a polymer. The conditions for polymerisations reactions are high temperature and pressure.
How are polymers named?
When alkene molecules are used in polymerisation they are often referred to as monomers. Standard way to write is ‘poly’ followed by the name of the monomer in brackets for example poly(ethene).
How are equations formed for polymerisations reaction?
Polymers formed do not have a fixed molecular formula need to find a different way to show what happens in the reaction. Usual way to do this is use the letter n to represent the number of monomer molecules reacting then to show a repeat unit of the polymer inside the bracket. The letter n is shown as a subscript or bracket, and there are covalent bonds shown passing through the to indicate there us another repeat unit joined to each side.
How are monomers identified?
If a repeat unit of a polymer is given, or a section of the polymer that contains several repeat units, can work out the structure of the corresponding monomer. need to identify the part that is repeated, this will be 2 carbon atoms in the chain and 4 atoms or groups joined to them. The monomer structure is all of these atoms, but with a double bond between the 2 carbon atoms.
What are monomers?
Monomers are the small molecules that combine together to forma polymer.
What are the reasons for the increase use of polymers?
Can be manufactured on a large scale in a variety of complex shapes with a wide range of physical properties. Plastic bottles that are rigid.
They are often lighter in weight than traditional alternatives like milk in a plastic bottle.
They are unreactive and so can contain substances for a long period of time
polymers are cheap to use when mass produced and have become disposable to some people. Polymers used to be thrown in the bin taken to landfill sites now there are many alternative solutions and rules preventing this. Polymer waste goes in a designated bin.
What are the solutions to polymer waste?
Reduce the use of polymers. Supermarkets are discouraging the use of free plastic bags and encourage bags for life.
The other uses include
Recycling
Incineration
Use of chemical feedstock.
How is recycling a solution to polymer waste?
Recycling means converting it into other materials like poly(ethylene terephthalate) known as PET is widely used in plastic bottles its now used on a wide scale to make carpets.
First stage is sorting there are many polymers in use and mixtures of these types can not be effectively processed together. A lot of this is done by hand. Polymer used to make plastic objects identified by a code.
Second stage is processing involved chopping the waste into small pieces and washing it. material then reused using methods like melting or molding.
How is incineration a solution to polymer waste?
Elements present in polymer wate are mostly hydrogen and carbon. So used as fuels, similar to hydrocarbons. An incinerator take is polymer waste and converts it into heat energy that can be used to heat homes and factories or generate electricity. Very little solid waste left after incineration most of the atoms end up in gaseous products.
Very local opposition when there is proposal to build an incinerator because of the concerns about air pollution. This is because as well as hydrogen and carbon other elements like in PVC there is chlorine and small amount of toxic heavy metals. Pollutants difficult to remove gases from atmosphere.
How is use as a chemical feedstock a solution to polymer waste?
This process similar to cracking used to break down the polymer waste into gases produces a feedstock that can be used in other chemical reactions.
How are biodegradable polymers a solution to polymer waste?
Traditional plastics into landfill down break down. Idea of using biodegradable polymers is to allow them to be broken down by microbes in the environment. Some of them are used in small scales in medicine However:
They are often made from plant material so there is the same issue to consider with biofuels land is needed
They are designed to break down in the environment so when they do, the hydrogen and carbon atoms they contain cannot be directly used. No recycling, incineration or use as a chemical feedstock is possible.
How can polymers life cycle be analysed?
All polymers have to be manufactured and then disposed of in the ways shown. Different stages in the life of a polymer have some environmental impact especially in terms of resource use, energy and disposal.
to help make decisions about the management of polymer waste in the future to reduce the long term impact on the environment. And to compare different polymer uses a life cycle analysis is carried out.
