3.3.4 - alkenes

Question 1

what is the general formula of an alkene

Answer 1

CnH2n

Question 2

what elements do alkenes contain

Answer 2

only carbon and hydrogen atoms, making them hydrocarbons

Question 3

what bonds do alkenes contain

Answer 3

alkenes contain at least one C=C double covalent bond

Question 4

are alkenes saturated or unsaturtaed

Answer 4

alkenes are unsaturated

Question 5

why are alkenes unsaturated

Answer 5

molecules with C=C double bonds are unsaturated as they can make more bonds with extra atoms in addition reactions

Question 6

what feature does the C=C bond in an alkene have

Answer 6

as there are 2 pairs of electrons in the C=C double bond, it has a really high electron density

Question 7

are alkenes reactive or unreactive?

Answer 7

alkenes are pretty reactive

Question 8

are alkenes saturated with hydrogen?

Answer 8

alkenes aren’t saturated with hydrogen

Question 9

how can double covalent bonds be formed?

Answer 9

by sharing 4 electrons between a pair of atoms

Question 10

what shape do alkenes have?

Answer 10

alkenes have a flat planar shape

Question 11

what is the reasoning for the shape of alkenes?

Answer 11

as there can be no rotation around the double bond, unlike with single bonds

Question 12

why is there no rotation around the double bonds in alkenes?

Answer 12

as in addition to the single bond between the carbons, there is a p orbital (containing a single electron) on each carbon that overlap to form a pie orbital, preventing rotation of the bonds

Question 13

what is sometimes called, when the rotation of the double bonds in alkenes is prevented?

Answer 13

this is sometimes called restricted rotation

Question 14

why are alkenes reactive?

Answer 14

because of the double bonds which have a greater bond enthalpy, but they are an electron rich area of the molecule that can easily be attacked by positively charged electrophiles

Question 15

why are alkenes more reactive than alkanes

Answer 15

carbon-carbon double bond gives the molecule an electron rich area that attracts positively charged electrophiles

Question 16

what are electrophiles?

Answer 16

electron pair acceptors

Question 17

electrophilic addition reaction:
step one

Answer 17

positively charged electrophile is attracted towards double bond in the alkene

Question 18

electrophilic addition reaction:
step two

Answer 18

electrophile accepts a pair of electrons from the double bond, forming a positive ion (carbocation)

Question 19

electrophilic addition reaction:
step three

Answer 19

carbocation reacts with a negative ion to form a bond

Question 20

what two types of products can from after an electrophilic addition reaction

Answer 20

major or minor products

Question 21

what are major products

Answer 21

product that forms most of the time

Question 22

what are minor products?

Answer 22

products that form less often

Question 23

secondary carbocations

Answer 23

secondary carbocation is more stable as it has 2 alkyl groups, this carbocation will form most of the time

Question 24

primary carbocations

Answer 24

primary carbocation is less stable as it has only 1 alkyl group, this carbocation forms less often

Question 25

how do you test for unsaturation

Answer 25

shake an alkene with orange bromine water

Question 26

what happens when you shake an alkene with orange bromine water

Answer 26

solution quickly turns from orange to colourless

Question 27

what is the reasoning for the result in the test for unsaturation?

Answer 27

bromine is added across the double bond to form a dibromoalkane, happens by electrophilic addition

Question 28

for electrophilic addition with hydrogen halides, what are the charges for the hydrogen halide

Answer 28

the hydrogen has a slightly positive charge, the halide has a slightly negative charge

Question 29

what are polymers

Answer 29

long chain molecules formed when lots of small molecules (monomers) join together

Question 30

what 2 types of polymers are there

Answer 30

polymers can be natural or synthetic

Question 31

give an example of a natural polymer

Answer 31

DNA

Question 32

give an example of a synthetic polymer

Answer 32

polythene

Question 33

what sort of things can polymers be used for?

Answer 33

plastic bags, non stick pans, car tyres, fabric

Question 35

describe addition polymerisation

Answer 35

double bonds in alkenes can open up and join together to make long chains called polymers (addition polymerisation)

Question 36

what is an addition polymer

Answer 36

addition polymer is a polymer made from monomers, containing C=C double bonds

Question 37

what happens when alkenes polymerise

Answer 37

when alkenes polymerise, the double bond is broken and links the alkenes to form a backbone of carbons all joined by single bonds

Question 38

how to name polymers

Answer 38

find the name of the monomer and add poly in front

Question 39

what happens to alkene monomers once they form polymers

Answer 39

once they form polymers they become saturated (only single bonds in carbon chain)

Question 40

reactivity of addition polymers

Answer 40

addition polymers are very unreactive - polyalkenes are chemically inert

Question 41

what is the reason for the reactivity of addition polymers

Answer 41

main carbon chain of polyalkenes is usually non polar

Question 42

what bonds do monomers within a polymer have

Answer 42

monomers within a polymer chain have strong covalent bonds

Question 43

intermolecular forces between polymer chains

Answer 43

intermolecular forces between polymer chains are much weaker which affects the properties of the polymer

Question 44

intermolecular forces of longer chains

Answer 44

longer chains with fewer branches have stronger intermolecular forces

Question 45

how does the intermolecular forces in longer chains affect the polymer material's properties

Answer 45

makes these polymer materials stronger and more rigid

Question 46

polyethene

Answer 46

polymer with few/no branches e.g. polyethene can pack closely together so polymer chains are attracted to each other by van der Waals forces, making a strong/rigid material

Question 47

what is polychloroethene also known as

Answer 47

polyvinyl chloride or PVC

Question 48

what is PVC made from?

Answer 48

PVC is an addition polymer formed from chloroethene monomers

Question 49

covalent bonds on polychloroethene

Answer 49

covalent bonds between the chlorine and carbon atoms are polar, chlorine is more electronegative

Question 50

polychloroethene: permanent dipole-dipole forces

Answer 50

slightly negative charges on the chlorine atoms and slightly positive charges on carbon atoms means there are permanent dipole-dipole forces between the polymer chains

Question 51

properties of PVC

Answer 51

PVC is a hard but brittle material

Question 52

what is PVC used to make

Answer 52

it is used to make drain pipes and window frames

Question 53

why add plasticisers to polymers?

Answer 53

adding plasticisers to polymers modifies their properties

Question 54

how does adding a plasticiser to a polymer affect its properties

Answer 54

adding a plasticiser to a polymer makes it more bendy

Question 55

how do plasticisers alter properties of polymers

Answer 55

plasticiser molecules get between the polymer chains and push them apart, reduces strength of intermolecular forces between the chains so the chains can slide around more, becoming more flexible

Question 56

give an example of a polymer containing plasticisers?

Answer 56

plasticised PVC

Question 57

how is plasticised PVC different to normal PVC

Answer 57

plasticised PVC is much more flexible than rigid PVC

Question 58

what is plasticised PVC used for

Answer 58

to make electrical cable insulation, flooring tiles and clothing