Basic Concepts and Hydrocarbons (Module 1)

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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Positional isomers:

Butan-1-olButan-2-ol


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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


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:


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:
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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.
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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)

Basic Concepts and Hydrocarbons (Module 1)

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