unit 2b: molecular orbitals

Question 1

molecular orbitals

Answer 1

formed when atoms approach each other to form bonds and their atomic orbitals combine e.g. two hydrogen atoms- the 1s orbitals from each atom combine

Question 2

the number of molecular orbitals that form

Answer 2

is equal to the number of atomic orbitals that combine

Question 3

bonding orbital

Answer 3

the highest occupied molecular orbital (HOMO)

Question 4

antibonding orbital

Answer 4

lowest unoccupied molecular orbital (LUMO)

Question 5

molecular orbitals can hold

Answer 5

a maximum of two electrons

Question 6

electrons fill

Answer 6

bonding MOs and leave the higher energy antibonding MOs unfilled

Question 7

in the bonding MO

Answer 7

the attraction of the positive nuclei and the negative electrons is the basis of bonding between atoms

Question 8

electrons fill from the

Answer 8

lowest energy orbital up

Question 9

non polar covalent bond

Answer 9

electrons are shared equally so the bonding MO is spread evenly over both nuclei, the bonding MO is symmetrical around the midpoint between the two atoms

Question 10

polar covalent bond

Answer 10

the two nuclei are sharing electrons but not evenly, the bonding MO is asymmetrical around the midpoint between the two atoms, more of the MO appears over the more electronegative atom

Question 11

ionic bond

Answer 11

an electron has transferred to the more electronegative atom, the bonding MO will sit almost fully over the negative ion, this is an extreme case of asymmetry

Question 12

sigma molecular orbitals

Answer 12

formed by the end-on overlap of atomic orbitals

Question 13

carbons ability to form 4 bonds

Answer 13

can be explained by a theory called hybridisation

Question 14

2s and 2p orbitals

Answer 14

are close in energy allowing one of the 2s electrons to be promoted to the empty 2p orbital

Question 15

bonding in alkanes

Answer 15

the three 2p orbitals and the 2s orbital mix with each other (hybridise) and create four new hybrid atomic orbitals

Question 16

sp3 orbitals

Answer 16

they are made of one s and three p orbitals and are degenerate

Question 17

sp3 hybridisation

Answer 17

one electron occupies each sp3 orbital resulting in 4 unpaired electrons in identical, degenerate orbitals

Question 18

four sp3 orbitals

Answer 18

take up a tetrahedral arrangement around the C atom

Question 19

forming methane

Answer 19

the s orbitals of four hydrogen atoms approach the four sp3 orbitals, end on overlap occurs between them, four new molecular orbitals are created

Question 20

bonds in methane

Answer 20

sigma molecular orbitals are formed therefore sigma bonds form between C and each H, one molecule of methane contains four sigma bonds

Question 21

bonding in ethane

Answer 21

end on overlap occurs between three of the sp3 orbitals of each carbon

Question 22

ethane has … bonds

Answer 22

seven sigma bonds, six C-H sigma bonds and one C-C sigma bond, both carbons are sp3 hybridised

Question 23

sideways overlap compared to end on overlap

Answer 23

weaker

Question 24

pi orbitals

Answer 24

formed from sideways overlap of orbitals, the bonding pi orbital exists in two parts from the top and bottom lobes

Question 25

bonding in alkene

Answer 25

only two of the p orbitals then mix with the 2s orbital

Question 26

sp2 hybridisation

Answer 26

occurs in alkenes where three degenerate sp2 hybrid orbitals are formed, the other p orbital remains unhybridised

Question 27

three sp2 hybrid orbitals

Answer 27

adopt a trigonal planar arrangement, the unhybridised p orbital is perpendicular to the sp2 hybrid orbitals

Question 28

ethene bonding

Answer 28

4 sigma bonds form between the sp2 orbitals and the s orbitals of the hydrogens, a sigma bond forms between both carbons, sideways overlap occurs between the two unhybridised p orbitals forming a pi bond

Question 29

a C=C double bond consists of

Answer 29

one sigma bond and one pi bond

Question 30

molecules that contain a benzene ring are known as

Answer 30

aromatics

Question 31

bonding in aromatics

Answer 31

the carbon atoms in benzene undergo sp2 hybridisation, end on overlap occurs between the carbon atoms to form sigma bonds and one hydrogen is bonded to each carbon with a sigma bond

Question 32

pi bonds in benzene

Answer 32

each carbon has an unhybridised p orbital, sideways overlap occurs forming a donut shaped pi bonding system above and below the carbon ring

Question 33

alkynes

Answer 33

unsaturated hydrocarbons which all contain a carbon to carbon triple bond

Question 34

sp hybridisation

Answer 34

takes place to allow a triple bond to form

Question 35

bonding in alkynes

Answer 35

an electron from the 2s is promoted to the empty 2p, the 2s orbital and one of the 2p orbitals hybridise to form two degenerate sp hybrid orbitals leaving two unhybridised p orbitals

Question 36

sp hybrid orbitals

Answer 36

are linear around the carbon, the two unhybridised p orbitals lie perpendicular to the sp hybrid orbitals and to each other

Question 37

C-C

Answer 37

sp3 hybridisation, 1 sigma bond

Question 38

C=C

Answer 38

sp2 hybridisation, 1 sigma bond and 1 pi bond

Question 39

C=-C

Answer 39

sp hybridisation, 1 sigma bond and 2 pi bonds

Question 40

molecular orbital theory

Answer 40

can be used to explain why organic molecules are colourless or coloured

Question 41

deltaE

Answer 41

the absorption of energy can cause electrons to be promoted from HOMO to LUMO

Question 42

in most organic molecules

Answer 42

deltaE is large meaning the wavelength of light absorbed lies outwith the visible spectrum so no colour is observed

Question 43

chromophores

Answer 43

a group of atoms within a molecule that is responsible for the absorption of light in the visible region of the spectrum

Question 44

coloured organic molecules contain

Answer 44

chromophores

Question 45

chromophore example

Answer 45

beta carotene

Question 46

chromophores exist in

Answer 46

molecules with a conjugated system

Question 47

conjugated system

Answer 47

adjacent unhybridised p orbitals that overlap sideways to form a molecular orbital across a number of carbon atoms where the electrons are delocalised

Question 48

a conjugated system often consists of

Answer 48

alternating double and single bonds (or multiple connected benzene rings)

Question 49

a chromophore/ conjugated system always starts and ends with

Answer 49

a double bond

Question 50

as the conjugated system gets larger

Answer 50

the energy gap between HOMO and LUMO decreases therefore lower energy light is required to promote an electron so the light absorbed is part of the visible spectrum

Question 51

the colour of the compound is

Answer 51

the complementary colour of the light absorbed

Question 52

different colours of organic compounds are the result of

Answer 52

differing sizes of the conjugated system

Question 53

explain fully how colour arises in molecules

Answer 53

electrons move from HOMO to LUMO, absorption of light means the light of the complementary colour is seen

Question 54

explain why a molecule with a shorted chromophore will absorb a shorter wavelength of light

Answer 54

there is less conjugation, larger energy gap

Question 55

explain what is meant by sp2 hybridisation

Answer 55

mixing an s orbital with two p orbitals

Question 56

explain how a conjugated system gives rise to the colour red

Answer 56

electrons promoted from HOMO to LUMO, the complementary colour is absorbed