Colour by Design - Colour and Conjugated Systems

Question 1

draw the colour wheel

Answer 1

Question 2

why do objects appear coloured?

Answer 2

the wavelengths of light for the complementary colour are absorbed

Question 3

why do some objects appear colourless?

Answer 3

no wavelengths of visible light are absorbed by the object - they are all transmitted

Question 4

why do some objects appear white?

Answer 4

no wavelengths of visible light absorbed - all reflected

Question 5

what is the equation linking wavelength and frequency?

Answer 5

c = ν λ

speed of light = frequency x wavelength

Question 6

what change in molecular energy do radiowaves cause?

Answer 6

translational

Question 7

describe the effect of translation on molecules

Answer 7

whole molecule moves

Question 8

what change in molecular energy do microwaves cause?

Answer 8

rotational

Question 9

describe the effect of rotational work done on a molecule

Answer 9

whole molecule rotates

Question 10

what change in molecular energy does infrared radiation cause?

Answer 10

vibrational

Question 11

describe the effect of vibrational work done on a molecule

Answer 11

the bonds vibrate

Question 12

what change in molecular energy does visible and UV light cause?

Answer 12

electronic excitation

Question 13

describe the effect of excitation on a molecule

Answer 13

electrons change energy level

Question 14

what change in molecular energy does UV light also cause?

Answer 14

electronic dissociation

Question 15

describe the effect of dissociation on a molecule

Answer 15

bonds break

Question 16

what change in molecular energy does x-ray and gamma cause?

Answer 16

electronic ionisaton

Question 17

describe the effect of ionisation on a molecule

Answer 17

electrons leave the molecule

Question 18

why are most organic compounds colourless?

Answer 18

the excited state of electrons is above visible light - so we can’t see

Question 19

what is a conjugated system?

Answer 19

a system containing alternating single and double bonds

Question 20

what is an example of a conjugated system?

Answer 20

aromatic molecules

Question 21

what does a conjugated system allow its electrons to do?

Answer 21

become delocalised across the many atoms in the molecule

Question 22

other than single and double carbon bonds, what else can conjugated systems include?

Answer 22

lone pairs on oxygen and nitrogen if aligned in correct direction to be able to overlap

the π electrons on C = O and C = N

one of the pairs in N₂

Question 23

how do conjugated systems affect the overall colour of a molecule?

Answer 23

it decreases the gap between the ground electronic state and excited state

this means excited state more likely to overlap with visible light, producing a coloured molecule

Question 24

as a general rule, what number of π bonds do there need to be in a conjugated system for it to become coloured?

Answer 24

5

Question 25

if the conjugated system is larger, what size will the energy gap be?

Answer 25

smaller