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7/28/2019 Basic Concepts and Hydrocarbons (Module 1)
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Chemistry: Unit 2: Section 1
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Isomerism
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Structural isomers compounds with same
molecular formula but different structural formulae.
Chain isomers:
butane methylpropaneC4H10
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Structural isomers compounds with same
molecular formula but different structural formulae.
Positional isomers:
Butan-1-olButan-2-ol
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Structural isomers compounds with same
molecular formula but different structural formulae.
Functional group isomers:
Butan-1-ol ethoxyethane
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Sterioisomers compounds with samestructural formulae but a different
arrangement in space
Alkenes have sterioisomers because of a lack of
rotation around the C=C double bond. When the double-bonded carbon atoms each
have 2 different atoms or groups attached to
them, you get an E-isomer or a Z-isomer.
E-isomer: same groups are across the doublebond
Z-isomer: same groups are both above or both
below the double bond.
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E-isomer: E-but-2-ene
Same groups are
across the double
bond
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Z-isomer: Z-but-2-ene
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Cis-trans isomerism as a special case E/Zisomerism in which 2 of the substituent
groups are the same
Cis = Z-isomer
Trans = E-isomer Each of the groups linked to the double-bonded
carbons is given a priority.
If the two carbon atoms have their higher priority
group on opposite sides = E-isomer Higher priority groups on same side = Z-isomer
Br has a higher priority than F
The names depend on where the Br atom is inrelation to the CH3 group.
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Percentage Yield
(k) carry out calculations to determine the
percentage yield of a reaction.
Percentage yield = actual yield X 100
theoretical yield
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Ethanol can be oxidised to form ethanal:C2H5OH + O CH3CHO + H2O9.2g of ethanol was reacted with an oxidising agent in excess and 2.1g of
ethanal was produced. Calculate percentage yield.
N = m(g) =
M
Moles of C2H5OH = 9.2 / (2x12) + (5x1) + (16) = 0.2 moles
1 mole of C2H5OH produces 1 mole of CH3CHO so 0.2
moles of C2H5OH will produce 0.2 moles of CH3CHO.
M of CH3CHO = 2x12 + 4x1 + 16 = 44gmol-1Theoretical yield (mass of CH3CHO = number of moles x
M = 0.2 x 44 = 8.8g
2.1/ 8.8 x 100 = 24%
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Atom economy is a measure of the
efficiency of a reaction
Atom economy is a measure of the proportion
of the reactant atoms that become part of thedesired product in a balanced symbol equation.
% atom economy = molecular mass of desired product x 100
sum of molecular masses of all products
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Addition reaction
reactants combine to form a single product
Atom economy is always 100% since no atomsare wasted
C2H4 + H2 C2H6
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Substitution reaction:
One where some atoms from one reactant are
swapped with atoms from another reactant.Always results in at least 2 products the
desired product and at least one by product.
CH3Br + NaOH CH3OH + NaBr
More wasteful than an addition reactionbecause the Na and Br are not part of the
desired product.
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Calculating atom economy
CH3Br + NaOH CH3OH + NaBr
Molecular mass of desired product x 100
Sum of molecular masses of all products
Mr of CH3Br x 100
Mr of CH3OH + Mr of NaBr
12+3+16
12+3+16+23+80
32/ 32+103 x 100 = 23.7%
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Explain that a reaction may have a high percentage
yield but a low atom economy
0.475g of CH3Br reacts with an excess of NaOH in:
CH3Br + NaOH CH3OH + NaBr
0.153g of CH3OH is produced. What is percentage yield?
Moles of CH3Br = 0.475 / (12+3+80) = 0.005 moles
Reactant to product ratio is 1:1
Theoretical yield = 0.005 moles x Mr of CH3OH = 0.005 x12 + 3 + 16) = 0.160g
0.153 / 0.160 x 100 = 95.6%
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Describe the benefits of developing chemicalprocesses with a high atom economy in terms of
fewer waste materials
Low atom economy = lots of waste produced
Costs money to separate desired product from waste
products and costs money to dispose of the waste
products safely so theres no harm to environment
Reactant chemicals are expensive
Its a waste of money is a high proportion of reactant
chemicals end up as useless products.
