1、 Boardworks Ltd 20051 of 45KS4 Chemistry Alkanes and Alkenes Boardworks Ltd 20052 of 45Alkanes and AlkenesAlkenesAlkanesSummary activitiesCombustion of alkanes ContentsCracking and polymerization Boardworks Ltd 20053 of 45Because the main use of hydrocarbons is as a fuel there is no point in going t
2、o the effort to separate them into individual hydrocarbons.It is,however,possible to obtain pure hydrocarbons by very careful distillation.This section is about pure hydrocarbons.Pure hydrocarbons Boardworks Ltd 20054 of 45Carbon is an unusual atom in that it is able to form very strong covalent bon
3、ds with other carbon atoms.When we then include its ability to also bond with other elements we open up the possibility of the highly diverse and complex molecules(like DNA)that have led to the possibility of life.Because of this,the chemistry of carbon containing compounds is often called Organic c
4、hemistry:carbon Boardworks Ltd 20055 of 45The simplest hydrocarbons form a series of compounds known as alkanes.These all consist of carbon and hydrogen only and every carbon has four single covalent bonds.HydrocarbonFormulaStructureMethaneCH4Ethane C2H6PropaneC3H8ButaneC4H10hydrogencarbon Alkanes B
5、oardworks Ltd 20056 of 45The names of the 4 simplest alkanes are methane,ethane,propane and butane.After that the names are systematic(like the words used to describe geometric shapes).E.g.5 carbons=pentane C5H126 carbons=hexaneC6H147 carbons=heptaneC7H168 carbons=octaneC8H189 carbons=nonaneC9H2010
6、carbons=decaneC10H22carbonhydrogenpentane Names of alkanes Boardworks Ltd 20057 of 45Alkanes all have very similar structures.They all consist of carbon and hydrogen only and every carbon has four single covalent bonds.They have a CH3 at each end of the molecule.What differs is the number of CH2 gro
7、ups between the two ends.It is possible to build up a series by simply adding an extra CH2 groupThis leads to a general formula of CnH2n+2hydrogencarbonn=1 n=2 n=3 n=4 Structure of alkanes Boardworks Ltd 20058 of 45What will be the formula for alkanes containing the following numbers of carbons?C12H
8、26C16H34C31H64C19H40Number of carbonsFormula1216 3119 Whats the formula?Boardworks Ltd 20059 of 45Although normal chemical formula-like C5H12-are used to describe alkanes,they do not convey which atom is joined to which other atom.To get around this we often used displayed formulae to describe organ
9、ic molecules.Displayed formulae show which 4 atoms each carbon is bonded to,but even these do not show the actual 3D shapes.For that we use models.methane,CH4CHHHH Formulae and models of alkanes Boardworks Ltd 200510 of 45CHHHHmethane,CH4CHHHCHHHethane,C2H6CHHHCHHCHHHpropane,C3H8 Formulae and models
10、 of alkanes Boardworks Ltd 200511 of 45butane,C4H10CHHHCHHCHHCHHHpentane,C5H12CHHHCHHCHHCHHCHHH Formulae and models of alkanes Boardworks Ltd 200512 of 45CHHHCHHCHHCHHCHHCHHHhexane,C6H14and so onNotice the carbon chainis not really straight Formulae and models of alkanes Boardworks Ltd 200513 of 45A
11、lkanes of the same formula can have different arrangements of atoms.Such different arrangements are known as Two isomers of C4H10 are shown:CCCHHHHHHHCHHHIsomers of butane Isomerism Boardworks Ltd 200514 of 45CHHHHCHHHHAlkanes contain atoms held together by single covalent bonds.In the displayed for
12、mula we show these bonds as a single line.Each line is really a pair of shared electrons.Bonding in alkanes:methane Boardworks Ltd 200515 of 45 Ethane is the simplest alkane containing a C-C single covalent bond.CHHHCHHHCHHCHHHH Bonding in alkanes:ethane Boardworks Ltd 200516 of 45 Complete the diag
13、ram below including its electrons.CHHCHHHHCHHCarbon electronHydrogen electron Bonding in alkanes:propane Boardworks Ltd 200517 of 45Alkanes and AlkenesAlkenesAlkanesSummary activitiesCombustion of alkanesCracking and polymerization Contents Boardworks Ltd 200518 of 45 Alkanes are not especially reac
14、tive but they do undergo one very important reaction:combustion.With an adequate supply of air they react to form carbon dioxide and water.methane+oxygen water +carbon dioxideCH4 +2O2 2H2O +CO2 Combustion of alkanes Boardworks Ltd 200519 of 45 In the absence of an adequate supply of air,alkanes may
15、react to form carbon monoxide and water.Carbon monoxide is highly poisonous and this is one reason why gas boilers must be serviced regularly.methane+oxygen water +carbon monoxide2CH4 +3O2 4H2O +2COA carbon monoxide detector Incomplete combustion of alkanes Boardworks Ltd 200520 of 45 Complete the e
16、quations below assuming an adequate supply of oxygen for complete combustion.(These are quite tricky!)1.2C2H6 +7O2 2.C3H8 +5O2 3.2C4H10 +13O2 1.2C2H6 +7O2 4CO2 +6H2O2.C3H8 +5O2 3CO2 +4H2O3.2C4H10 +13O2 8CO2 +10H2O Complete the equation Boardworks Ltd 200521 of 45Alkanes and AlkenesAlkenesAlkanesSumm
17、ary activitiesCombustion of alkanesCracking and polymerization Contents Boardworks Ltd 200522 of 45 When carbon forms compounds each carbon atom always forms four bonds.This does not,however,mean that each carbon is joined to four other atoms.It is possible to have bonds grouped into pairs.These are
