alkanes

what are alkanes

saturated hydrocarbons among the least reactive organic compounds

what does it mean if a hydrocarbon is saturated

the only contain carbon-carbon and carbon-hydrogen single bonds

what are alkanes used as

fuels and lubricants we starting materials for a range of other compounds

why are alkanes very important to industry

they are used as fuels and lubricants and as starting materials for a range of other compounds

main source of alkanes

crude oil

general formula of alkanes

CnH2n+2

hydrocarbons may be…

• unbranched

• branched

• rings

unbranched chains are often called….

straight chains even though the C-C-C angle is 109.5

why are straight chain molecules not actually straight

the C-C-C angle is 109.5

in an unbranched alkane, each carbon atom has….

two hydrogen atoms except the end carbons which have one extra

branched chains aren’t ____ chains

straight

ring alkanes general formula

CnH2n

why do ring alkanes have the general formula CnH2n

because the end hydrogens are not required

how to name straight chain alkanes

• named from the root

• suffix -ane

how to name a branched alkane

• longest unbranched chain gives root

• use prefix for branches/side chains

• add numbers to say which carbon atoms the side chains are attached to

branches/side chain prefixes

• methyl

• ethyl

• propyl

• butyl

methane, ethane and propane all have no….

isomers

after propane, the possible number of isomers…

increases with the number of carbons in the alkane

how many isomers does butane have

two isomers

how many isomers does pentane have

3

polarity of alkanes

• almost non polar

• this is because the electronegativities of carbon and hydrogen are so similar

• as a result the only intermolecular forces between their molecules are weak van der waals

boiling point of alkanes

• increasing intermolecular forces is why boiling point of alkanes increases with chain length

• alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms

• this is because they cannot pack together as closely as unbranched chains so the van der waals forces are not so effective

shorter chains are______ at room temperature

gases

what state is pentane at room temperature

liquid

at what chain length do alkanes become solid at room temperature

about 18 carbons

solid alkanes have a ____ feel

waxy

solubility of alkanes

• insoluble in water

• this is due to water molecules being held together by hydrogen bonds which are much stronger than the van der waals forces that act between alkane molecules

• alkanes do mix with other relatively non-polar liquids

how do alkanes react

• relatively unreactive

• strong carbon-carbon and carbon-hydrogen bonds

• do not react with acids, bases. oxidising agents, reducing agents

• they do burn and react with halogens under suitable conditions

• burn in a plentiful supply of oxygen to form co2 and water

• burn in restricted oxygen to form carbon monoxide or carbon

crude oil is at present the worlds main source of….

organic chemicals

why is crude oil called a fossil fuel

because it was formed millions of years ago by the breakdown of plant and animal remains at the high pressures and temperatures deep below the earths surface

why is crude oil effectively non-renewable

as it forms very slowly

what is crude oil

a mixture mostly of alkanes both unbranched and branched

crude oils from different sources have different….

compositions

crude oil contains small amounts of….

other compounds dissolved in it which come from other elements in the original plants and animals the oil was formed from

why is sulfur one of the causes of acid rain

when burnt, produces sulfur dioxide which reacts with oxygen high in the atmosphere to form sulfur trioxide which reacts with water in the atmosphere to form sulfuric acid

how do you convert crude oil into useful products

you have to separate the mixture by heating it and collecting the fractions that boil over different ranges of temperatures

what is each fraction

a mixture of hydrocarbons of similar chain length and therefore similar properties

what is the process of splitting crude oil into useful products called

fractional distillation

where does fractional distillation occur

a fractionating tower

process of fractional distillation of crude oil

• crude oil is heated in furnace

• mixture of liquid and vapour passes into a tower that is cooler at the top than the bottom

• the vapours pass up the tower via a series of trays containing bubbles caps until they arrive at a tray that is sufficiently cool (lower temperature than their boiling point) where they condense to liquid

• the mixture of liquids that condenses on each tray is piped off

• shorter chain hydrocarbons condense in the trays nearer to the top of the tower where it is cooler as they have lower boiling points

• 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 supply often exceeds demand, this fraction is often further processed to give more valuable products

what is gasoline/petrol used for

cars

what is kerosene/paraffin used for

jet fuel, lighting

what is diesel oil used for

lorrie’s and taxis

what is lubricating oil and waxes used for

candles and engine oil

what is fuel oil used for

ships, power stations

what is tar/bitumen used for

roads and roofing

what is broken during fractional distillation

the van der waals forces between the molecules during vaporisation then reform on condensing

what does it mean as fractional distillation is a physical technique

no covalent bonds within molecules are broken, it is the van der waals that are

what is almost half the uks electricity generated from

natural gas (largely methane)

where does around half of natural gas come from in uk

the north sea, but this percentage is decreasing as these wells become depleted and more and more gas is being imported via pipeline from europe

many areas of the uk have resources of natural gas which aren’t…

caught under impervious rock layers as the north sea but trapped within shale rock rather like water in a spogg ng e

how to extract natural gas not in north sea

• gas can be extracted by drilling into the shale and forcing pressurised water mixed with sand into the shale

