U3-1 - Molecular Orbitals

Question 1

How can you tell the number of carbons in this structure?

Answer 1

Count each ‘kink’ on the zigzag, including the ends.

This structure has 7 carbons.

Question 2

How can you tell how many hydrogens are atached to the circled carbon?

Answer 2

No. of H = 4 – No. of bonds made

This carbon makes 3 bonds, so must have 1 H.

Question 3

Two atomic orbitals combine to make ___ molecular orbitals.

Answer 3

two

Question 4

What is the difference between bonding and antibonding orbitals?

Answer 4

Antibonding orbitals are higher in energy.

Question 5

How do sigma bonds form?

Answer 5

End on overlap of atomic orbitals

Question 6

How do pi bonds form?

Answer 6

Side on overlap of atomic orbitals.

Question 7

Which bonds are stronger, sigma or pi?

Answer 7

Sigma (better overlap of atomic orbitals)

Question 8

How do you count the number of sigma and pi bonds in a molecule?

Answer 8

Single bonds are always sigma.

In double and and triple bonds, first bond is sigma, the rest are pi.

Question 9

Type of hybridisation found in alkanes

Answer 9

sp³

Question 10

Type of hybridisation found in alkenes and aromatic rings

Answer 10

sp²

Question 11

Type of hybridisation found in alkynes (C≡C)

Answer 11

sp

Question 12

Bond angle associated with sp³ hybridisation

Answer 12

109.5 (tetrahedral)

Question 13

Bond angle associated with sp² hybridisation

Answer 13

120 (trigonal planar)

Question 14

Bond angle associated with sp hybridisation

Answer 14

180 (linear)

Question 15

Explain how sp³ hybrid orbitals form

Answer 15

One s and three p orbitals mix to form four degenerate sp³ atomic orbitals

Question 16

Explain how sp² hybrid orbitals form

Answer 16

One s and two p orbitals mix to form three degenerate sp² atomic orbitals

Question 17

Explain how sp hybrid orbitals form

Answer 17

One s and one p orbital mix to form two degenerate sp atomic orbitals

Question 18

Which orbitals overlap to form sigma bonds in alkanes?

Answer 18

sp³ hybrid orbitals (overlap end on)

Question 19

Which orbitals overlap to form pi bonds in alkenes and aromatic rings?

Answer 19

unhybridised 2p orbitals (overlap side on)

Question 20

Which orbitals overlap to form sigma bonds in aromatic rings?

Answer 20

sp² hybrid orbitals (overlap end on)

Question 21

Formula for benzene

Answer 21

C₆H₆

Question 22

Benzene structure (skeletal)

Answer 22

Question 23

Benzene structure (full)

Answer 23

Question 24

State the feature that provides stability to aromatic rings.

Answer 24

Delocalised electrons

Question 25

Explain what is meant by the highest occupied molecular orbital (HOMO).

Answer 25

Highest bonding molecular orbital containing e⁻.

Question 26

Explain what is meant by the lowest unoccupied molecular orbital (LUMO).

Answer 26

Lowest antibonding molecular orbital (no e⁻ present)

Question 27

An organic complex is colourless if it absorbs ______ light.

Answer 27

UV

Question 28

An organic complex absorbs UV light. Why is this?

Answer 28

Large energy gap between HOMO and LUMO

Question 29

Chromophore

Answer 29

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

Question 30

State what happens to e^– when a chromophore absorbs light

Answer 30

An e^– is promoted from HOMO to LUMO

Question 31

Chromophores exist in molecules containing a _______________ _________.

Answer 31

conjugated system

Question 32

Conjugated system

Answer 32

A system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of C atoms.

Question 33

What structural feature is present in conjugated systems?

Answer 33

Alternating single and double bonds

Question 34

A system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of C atoms.

Answer 34

Conjugated system

Question 35

Electrons in a conjugated system can be described as __________.

Answer 35

delocalised

Question 36

How can you compare the sizes of conjugated systems in different molecules?

Answer 36

Count the no. of (alternating) double bonds. More double bonds = larger conjugated system.

Question 37

The more atoms in the conjugated system, the ___________ the energy gap between HOMO and LUMO.

Answer 37

smaller

Question 38

The larger the conjugated system, the _________ the wavelength of light absorbed.

Answer 38

longer (lower E)

Question 39

When the wavelength of light absorbed is in the visible region, the compound will exhibit the __________________ colour.

Answer 39

complementary