Low atom economy = less sustainable
Raw materials are in limited supply
Better to environment if less is produced,
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Alkanes
Alkanes and cycloalkanes e.g. C6H12 are saturated
hydrocarbons. Alkanes have the general formula CnH2n+2
Hydrocarbons compound of hydrogen and
carbon only
Every carbon atom in an alkane has 4 single
bonds with other atoms
Its impossible for carbon to make more than 4
bonds so alkanes are saturated
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Examples of alkanes:
methaneethane
propane
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State and explain the tetrahedral shape
around each carbon atom
Each carbon atom has 4 pairs of bonding
electrons around it. They repel each other equally.
The molecule forms a tetrahedral shape
around each carbon
Methane: 1
tetrahedral carbon
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Explain, in terms of van der Waals forces, thevariations in boiling points of alkanes with different
C-chain length and branching
The smallest alkanes, like methane, are gas at room temperatureand pressure low boiling point.
Larger alkanes are liquids higher boiling points Explanation: alkanes have covalent bonds inside the molecules.
Between the molecules there are van der Waals forces that holdthem together.
Longer the carbon chain = stronger van der Waals.
Because theres more molecular surface area and moreelectrons to interact.
Branched-chain alkane = lower boiling point than its straight chainisomer.
Branched-chain alkanes cant pack closely together and havesmaller molecular areas van der Waals reduced.
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Describe the combustion of alkanes, leadingto their uses as fuels in industry in home and
transport
Alkanes burn completely in oxygen
If you oxidise alkanes with oxygen, you get carbon dioxide
and water (combustion reaction) C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)
Combustion reactions happen between gases, so liquidalkanes have to be vaporised first.
Smaller alkanes turn into gases easier (more volatile) sotheyll burn easily.
Alkanes are good fuels.
Propane central heating and cooking
Butane camping gas +
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Explain, using equations, the incompletecombustion of alkanes in a limited supply of
oxygen
Without enough oxygen, alkanes will burn and
produce carbon monoxide and water. For example:
2CH4(g) + 3O2 2CO9G) + 4H2O(g)
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Outline the potential dangers of CO
production in home and car use
Oxygen in bloodstream is carried by haemoglobin.
Carbon monoxide binds to haemoglobin easily. If you breathe in a high concentration of carbon
monoxide, it will bind to haemoglobin in your
bloodstream before the oxygen can.
Therefore, less oxygen reaches your cells suffer
oxygen deprivation and fatigues, headaches,
nausea.
A high concentration of carbon monoxide is fatal.
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Outline the potential dangers of CO
production in home and car use
Any appliance that burns alkanes produces carbon
monoxide: gas or oil-fired boilers and heaters, gas
stoves, coal or wood fires produce carbon
monoxide.
All appliances that use an alkane-based fuel need
to be properly ventilated. They need to be checkedand maintained regularly sources of ventilation
should never be blocked.
Use a carbon monoxide detector.
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Explain the use of crude oil as a source of hydrocarbons, separated as fractionswith different boiling points by fractional distillation, which can then be used as
fuels for processing into petrochemicals
Fractional distillation:
Crude oil is vaporised at 350 degrees Celsius.
Vaporised crude oil goes into the fractioning column andrises up through the trays. Largest hydrocarbons dontvaporise at all because boiling points are too high.
As the crude oil vapour goes up the fractionating column, it
gets cooler. Because of the different chain lengths, eachfraction condenses at a different temperature. Fractionsare drawn off at different levels in the column.
Hydrocarbons with the lowest boiling points dontcondense. They are drawn off as gases at the top of the
column,
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Fractional Distillation of crude oil
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Uses of fractions
Most of the fractions are either used as fuels or
processed to make petrochemicals.