18、 called double bonds.Alkenes contain carbon atoms joined by double covalent bonds.Single covalent bondCCDouble covalent bondCC Alkenes Boardworks Ltd 200523 of 45 A series of alkanes exist,differing only in the number of CH2 groups.The same is true for alkenes.This leads to a homologous series with
19、the general formula CnH2n.hydrogencarbonn=1 n=2 n=3 n=4Alkenesnone=Structure of alkenes Boardworks Ltd 200524 of 45What will be the formula for alkenes containing the following numbers of carbons?Number of carbonsFormula1113 3221C11H22C13H26C32H64C21H42 Whats the formula?Boardworks Ltd 200525 of 45
20、The simplest alkene is ethene.It has the formula C2H4 The carbon atoms are joined together by a double bond.Its displayed formula may be drawn in slightly different forms but should always clearly show the double bond.CHHCHHor The formula of ethene Boardworks Ltd 200526 of 45 In all alkenes there ar
21、e two carbon atoms that are joined by pairs of electrons.This is the double bond.CHHCHH The electron structure of ethene Boardworks Ltd 200527 of 45 Displayed formula for propene and butene are shown.Draw a displayed structure for pentene.CCCHHHHHHCCHHHCCHHHHHCCHHHCCHHHHHHHC The structure of pentene
22、 Boardworks Ltd 200528 of 45 Saturated means“full up”.They have a double bond that could instead become two single bonds.This means that other atoms can be added.It is not“full up”.Alkenes are.Every carbon atom has already used all four of its bonds to join to four other atoms.No other atoms can be
23、added.Alkanes are.Saturated or unsaturated?Boardworks Ltd 200529 of 45 Alkenes are unsaturated and so extra atoms can be added to alkene molecules.This forms the basis of a test to distinguish between alkanes and alkenes.When bromine water is added to an alkane nothing happens,but when bromine is ad
24、ded to an alkene the red colour of the bromine disappears.+Br2 gas red colourlessBromine loses this red colour Testing for alkanes and alkenes Boardworks Ltd 200530 of 45Copy the table and complete the empty boxes.Number of carbonsNameAlkane or alkeneFormula5pentene8octane4butene10decanealkeneC5H10a
25、lkaneC8H18alkeneC4H8alkaneC10H22 Identify the compound Boardworks Ltd 200531 of 45Alkanes and AlkenesAlkenesCracking and polymerization AlkanesSummary activitiesCombustion of alkanes Contents Boardworks Ltd 200532 of 45 Crude oil contains many large molecules.If these are to be used as fuels or feed
26、stock for the chemical industry then they have to be broken down(or cracked)into smaller molecules.Big moleculesSmall moleculesMedium molecules Source of alkenes Boardworks Ltd 200533 of 45 Large hydrocarbons are broken down into smaller molecules using heat and a catalyst.This process is known as c
27、atalytic cracking.The small molecules produced are then separated by distillation.CatalyticcrackerHeat tovaporizeDistillationtowerpressureBig moleculesSmaller moleculesMolecules break up Catalytic cracking Boardworks Ltd 200534 of 45 In the catalytic cracker long chain molecules are split apart or c
28、racked.An example of such a reaction is:C8H18 C6H14+C2H4+etheneoctanehexaneUsedto makeplasticsheat&pressurecatalystUsed as a fuel Catalytic cracking Boardworks Ltd 200535 of 45 Draw out displayed formulae of a pair of products formed by cracking decane.Heat&pressurecatalystHHC CHH HH HC CH HHHC CH H
29、HHC CH HHHC CH HHdecane Cracking decane+etheneHHC CHH HH HC CH HHHC CH HHCHHCHHoctane Boardworks Ltd 200536 of 45 Making polymers How do monomers become polymers?Boardworks Ltd 200537 of 45 One important reaction of alkenes involves the joining together of alkene molecules.HHC CHH HH HC CH HHHC CH H
30、HHC CH HHHC CH HAndlotsmore.This is called and is written as:pressurehigh temperaturecatalystnnethenepoly(e)thene1 2 3 4 5 thousands Production of polythene Boardworks Ltd 200538 of 45 Ethene is only one alkene.Other unsaturated molecules such as propene,vinyl chloride and styrene can also be to pro
31、duce a range of plastics.E.g.propene:poly(propene)nnpropene Polypropene Boardworks Ltd 200539 of 45 Tetrafluoroethene is another alkene that is made into an important plastic used to coat non-stick pans:polytetrafluoroethene or PTFE.ntetrafluoroethenepoly(tetrafluoroethene)or PTFEn PTFE Boardworks L
32、td 200540 of 45pvcCCHHHClnnVinyl chlorideC CClHHH Fill in the products that will be obtained from vinyl chloride Polymerization of vinyl chloride Boardworks Ltd 200541 of 45Poly(e)theneShopping bagsBottlesBucketsWashing-up bowlsPolypropeneMilk cratesRopeCarpet fibresPolystyrenePackingInsulationBall
33、pens Uses of plastics Boardworks Ltd 200542 of 45Alkanes and AlkenesAlkenesAlkanesSummary activitiesCombustion of alkanesCracking and polymerization Contents Boardworks Ltd 200543 of 45 Glossaryl alkanes A family of saturated hydrocarbons used as fuels.l alkenes A family of unsaturated hydrocarbons
34、containing one double bond,and which are used to make plastics.l cracking The process in which large molecules are broken down into smaller molecules by heating.l monomer A molecule that joins with others to form a polymer.l polymerization The process in which small molecules are joined together to form a much larger molecule.l saturated A molecule in which all the bonds are single.l unsaturated A molecule in which at least one bond is double or triple.Boardworks Ltd 200544 of 45 Anagrams Boardworks Ltd 200545 of 45 Multiple-choice quiz
