CD.1

Question 1

Why do white opaque objects appear white?

Answer 1

None of the visible light wavelength is absorbed by the surface of the object. All visible light wavelengths are reflected and we perceive the object as white.

Question 2

Why do colourless transparent substances appear colourless?

Answer 2

None of the visible light wavelength is absorbed. All visible light wavelengths are transmitted and therefore it looks colourless.

Question 3

Why are some objects coloured?

Answer 3

Because they absorb certain frequencies of the visible region, corresponding to different colours.

Question 4

When an opaque object absorbs wavelength corresponding to a colour, what colour would it appear to be?

Answer 4

The complementary colour, which corresponds to the colour of the wavelengths that has been reflected.

Question 5

When a transparent substance absorbs wavelength corresponding to a colour, what colour would it appear to be?

Answer 5

The complementary colour, which corresponds to the colour of the wavelengths that has been transmitted.

Question 6

What is the complementary colour of red?

Answer 6

Green

Question 7

What is the complementary colour of yellow?

Answer 7

Violet

Question 8

What is the complementary colour of blue?

Answer 8

Orange

Question 9

The lower frequency/longer wavelength end of the visible light spectrum corresponds to which colour?

Answer 9

Red

Question 10

The higher frequency/shorter wavelength end of the visible light spectrum corresponds to which colour?

Answer 10

Violet

Question 11

What kind of electronic energy change happens when molecules absorb visible light radiation?

Answer 11

Excitation.

Question 12

What happens when molecules absorb visible light radiation?

Answer 12

Electrons are excited from their ground state to an excited state.

Question 13

What happens when molecules require greater energy that visible light to cause excitational energy change?

Answer 13

They would absorb UV radiation rather than visible light.

Question 14

What colour would molecules be if they require UV radiation to raise electron to their excited state?

Answer 14

They would be white or colourless.

Question 15

What is a conjugated system?

Answer 15

In an organic molecule, where it consists a system of alternating double and single bonds.

Question 16

How are electrons arranged in a conjugated system?

Answer 16

The electrons in the p-orbitals that make up the π-bonds of the double bonds are able to spread out over all atoms of the system. The electrons are said to be delocalised in a conjugated system.

Question 17

As well as the normal C=C double bonds, what other bonds or atoms can contribute to the delocalisation of electrons in a conjugated system?

Answer 17

C≡C, C=O, N=N and other multiple bonds. Lone pairs on oxygen and nitrogen atoms can also be part of the conjugated system if they are aligned in the correct direction to overlap with other delocalised electrons in the system.

Question 18

What is a very important example of a conjugated system?

Answer 18

Benzene.

Question 19

In terms of molecular structure, what explains the colours in organic molecules?

Answer 19

The presence of conjugated systems.

Question 20

What effect does a conjugated system have on the energy required for a molecule to undergo excitation electronic energy change?

Answer 20

It decreases the energy needed to raise electrons from their ground state to the excited state.

Question 21

Explain in three steps why a large conjugated system means a molecules is more likely to be coloured?

Answer 21

1. The more electrons that are delocalised in the conjugated system, the smaller the energy gap between the ground stated and the excited state.
2. A smaller energy gap results in the absorption of longer wavelength/lower frequency radiation.
3. This means the larger the conjugated system, the more likely the molecule will absorb in the visible light region rather than UV region and be coloured.

Question 22

What does the basic rule of thumb state about the least number of π bonds a conjugated system should have for it to likely absorb in the visible light region and be coloured?

Answer 22

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