A petrochemical is any compound that is made
from crude oil or any of its fractions and is not
a fuel.
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Describe the use of catalytic cracking to
obtain more useful alkanes and alkenes
Heavy fractions can be cracked to makesmaller molecules
Cracking breaking long-chain alkanes intosmaller hydrocarbons.
Cracking involves breaking the C-C bonds .
E.g. cracking decane: C10H22 C2H4 + C8H18 Decane ethene + octane
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Catalytic cracking:
Heavier fractions are passed over a catalyst at ahigher temperature and moderate pressure.
This breaks them up into smaller molecules
Using a catalyst cuts costs down because thereaction can be done at a lower temperature and
pressure. The catalyst speeds up the reaction.
Gives a high % of branched hydrocarbons andaromatic hydrocarbons (contain benzene rings)
useful for making petrol.
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Hydrocarbons with a high octane
rating burn more smoothly
How a petrol-engine works:
Fuel/air mixture is squashed by a piston and ignited with a
spark, creating an explosion. This drives the piston up again,
turning the crankshaft. Four pistons work one after the other,
so the engine runs smoothly.
Problem: straight-chain alkanes in petrol auto-ignite when
the fuel/air is compressed they explode without being ignitedby the spark. This extra explosion causes knocking in the
engine. To get rid of knocking and make combustion more
efficient, shorter branched-chain alkanes, cycloalkanes and
arenes are included in petrols, creating a high octane rating.
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Explain that the petroleum industry processes straight-chainhydrocarbons into branched alkanes and cyclic hydrocarbons
to promote efficient combustion
Straight-chain alkanes are made into branched or cyclic
hydrocarbons.
Using isomerisation and reforming.
Isomerisation straight-chain to branched-chain:
When you heat straight-chain alkanes with a catalyst
stuck on inert aluminium, oxide. The alkanes break up
and join back together as branched isomers.
A molecular sieve (zeolite) is used to separate the
isomers. Straight-chain molecules go through the sieve
and are recycled.
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Isomerisation:
Pt
CH3CH2CH2CH3
2-methylpropane
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Reforming straight-chain to cyclic
Reforming converts alkanes into cyclic hydrocarbons.
It uses a catalyst made of platinum and another metal.
You need to stick the catalyst on inert aluminium oxide.
Pt + metal
Hexane -------- cyclohexane + H2 benzene + 3H2
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Alkanes
Alkanes are saturated hydrocarbons
Alkanes have the general formula of CnH2n+2 They only have carbon and hydrogen atoms so
are hydrocarbons.
Every carbon atom has 4 single bonds with
other atoms it is impossible to make more so
it is saturated.
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Examples of alkanes:
Methane:Ethane:
http://www.google.co.uk/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/7/7c/Ethane-2D.png&imgrefurl=http://commons.wikimedia.org/wiki/File:Ethane-2D.png&usg=__TVAN42n3DR7BJIRlA90nJ54TdFE=&h=871&w=1100&sz=15&hl=en&start=5&itbs=1&tbnid=IE-rnQQBS6vFeM:&tbnh=119&tbnw=150&prev=/images%3Fq%3Dethane%26hl%3Den%26gbv%3D2%26tbs%3Disch:17/28/2019 Basic Concepts and Hydrocarbons (Module 1)
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Alkane molecules are
TETRAHEDRAL
Each carbon atom has 4 pairs of bonding
electrons.
Repel each other equally.
Forms a tetrahedral shape around each carbon.
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Boiling point of an alkane depends
on its size and shape:
Small alkanes, like methane, are gas at room temperature and pressure =low boiling points.
Larger alkanes are liquids = higher boiling points. Alkanes have covalent bonds inside. Between the molecules, there are
van der Waals forces that hold them together.
Longerthe chain = strongervan der Waals.
