DF 9 - what do the molecules look like

Question 1

what do chemists use to explain how molecules behave

Answer 1

models of molecular structures.

Question 2

why is representing structures in a two dimensional way on paper a bad thing

Answer 2

it can give a misleading picture of what the molecules look like.

Question 3

what do pairs of electrons in covalent bond do

Answer 3

they repel one another so they arrange themselves around the central atom as far as possible to minimise repulsion

Question 4

how are the C-H bonds arranged in methane

Answer 4

they are directed so they point towards the corners of a regular tetrahedron. the carbon atom is at the centre of the tetrahedron

Question 5

what is the bond angle of H-C-H bonds in methane

Answer 5

109.5°

Question 6

what does a normal line show

Answer 6

the bond is in the plane of the paper

Question 7

what does a dotted line show

Answer 7

the bond is in a direction behind the plane of the paper

Question 8

what does a wedged line show

Answer 8

the bond is in a direction in front of the plane of the paper

Question 9

what is the electron pair repulsion theory

Answer 9

the shape of a molecule or ion is determined by the number of electron pairs in the outer shell surrounding the central atom. as electrons all have a negative charge, each electron pair repels other electron pairs. the electron pairs push one another as far apart as possible

Question 10

which is more electron dense - a lone pair or bonded pair

Answer 10

a lone pair. this means it repels more than a bonded pair

Question 11

what is the shape and bond angle of a molecule with 2 areas of electron density

Answer 11

linear - 180°

Question 12

what is the shape and bond angle of a molecule with 3 areas of electron density

Answer 12

trigonal planar - 120°

Question 13

what is the shape and bond angle of a molecule with 4 areas of electron density

Answer 13

tetrahedral - 109.5°

Question 14

what is the shape and bond angle of a molecule with 5 areas of electron density

Answer 14

trigonal bipyramid - 120° and 90°

Question 15

what is the shape and bond angle of a molecule with 6 areas of electron density

Answer 15

octahedral - 90°

Question 16

what are the bond angles of hydrocarbons

Answer 16

109.5°

Question 17

are hydrocarbon chains straight

Answer 17

no they are a zigzag of carbon atoms

Question 18

what is the shape of a hydrocarbon chain often represented by

Answer 18

a skeletal formula

Question 19

what does skeletal formula show

Answer 19

they only show the carbon skeleton and associating functional groups of the molecule. they do not assist in visualising shapes of molecules

Question 20

what is an isomer

Answer 20

molecules which have the smae molecular formula but differ in the way their atoms are arranged. they have different physical and chemical properties

Question 21

why is isomerism common in carbon compounds

Answer 21

because of the variety of ways in which carbon atoms can form chains and rings

Question 22

what are the two ways in which atoms can be arranged differently in isomers

Answer 22

structural isomers and stereoisomers

Question 23

what is a structural isomer

Answer 23

the atoms are bonded together in a different order in each isomer

Question 24

what is a stereoisomer

Answer 24

the order of the bonding in the isomers is the same but the arrangement of the atoms in space is different in each isomer

Question 25

which isomers have the same molecular formula but different structural arrangement of atoms

Answer 25

structural isomers

Question 26

what are the three types of structural isomerism

Answer 26

chain, position and functional group isomerism

Question 27

what is a chain isomer

Answer 27

two or more compounds with similar chemical formulas but distinct carbon atom arrangements in straight or branched chains

Question 28

when does chain isomerism occur

Answer 28

in methane, ethane, and propane only one alkane corresponds to the molecular formula
with four or more carbon atoms in a chain different arrangements are possible.

Question 29

give two structural chain isomers

Answer 29

butane and 2-methylpropane have the molecular formula C4H10

Question 30

what does the different structure in the chain isomers lead to

Answer 30

different properties

Question 31

as the number of carbon atoms increases..

Answer 31

.. the number of possible isomers increases

Question 32

what is positonal isomerism

Answer 32

occurs when a functional group can occupy different positions on the same carbon chain

Question 33

when does positional isomerism occur

Answer 33

when there is an atom, or groups of atoms substituted in a carbon chain or ring, these are functional groups.

Question 34

give an example of positional isomers

Answer 34

propan-1-ol and propan-2-ol
the functional group is OH and is situated at different places on the hydrocarbon chains

Question 35

what is functional group isomerism

Answer 35

when substances have the same molecular formula but different functional groups.

Question 36

what does it mean if compounds have different functional groups

Answer 36

they belong to a different homologous series

Question 37

give an example of functional group isomers

Answer 37

propan-1-ol and methoxyethane have the same molecular formula (C3H8O) but different functional groups
propan-1-ol has the OH functional group (alchohol)
methoxyethane has the -O functional group (ether)
they have different physical and chemical properties due to the different functional groups

Question 38

which isomers have the same structural formula but differ in the way their atoms are arranged in space

Answer 38

stereoisomers

Question 39

what are the two types of stereoisomerism

Answer 39

E/Z or cis/trans isomerism
optical isomerism

Question 40

name a compound with two E/Z isomers

Answer 40

but-2-ene

Question 41

show the isomers of but-2-ene

Answer 41

H CH3 H H
\ / \ /
C=C C=C
/ \ / \
CH3 H CH3 CH3

Question 42

how would we turn make both methyl side chains situate on the same side of the C=C bond in but-2-ene

Answer 42

you would have to spin one end of the molecule round in relation to the other end. this can only be done by breaking the 𝛑 bond in the double bond

Question 43

what is the average bond enthalpy to break the 𝛑 bond in but-2-ene

Answer 43

+270kJmol-1
—> not available at room temperature

Question 44

what bond has to be broken to interconvert the forms of but-2-ene

Answer 44

covalent has to be broken and then another reformed

Question 45

what is the form of but-2-ene with the same groups on the same side of the double bond

Answer 45

cis-but-2-ene or (Z)-but-2-ene

Question 46

what is the form of but-2-ene with the same groups on opposite sides of the double bond

Answer 46

trans-but-2-ene or (E)-but-2-ene

Question 47

when does the cis/trans rule work

Answer 47

when either of the two groups of either side of the double bond are the same or if there is a hydrogen on both carbon atoms

Question 48

give an example of where the cis/trans naming system cannot be used

Answer 48

naming haloalkanes
cis (Z isomer)
trans (E isomer)

Question 49

why do E and Z isomers have different properties

Answer 49

they are different compounds

Question 50

we use the cis/trans naming system when…

Answer 50

…one of the groups on each carbon of C=C double bond is the same

Question 51

we use the E/Z naming system for..

Answer 51

..compounds that have an H on each carbon of C=C

Question 52

why is there a lack of free rotation around the C=C double bond

Answer 52

Due to the pi-bond formation, rotation is restricted because if we rotate the atoms around pi bond, then the overlapping of the p orbitals will get disturbed.