CD - Energy and matter

Question 1

how can we see colour?

Answer 1

by absorbing the right amount of energy, electrons can move from a filled molecular orbital to an empty one - forming an excited state
- this energy can be in the form of UV or visible light

if the energy gap between filled and empty orbitals correspond to frequencies of visible light, then the molecule appears coloured
- its colour will be complementary to the frequency it absorbs

Question 2

What affects the energy needed to excite electrons?

Answer 2

delocalisation

as delocalisation increases more molecular orbitals form and are closer in energy

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

this is why functional groups that extent the delocalisation in chromophores cause colour change

Question 3

What affects the energy needed to excite electrons: single covalent bond

Answer 3

single covalent bond forms when two atomic orbitals each hold one electron

since each molecular orbital can hold 2 electrons only one is filled

the energy gap between these orbitals is very large

high frequency UV is needed to excite the electrons

Question 4

What affects the energy needed to excite electrons: double bond

Answer 4

contains four molecular orbitals

smaller energy gap between the highest filled orbital and the lowest empty orbital than in a single bond

requires lower frequency UV to excite the electrons

Question 5

What affects the energy needed to excite electrons: delocalised system

Answer 5

in a delocalised system (like benzene) many molecular orbitals are formed

these orbitals are closer in energy than in a double bond

the electrons in them absorb very low frequency UV and visible light to become excited

Question 6

What is a delocalised system?

Answer 6

not all delocalised systems involve benzene rings

when C-C and C=C alternate electrons become delocalised - this is known as conjugation