Because theres more molecular surface area and more electrons to
interact. As the molecules get longer, it takes more energy to overcome the van
der Waals and separate them so boiling point rises.
Branched-chain alkane has a lower boiling point than its straight-chainisomer. Branched chain alkanes cant pack closely together and havesmaller molecular surface areas.
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Alkanes burn completely in
oxygen:
If you oxidise alkanes with enough oxygen, you get carbondioxide and water (combustion).
Combustion of propene: C3H8(g) + 5O2 3CO2 (g) + 4H2O
Combustion reactions happen between gases, so liquidalkanes have to be vaporised first. Smaller alkanes turn
into gases more easily more volatile burn more easily. Larger alkanes release more energy per mole because of
more bonds to react (good fuel).
Propane is used as central heating and cooking fuel,butane is bottled and sold as a camping gas.
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Alkanes can be burned in limited oxygen but
produce carbon monoxide
Burning methane with not much oxygen:
2CH4(g) + 3O2(g) 2CO (g) + 4H2O (g)
Carbon monoxide is poisonous. CO is better at binding
with haemoglobin than oxygen is if you breathe in air with
a high conc of CO, it will bind to haemoglobin before
oxygen.
Less oxygen reaches cells. Start to suffer oxygen
deprivation fatigue, headaches, nausea, death!
Appliances producing CO must be properly ventilated and
have a CO detector.
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Fossil Fuels
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Fossil Fuels
Combustion of fossil fuels is exothermic give out lots ofenergy.
Different alkanes are used as fuels for transport. Fossil fuels are used to generate electricity in worlds
power stations.
Coal, oil and gas are important raw materials in chemical
industry. Hydrocarbons obtained from fossil fuels are used,especially oil.
Modern plastics are polymers made with organic chemicalsfrom fossil fuels.
Products of petrochemical industry: solvents, detergents,
adhesives, lubricants.
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Burning fossil fuels = greenhouse
gases
Burning carbon-based fossil fuels in transport,
power stations is used a lot today = increased
carbon dioxide (greenhouse gas).
Extra CO2 is contributing to global warming and
climate change by enhancing the greenhouse
effect.
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Fossil Fuels are non-renewable:
Theyll run out because of over-reliance.
Oil first to goits scarce more expensive
Its not sustainable to keep using fossil fuels. Fossil fuels relied on to provide energy for transport,
heating and electricity. Fossil fuels are used to makechemicals like plastics and fibres.
45 years worth of oil, 70 years worth of gas, 250 years ofcoal.
Countries like China and India are developing rapidly andincreasing energy needs
Alternate sources of energy can be used most chemicalsmade from crude oil are made from coal or plants.
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Alternatives to fossil fuels:
Plants are an important source of fuels for the
future
Theyre renewable grow more if you need to.
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Alternative to fossil fuel:
BIOETHANOL
Bioethanol is ethanol produced from plantsmade by fermentation of sugar from crops like
maize. Bioethanol is carbon-neutral = no overall carbon
emission into the atmosphere. All of the CO2released when fuel is burned is removed by crop
as it grows. BUT making fertilisers and powering agricultural
machinery produces CO2.
Its better than petrol and it conserves crude oil.
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Bioethanol
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Alternative fuel (2): BIODIESEL
Comes from plants
Used in diesel engines Made by refining renewable fats and oils like
vegetable oils.
Biodiesel is carbon neutral
BUT carbon dioxide is produced when making
fertilisers and powering agricultural machinery.
Its better than petrol and conserves crude oil.
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Problems using crops to make fuel:
Developed countries like UK will create a huge
demand as they try to find fossil fuel alternatives.
Poorer developing countries like South America
will use this as a way of earning money and
convert their farming land to produce biofuels, so
they wont grow enough food to eat. Forests are being cleared to make room for biofuel
crops. The crops absorb a lot less CO2 than the
forest.
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Alkanes substitution reaction
Heterolytic fission two different substances
are formed a cation (+) and an anion (-).
