CbD - Colour

Question 1

What is an electron configuration?

Answer 1

The arrangement of electrons in an atom.

Question 2

What is electron configuration described in terms of?

Answer 2

Shells and sub-shells.

Question 3

What happens when electrons move between shells?

Answer 3

Their energy levels change.

Question 4

What happens in an atom when a covalent bond forms?

Answer 4

Atomic orbitals link up to form molecular orbitals.

Question 5

Which molecular orbitals fill up first?

Answer 5

The lowest energy orbitals.

Question 6

What are molecular orbitals?

Answer 6

Atomic orbitals link up to form molecular orbitals when a covalent bond is formed.

Question 7

By absorbing exactly the right amount of energy what can electrons do?

Answer 7

Move from a filled molecular orbital to an empty one - forming an excited state.

Question 8

What can the energy absorbed be in the form of?

Answer 8

UV or visible light.

Question 9

What happens if the energy gap filled and the empty molecular orbitals correspond to frequencies of light?

Answer 9

The molecule appears coloured.

Question 10

When does a molecule appear coloured in terms of energy levels?

Answer 10

If the energy gap filled and the empty molecular orbitals correspond to frequencies of light.

Question 11

What does the colour of the molecule match?

Answer 11

The complement of the frequency it absorbs.

Question 12

What does delocalisation affect?

Answer 12

The energy needed to excite electrons.

Question 13

The energy needed to excite electrons will be affected by what?

Answer 13

Delocalisation.

Question 14

When does a single covalent bond form?

Answer 14

When two atomic orbitals, each holding one electron, come together to form two molecular orbitals.

Question 15

What needs to be absorbed to excite the electrons if there is a large energy gap (in a single bond)?

Answer 15

High-frequency UV.

Question 16

Where do the electrons in a single covalent bond go in the molecular orbital?

Answer 16

Since each molecular orbital can hold two electrons, only one is filled.

Question 17

What is the energy gap like between the filled and unfilled molecular orbital in a single covalent bond?

Answer 17

Very large.

Question 18

How many molecular orbitals does a double covalent bond contain?

Answer 18

4

Question 19

How many molecular orbitals does a single covalent bond contain?

Answer 19

2

Question 20

What is the energy gap like between the highest filled molecular orbital and the lowest empty molecular orbital in a double covalent bond compared to the energy gap in a single covalent bond?

Answer 20

There is a smaller energy gap between the highest filled molecular orbital and the lowest empty molecular orbital in a double covalent bond.

Question 21

What needs to be absorbed to excite the electrons if there is a small energy gap (in a double bond)?

Answer 21

Lower frequency UV.

Question 22

In a delocalised system (like benzene), how many molecular orbitals are formed?

Answer 22

Many.

Question 23

What is the energy gap like between orbitals in a delocalised system?

Answer 23

The orbitals are even closer in energy than in a double bond.

Question 24

What needs to be absorbed to excite the electrons if there is a VERY small energy gap (in a delocalised system)?

Answer 24

VERY low frequency UV and visible light.

Question 25

What happens as delocalisation increases?

Answer 25

More molecular orbitals form and are closer in energy.

Question 26

If more molecular orbitals that are closer in energy form as delocalisation increases, what does this mean needs to happen for the electrons to be excited?

Answer 26

Less energy is needed to excite electrons and the frequency of light absorbed drops.

Question 27

Why do functional groups that extend the delocalisation in chromophores cause the colour to change?

Answer 27

Because more molecular orbitals that are closer in energy form as delocalisation increases meaning less energy is needed to excite electrons and the frequency of light absorbed drops.

Question 28

Do all delocalised systems involve benzene rings?

Answer 28

No

Question 29

What happens when C=C double and C-C single bonds alternate?

Answer 29

Electrons become delocalised.

Question 30

What is conjugation?

Answer 30

When C=C double and C-C single bonds alternate causing electrons to become delocalised.

Question 31

Equation linking wave speed, wavelength and frequency

Answer 31

wave speed = wavelength x frequency

Question 32

What is a conjugated system?

Answer 32

Alternating single and double bonds.

Question 33

What does a conjugated system allow?

Answer 33

Delocalisation of electrons across many atoms in the molecule.

Question 34

What can a conjugated system contain?

Answer 34

π (Pi) electrons from C=O and C=N bonds.

Can also include lone pairs on oxygen and nitrogen if they are aligned in the correct direction to allow overlap with the system.

Question 35

Relationship between energy and level of conjugation

Answer 35

Lower in energy (smaller the energy gap) if more conjugated.

Question 36

Why does a conjugated system produce colour?

Answer 36

A conjugated system decreases the energy gap between ground electron state and the excited state. So electrons in the molecule are allowed to absorb visible light frequencies.

Question 37

What do the size of the energy gaps mean for the wavelengths and frequencies of light that are absorbed?

Answer 37

Smaller energy gaps = longer wavelengths ( and lower frequencies) of light being absorbed.

Question 38

How many π (Pi) bonds must be in a conjugated system for it to absorb light in the range of the visible light spectrum?

Answer 38

5+ (7 is best).