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1、KS4 Chemistry,Alkanes and Alkenes,Alkanes and Alkenes,Alkenes,Alkanes,Summary activities,Combustion of alkanes,Contents,Cracking and polymerization,Because the main use of hydrocarbons is as a fuel there is no point in going to the effort to separate them into individual hydrocarbons. It is, however
2、, possible to obtain pure hydrocarbons by very careful distillation. This section is about pure hydrocarbons.,Pure hydrocarbons,Carbon is an unusual atom in that it is able to form very strong covalent bonds with other carbon atoms. When we then include its ability to also bond with other elements w
3、e 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 chemistry.,Organic chemistry: carbon,The simplest hydrocarbons form a series of compounds k
4、nown as alkanes. These all consist of carbon and hydrogen only and every carbon has four single covalent bonds.,hydrogen,carbon,Alkanes,The 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).
5、 E.g. 5 carbons = pentane C5H12 6 carbons = hexane C6H14 7 carbons = heptane C7H16 8 carbons = octane C8H18 9 carbons = nonane C9H20 10 carbons = decane C10H22,pentane,Names of alkanes,Alkanes all have very similar structures. They all consist of carbon and hydrogen only and every carbon has four si
6、ngle covalent bonds. They have a CH3 at each end of the molecule. What differs is the number of CH2 groups between the two ends. It is possible to build up a series by simply adding an extra CH2 group This leads to a general formula of CnH2n+2,Structure of alkanes,What will be the formula for alkane
7、s containing the following numbers of carbons?,C12H26,C16H34,C31H64,C19H40,Whats the formula?,Although 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 org
8、anic 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, CH4,Formulae and models of alkanes,Formulae and models of alkanes,Formulae and models of alkanes,and so on,Notice the carbon chain is not
9、really straight,Formulae and models of alkanes,Alkanes of the same formula can have different arrangements of atoms. Such different arrangements are known as isomers. Two isomers of C4H10 are shown:,Isomers of butane,Isomerism,Alkanes contain atoms held together by single covalent bonds. In the disp
10、layed formula we show these bonds as a single line. Each line is really a pair of shared electrons.,Bonding in alkanes: methane,Ethane is the simplest alkane containing a C-C single covalent bond.,Bonding in alkanes: ethane,Complete the diagram below including its electrons.,Bonding in alkanes: prop
11、ane,Alkanes and Alkenes,Alkenes,Alkanes,Summary activities,Combustion of alkanes,Cracking and polymerization,Contents,Alkanes are not especially reactive but they do undergo one very important reaction: combustion. With an adequate supply of air they react to form carbon dioxide and water.,Combustio
12、n of alkanes,In the absence of an adequate supply of air, alkanes may react to form carbon monoxide and water. Carbon monoxide is highly poisonous and this is one reason why gas boilers must be serviced regularly.,A carbon monoxide detector,Incomplete combustion of alkanes,Complete the equations bel
13、ow assuming an adequate supply of oxygen for complete combustion. (These are quite tricky!),2C2H6 + 7O2 C3H8 + 5O2 2C4H10 + 13O2 ,2C2H6 + 7O2 4CO2 + 6H2O,C3H8 + 5O2 3CO2 + 4H2O,2C4H10 + 13O2 8CO2 + 10H2O,Complete the equation,Alkanes and Alkenes,Alkenes,Alkanes,Summary activities,Combustion of alkan
14、es,Cracking and polymerization,Contents,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 called double bonds. Alkenes contain carbon atoms join
15、ed by double covalent bonds.,Alkenes,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 the general formula CnH2n.,Structure of alkenes,What will be the formula for alkenes containing the following numbers of ca
16、rbons?,C11H22,C13H26,C32H64,C21H42,Whats the formula?,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.,The formula of ethene,In
17、all alkenes there are two carbon atoms that are joined by two pairs of electrons. This is the double bond.,The electron structure of ethene,Displayed formula for propene and butene are shown. Draw a displayed structure for pentene.,The structure of pentene,Saturated means “full up”.,They have a doub
18、le bond that could instead become two single bonds. This means that other atoms can be added. It is not “full up”.,Alkenes are unsaturated.,Every carbon atom has already used all four of its bonds to join to four other atoms. No other atoms can be added.,Alkanes are saturated.,Saturated or unsaturat
19、ed?,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 added to an alkene the red colour of the bromine disappears.,Testi
20、ng for alkanes and alkenes,Copy the table and complete the empty boxes.,alkene,C5H10,alkane,C8H18,alkene,C4H8,alkane,C10H22,Identify the compound,Alkanes and Alkenes,Cracking and polymerization,Summary activities,Contents,Crude oil contains many large molecules. If these are to be used as fuels or f
21、eedstock for the chemical industry then they have to be broken down (or cracked) into smaller molecules.,Big molecules,Small molecules,Medium molecules,Source of alkenes,Large hydrocarbons are broken down into smaller molecules using heat and a catalyst. This process is known as catalytic cracking.
22、The small molecules produced are then separated by distillation.,Big molecules,Smaller molecules,Molecules break up,Catalytic cracking,In the catalytic cracker long chain molecules are split apart or cracked. An example of such a reaction is:,C8H18 C6H14 + C2H4,+,ethene,octane,hexane,Used to make pl
23、astics,Used as a fuel,Catalytic cracking,Draw out displayed formulae of a pair of products formed by cracking decane.,Cracking decane,Making polymers,How do monomers become polymers?,One important reaction of alkenes involves the joining together of alkene molecules.,And lots more.,This is called ad
24、dition polymerization and is written as:,1 2 3 4 5 thousands,Production of polythene,Ethene is only one alkene. Other unsaturated molecules such as propene, vinyl chloride and styrene can also be polymerized to produce a range of plastics. E.g. propene:,Polypropene,Tetrafluoroethene is another alken
25、e that is made into an important plastic used to coat non-stick pans: polytetrafluoroethene or PTFE.,PTFE,Fill in the products that will be obtained from vinyl chloride,Polymerization of vinyl chloride,Poly(e)thene Shopping bags Bottles Buckets Washing-up bowls,Polypropene Milk crates Rope Carpet fi
26、bres,Polystyrene Packing Insulation Ball pens,Uses of plastics,Alkanes and Alkenes,Alkenes,Alkanes,Summary activities,Combustion of alkanes,Cracking and polymerization,Contents,Glossary,alkanes A family of saturated hydrocarbons used as fuels. alkenes A family of unsaturated hydrocarbons containing
27、one double bond, and which are used to make plastics. cracking The process in which large molecules are broken down into smaller molecules by heating. monomer A molecule that joins with others to form a polymer. polymerization The process in which small molecules are joined together to form a much larger molecule. saturated A molecule in which all the bonds are single. unsaturated A molecule in which at least one bond is double or triple.,Anagrams,Multiple-choice quiz,