Homolytic fission two electrically uncharged
radicals are formed.
Radicals are species that have an unpaired
electron. Radicals are very reactive.
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(K) Describe the substitution of alkanes usingUV radiation by Cl2 and BR2 to form
halogenoalkanes
Halogens react with alkanes in photochemicalreactions started by UV light.
A hydrogen atom is substituted (replaced) bychlorine or bromine (free-radical substitutionreaction).
E.g.
CH4 + Cl2 CH3Cl + HCl
CH4 + Br2 CH3Br + HBr
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(m) Describe how homolytic fission leads tomechanism of radical substitution in alkanes in terms
of initiation, propogation and termination
INITIATION:
Free radical produced. Sunlight provides enough energy to break the Cl-
Cl bond by photodissociation.
Cl2 2Cl
Bond splits equally and each atom keeps one
electron by homolytic fission.
Atom becomes a highly reactive free radical,
Cl because of its unpaired electron.
http://www.google.co.uk/imgres?imgurl=http://www.mediabistro.com/unbeige/original/big%2520black%2520dot.jpg&imgrefurl=http://www.mediabistro.com/unbeige/graphic_design/new_york_city_opera_will_make_its_mark_with_big_black_dot_115558.asp&usg=__JKLnE8_LmknlFJ86glDTcJggY0o=&h=300&w=300&sz=6&hl=en&start=25&itbs=1&tbnid=wBy2pD7vGxtSTM:&tbnh=116&tbnw=116&prev=/images%3Fq%3Ddot%26start%3D20%26hl%3Den%26sa%3DN%26gbv%3D2%26ndsp%3D20%26tbs%3Disch:1http://www.google.co.uk/imgres?imgurl=http://www.mediabistro.com/unbeige/original/big%2520black%2520dot.jpg&imgrefurl=http://www.mediabistro.com/unbeige/graphic_design/new_york_city_opera_will_make_its_mark_with_big_black_dot_115558.asp&usg=__JKLnE8_LmknlFJ86glDTcJggY0o=&h=300&w=300&sz=6&hl=en&start=25&itbs=1&tbnid=wBy2pD7vGxtSTM:&tbnh=116&tbnw=116&prev=/images%3Fq%3Ddot%26start%3D20%26hl%3Den%26sa%3DN%26gbv%3D2%26ndsp%3D20%26tbs%3Disch:17/28/2019 Basic Concepts and Hydrocarbons (Module 1)
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(m) Describe how homolytic fission leads tomechanism of radical substitution in alkanes in terms
of initiation, propogation and termination
PROPOGATION:
Free radicals used up and created in a chainreaction.
Cl attacks a methane molecule:
Cl + CH4 CH3 + HCl
The new methyl-free radical, CH3 attacksanother Cl2 molecule
CH3 + Cl2 CH3Cl + Cl
New Cl can attack another CH4 molecule until
all Cl2 or CH4 molecules are wiped out.
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Termination!!!
Free radical mopped up.
If two fee radicals join together, they make astable molecule.
Cl + CH3 CH3Cl
CH3 + CH3 C2H6
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(n) Explain the limitations of radical substitution insynthesis, arising fro further substitution with
formation of a mixture of products.
In the reaction of methane and chlorine,
chloromethane is formed in the propogation
stage.
In the termination stage, chlorine, ethane and
chloromethane are produced.
Chloromethane made in the propagation stage,may react with further chlorine radicals until all
the hydrogen atoms have been replaced.
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(a) State that alkenes and cycloalkenes
are unsaturated hydrocarbons
Alkenes have the general formula CnH2n.
They are made up of hydrogen and carbon only,so are hydrocarbons.
Alkenes all have at least one C=C double
covalent bond, so are unsaturated because they
can make more bonds with extra atoms inaddition reactions.
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Examples of alkenes:
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(b) Describe the overlap of adjacent
p-orbitals to form a TT bond.