• this causes the rather soft shale rock to break up or fracture (giving the term fracking) releasing the trapped gas which flows to the surface

• number of chemicals added to the water such as HCl to help break up the shale and methanol to prevent corrosion in the system

why are people opposed to fracking

• they do not like the infrastructures of wells and the associated traffic near

• concern about amount of water used

• they worry the chemical additives polluting water supplies

• occasionally fracking appears to have caused small earthquakes

• burning natural gas produced co2 which is a cause of global warming

the ____ fraction from the fractional distillation of crude oil is in huge demand

naphtha

what is the naphtha fraction in huge demand for

petrol and by the chemical industry

longer chain fractions are not as useful and therefore of lower value….

economically

most crude oil has more of the longer chain fractions than is wanted and not enough of…

the naphtha fraction

shorter chains products are….

economically more valuable than the longer chain material

what happens to longer chain fractions to meet demand for the shorter chain hydrocarbons

many of the longer chain fractions are broken into shorter lengths by cracking

two results of cracking

• shorter more useful chains are produced especially petrol

• some of the products are alkenes which are more reactive than alkanes

what is petrol a mixture of

mainly alkanes containing between 4 and 12 carbon atoms

what does it mean if alkenes are used as chemical feedstock

they supply industries with the starting materials to make different products and are converted into a huge range of other compounds including polymers

examples of polymers alkenes get turned into

paints

drugs

why is ethene the most important alkene

it’s the starting materials for polyethene/polythene and a wide range of other everyday materials

why are harsh materials required to break alkanes

they are very unreactive

types of cracking for alkanes

• thermal

• catalytic

thermal cracking conditions

• high temperature 700-1200K

• high pressure up to 7000kPa

what does thermal cracking involve

heating alkanes to a high temperature to break bonds

how do bonds break in thermal cracking

• carbon-carbon bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom

• so initially two shorter chains are produced, each ending in a carbon atom with an unpaired electron

• these fragments are called free radicals

what are free radicals

highly reactive intermediates and react in a number of ways to form a variety of shorter chain molecules

why do alkenes form in thermal cracking

• there are not enough hydrogen atoms to produce two alkanes

• one of the new chains must have a C=C

in thermal cracking, any number of carbon-carbon bonds may break and the chain does not necessarily break…

in the middle

what may also be produced in thermal cracking

hydrogen

thermal cracking tends to produce a high proportion of…

alkenes

what happens to avoid decomposition in thermal cracking

the alkanes are kept in these conditions for a very short time, typically one second

catalytic cracking conditions

• takes place at a lower temperature 720K

• lower pressure but more than atmospheric

• uses zeolite catalyst

what does a zeolite catalyst consist of

silicon dioxide and aluminium oxide

structure of zeolite catalyst

honeycomb structure with an enormous surface area

are zeolite catalysts alkaline or acidic

acidic

catalytic cracking is used to produce….

motor fuels

products of catalytic cracking

• mostly branches alkanes

• cycloalkanes

• aromatic compounds

how are the products obtained from cracking separated

fractional distillation

test for alkenes

bromine water turns colourless

shorter chain alkanes burn completely in a plentiful supply of oxygen to give….

carbon dioxide and water

equation for complete combustion of methane

CH4 + 2O2 → Co2 + 2H2O

what do combustion reactions give out

heat and have large negative enthalpies of combustion, the more carbons presents, the greater the heat output

what are fuels

substances that release heat energy when they undergo combustion

alkanes store a large amount of energy…

for a small amount of weight

why are alkanes important as fuels

they have large negative enthalpies of combustion and the more carbons present the greater the heat output

examples of alkane fuels

• methane

• propane

• butane

• petrol

• paraffin

propane is considered ____ gas

camping

butane is considered a _____ gas

calor

what is petrol

a mixture of hydrocarbons of approximate chains length C8

what is paraffin

a mixture of hydrocarbons of chain lengths C10 to C18

what is produced in combustion with a limited supply of oxygen

carbon monoxide which is a poisonous gas

what is incomplete combustion

the burning of alkanes in limited supply of oxygen producing the poisonous gas carbon monoxide

even with less oxygen, in combustion what is produced

carbon (soot)

example of when soot is produced in incomplete combustion

• when a bunsen burner is used with a closed air hole the flame is yellow

• black sooty deposit appears on the apparatus

why does incomplete combustion happen with longer chain hydrocarbons

they need more oxygen to burn compared with shorter chains

all hydrocarbon based fuels derived from crude oil may produce pollution products when they…,

burn