A TT bond is formed when two p orbitals
overlap.
Its dumb-bell shaped.
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A double bond is made up of a
sigma bond and a Pi bond
A sigma bond is formed when two s orbitals
overlap.
Two s orbitals overlap in a straight line this
gives the highest possible electron density
between the two nuclei.
Single covalent bond.
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explain the trigonal planar shape around
each carbon in the C=C of alkenes
Because theres two pairs of electrons in the bond, the
C=C double bond has a high electron density so they
are reactive. Highly reactive because the Pi bond sticks outabove the
rest of the molecule, so is likely to be attacked by
electrophiles.
Double bonds cant rotate because the p orbitals haveto overlap to form a Pi bond. The C=C double bond
and the atoms bonded to these carbons are planar and
rigid.
Restricted rotation causes cis-trans or E/Z isomerism.
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Pi and Sigma bond
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(g) Describe the addition
polymerisation of alkenes
Alkenes join up to form addition polymers.
Double bonds in alkenes open up andjointogetherto make polymers.
Individual, small alkenes are monomers.
This is addition polymerisation.
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(h) Deduce the repeat unit of an addition
polymer obtained from a given monomer.
To find the monomer, take the repeat unit and
add a double bond.
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(j) Outline the use of alkenes in the industrial
production of organic compounds
The manufacture ofmargarine by catalytic
hydrogenation
ofunsaturated vegetable oils
using hydrogen and a nickel catalyst.
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(j) Outline the use of alkenes in the industrial
production of organic compounds
Chloroethene produces poly(chloroethene)
Poly(chloroethene) is used to make water pipes +
insulation on electrical wires and as a buildingmaterial.
Tetraflouroethene produces poly(tetraflouroethene)
PTFE which is chemically inert and has non-stick
properties, so is used for coating on frying pans. H2C=CHCl and F2C=CF2
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(k) Outline the processing of waste
polymers
Not biodegradable
Waste plastics can be BURIED:
Landfill used when the plastic is: difficult to
separate from other waste, not in sufficient
quantities to make separation financially
worthwhile, too difficult to recycle.
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(k) Outline the processing of waste
polymers: RECYCLING
Plastics are made from non-renewable oil-
fractions, recycle them as much as possible.
Sort into different types: some plastics can be
melted orremoulded,
some plastics can be cracked into
monomers
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Waste plastics can be BURNED
If recycling not possible, you burn.
Waste plastics are burned and the heat can be
used to generate electricity.
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Outline the role of chemists in
minimising environmental damage by:
The process needs to be carefully controlled to
reduce toxic gases.
Polymers that contain chlorine produce HCl
when burned, which must be removed by
combustion of halogenated plastics.
Waste gases are passed through scrubberswhich neutralise gases like HCl by allowing
them to react with a base.
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The development of biodegradable
and compostable polymers
Biodegradable polymers naturally decompose
because organisms can digest them.
Biodegradable polymers can be made from
materials such as starch (from maize) and from
the hydrocarbon isoprene (2-methyl-1,3-
butadiene).
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Biodegradable polymers can be produced from
renewable raw materials or from oil fractions.
Using renewable raw material has advantages:
Raw materials are renewable.
When polymers biodegrade, CO2 is produced.
If your polymer is plant-based, the CO2
released as it decomposes is the same as CO2
absorbed by the plant when it grew. Plant-based polymers save energy.
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Addition of alkenes:
Describe addition reactions of alkenes by ethene andpropene with:
Hydrogen in the presence of a suitable catalyst, Nickel, toform alkanes.
Halogens to form dihalogenoalkanes, including the use ofbromine to detect the presence of a double C=C bond as atest for unsaturation.
Hydrogen halides to form halogenoalkanes.
Steam in the presence of an acid catalyst to form alcohols.
Define an electrophile as an electron-pair acceptor.
Describe how heterolytic fission leads to the mechanism of
electrophilic addition in alkenes.
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End of Module (1)